ORIGIN OF THE EARTH
22.1 SOLAR SYSTEM
- Our solar system is the only planetary system which is known to us at present though there may be numerous such systems scattered in the space.
- The earth is a member planet of our solar system.
- It is, therefore, necessary to have some elementary knowledge of the solar system to understand the origin of the earth.
- Planets are non – luminous bodies whereas stars are luminous bodies of the universe around us.
- In other words, the planets do not have their own light, rather they reflect the light of the stars while the stars have their own light and energy due to thermonuclear reactions wherein hydrogen nuclei combine under intense temperature and pressure to form helium nuclei which release vast amount of energy.
- The congregation of stars and planets is known as solar system.
- like shape consists of 8 planets (e.g. Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune) and one star (the sun).
- Besides, there are numerous minor planets (Planetoids or asteroids). All these bodies revolve around the sun almost in the same plane and in the same direction along the near circular elliptical orbits.
- Most of the satellites of the planets also revolve in the same direction.
- The planetoids with some exceptions have their orbits between the orbits of Mars and Jupiter.
- The rotatory motion of the planets (except Venus and Uranus) is in the same direction as their revolution around the sun i.e. anticlockwise for one who looks at the earth from the North Pole to the South Pole.
According to the Titius Bode Rule, the distance of each planet from the sun is approximately twice that of next planet close to the sun.
- The diameter of the whole solar system is about 1,173 crore kilometres.
- The earth is about 149,600,000 Km (1.496 away from the sun.
- The diameter and average density of the earth are 12,742km and 5.52 respectively.
- Average distances from the sun, period of orbital revolution, period of rotation, diameter, average density and number of satellites of the member planets of our solar system are presented.
I. THE INNER PLANETS
- MERCURY is the smallest planet and closest to the Sun.
- It has the longest period of rotation and the shortest “year” taking only 88 Earth-days to orbit the sun.
- For a long time, astronomers thought that Mercury always kept the same face toward the Sun. But new observations show that the planet rotates once every 58 or 59 days.
- It has very thin atmosphere which cannot support life.
- VENUS is the brightest of all planets because it reflects more of the sun’s light which falls upon it than other planets do.
- As its orbit is within the earth’s, it never gets very far from the sun and it is easily seen after sunset.
- It is called the “Evening Star” when it appears in the western sky and “Morning Star” when it appears in the eastern sky.
- The thick atmosphere that hangs over Venus traps the Sun’s energy and helps build up the most furnace-like heat yet found on any planet.
- So, with its very high surface temperatures, and its atmosphere consisting largely of carbon dioxide, Venus certainly cannot support life as it exists on earth.
- In general, the planets and their satellites rotate from west to east, but Venus rotates on its axis from east to west.
3. THE EARTH
- THE EARTH appears as a large greenish blue disc with its blue oceans and sparkling white polar ice caps.
- Our earth’s orbit lies between those of Mars and Venus.
- Its average distance from the Sun is 150 million km.
- It takes 23 hours, 56 minutes and 4 seconds to complete one rotation and 362 days, 5 hours, 48 minutes and 46 seconds (i.e. approximately 365 ¼ days) to complete its orbit around the Sun. This revolution around the Sun gives us our year.
- For convenience, one-quarter of a day is ignored for three years and then an extra day is added to every fourth year to give us our Leap Year of 366 days.
- The Earth’s circumference is about 40,000 km and its diameter is 12,756.8 km across the equator and 12,713.8 km across the poles.
- Its surface area is about 510 million sq km.
- The size of the earth is therefore great but not vast in comparison with other planets such as Jupiter or Saturn.
- Our planet has one satellite, the Moon.
- MARS is “the Red Planet”, is smaller than the earth, and has very thin atmosphere and a very small amount of oxygen.
- It is farther away from the Sun than the earth and so it is much colder.
It takes 24 hours and 37 minutes to rotate on its axis and 687 days to complete one revolution.
- Mars presents its poles alternately to the Sun in the same way that the earth does because its axis is titled in a similar manner.
- Temperatures on Mars range from 2.670C to a low of -700C.
- About three-quarters of the planet’s surface is covered with bright reddish or yellowish patches.
THE ASTEROIDS (OR PLANETOIDS)
- There is a gap of more than 550 million km between the two (Mars & Jupiter). This gap is filled by millions of tiny planets known as Planetiods or Asteroids.
- They are mostly tiny lumps of rocks and altogether about 6,000 of them have been discovered.
- The largest, Ceres, has a diameter of 768 km.
- Inspite of their great numbers their combined mass is only a fraction (about 1/1000) of the Earth’s mass.
- All of them revolve round the sun in the same direction as the planets and their mean period of revolution is 4.5 years.
- The orbits of some of the asteroids come within the orbits of all the inner planets.
- Hermes, one of the small asteroids has come within 7,76,000 km of the earth
II. THE OUTER PLANETS
- JUPITER is the largest planet of the Solar System.
- If it were hollow there would be room for thirteen hundred earths inside it.
- It is only about 300 times as heavy as the earth, indicating that it must be much less dense. However, its mass is more than twice that of the eight other planets combined.
- Jupiter is nearly 780 million km from the Sun.
- One special feature of this planet is the Great Red Spot, 30,000 km long and 13,000 km wide.
- Jupiter’s upper cloud layers seem to have ammonia and methane, both poisonous gases.
- Hydrogen and helium are also probably present.
- It is the fastest rotating planet, taking less than 10 hours for rotation.
- SATURN is the sixth planet from the Sun and the second largest in the solar system.
- It is 9 times as large as the earth and is visible to the naked eye.
- Scientists say that the rings around Saturn are made up of millions of small bits of rocks and dust moving together round the planet.
- Outside the rings there are ten satellites, one of them, called Titan, is the largest known satellite of any planet.
- Saturn’s specific gravity is less than 1.0. (If a large ocean were available, Saturn would float in it).
- URANUS has 5 known satellites; one of them is very large.
- Uranus is tilted 980 as compared to only 23 1/20 for earth.
- It takes 84 years for revolution, so that its south polar regions receive sunlight for 42 years and then the North Pole gets its turn.
- Neptune appears blue in colour, has two satellites, and takes nearly 165 years to travel once round the sun.
- It has no air and is very cold, dark and desolate.
22.3 HEAVENLY BODIES
- Comets are the most exciting of the heavenly bodies.
- They are quite different from stars or from any of the planets but they are also a part of the Solar System.
- They are made up of small icy particles of gas and meteoric fragments.
- A well-developed comet has a tail which is always turned away from the Sun.
- The head forms the brightest portion of the comet.
- The tail which may extend millions of kilometers, even more than the distance of the Earth from the Sun, is made of gases blown out of the comet by the Sun’s rays.
- Comets move round the Sun in regular orbits but their orbits are very long oval paths (elongated ellipses).
- The orbits of many of the comets are so greatly elongated that it takes them hundreds and sometimes thousands of years to complete one revolution around the Sun.
- Halley’s Comet is one of the important comets which visits the earth regularly. It is visible once in 76 years.
- Meteors enter the earth’s atmosphere with such speed that the heat generated from friction with the air causes them to vaporise with a brief flash of brilliantly glowing gas.
- This is very fortunate for they could cause great damage if they fell directly on the earth.
- Meteors fragments that reach the ground are known as Meteorites.
- They consist of rock, iron or both combined.
- Most of them are quite small but a few weigh many tonnes.
- The Great Siberian Meteor of 1908, on reaching the earth, exploded so violently that trees were laid flat out to 50 kilometers from the area of impact.
WHY PLUTO IS NOT A PLANET?
- Pluto is called a “dwarf planet”. A dwarf planet orbits the sun just like other planets, but it is smaller. A dwarf planet is so small it cannot clear other objects out of its path.
- There are 5 officially recognised dwarf planets in our solar system, they are Ceres, Pluto, Haumea, Makemake and Eris.
- With the exception of Ceres, which is located in the asteroid belt, the other dwarf planets are found in the outer solar system.
- Dwarf planets share many of the same characteristics as planets though there is one significant difference.
- (a) A “dwarf planet” is a celestial body
that is in orbit around the Sun,
- (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape,
- (c) has not cleared the neighbourhood around its orbit, and
- (d) is not a satellite.
- (a) A “dwarf planet” is a celestial body
Shooting star is a common name for the visible path of a meteoroid as it enters the atmosphere, becoming a meteor. These are not stars at all. They are believed to come from different sources. The majority of them are small fragments like those in the belt of asteroids between Mars and Jupiter.
22.4 ORIGIN OF THE EARTH
- Various scientists and philosophers have propounded from time to time their concepts, hypotheses and theories to unravel the mystery and to solve the riddle of the problems of the origin and evolution of our solar system in general and of the earth in particular but none of these could be accepted by majority of the scientific community.
- Though there is no common consensus among the scientists about the origin of our solar system but it can be safely argued that all planets of our solar system are believed to have been formed by the same process.
- It means that all the concepts, hypotheses and theories propounded for the origin of the solar system are also applicable for the origin of the earth.
22.5 SCIENTIFIC CONCEPTS
- The scientific concepts, generally based on sound principles of hard sciences, are usually divided into two schools e.g. (i)’hot origin concepts’ and (ii) ‘cold origin concepts’.
HOT ORIGIN CONCEPTS
- According to the school of ‘hot origin’, our solar system and therefore the earth is believed to have been formed from the matter which as either initially
hot or was heated up in the process of the origin of the earth.
COLD ORIGIN CONCEPTS
- On the other hand, according to the school of ‘cold origin’ our solar system together with the earth was formed of the matter which was either initially cold or always remained cold.
- After the formation the earth was formed of the matter which was either initially cold or always remained cold.
- After the formation the earth might have been heated up due to the presence of radioactive elements or only the interior of the earth might have been heated up due to intense pressure exerted by the superincumbent load of the upper layers.
22.6 CLASSIFICATION OF SCIENTIFIC CONCEPTS
- On the basis of the number of heavenly bodies involved in the origin of the solar system and the earth the scientific concepts are further divided into three groups.
- (i) monistic concept (involving only one heavenly body,
- (ii) dualistic concept (involving two heavenly bodies) and
- (iii) binary star concept or trihybrid concept (involving more than two heavenly bodies).
BIG BANG THEORY
- The Big Bang Theory postulated in 1950’s -1960’s and validated in 1972 through convincing evidences received from COBE (Cosmic Back ground Explorer) explains the origin of universe and everything in it including ourselves on the premise that the universe contained many millions of galaxies, each on ‘having thousands of millions of stars and each star having numerous planets around them’.
- According to this theory, everything in the universe emerged from a point known as singularity, 15 billion years ago.
- The galaxies moved apart from one another as the empty space between them expanded.
- In the beginning the universe was much smaller as there was less space between the galaxies.
- All of the matter in the universe was created in one instant at a fixed moment in time.
- “As the universe expanded for 15 billion years, the hot radiation in the original fireball also expanded with it, and cooled as a result.”
- It may be summarized that there was a single fireball some 15 billion years ago.
- ‘There were already wispy clouds of matter stretching across vast distances, upwards 500 million light years across.
- As those clouds collapsed in upon themselves, pulled together by their own gravity, they would have broken up and formed clusters of galaxies with the galaxies themselves breaking up into stars like those of the Milky Way’.
- The stars might have broken up to form their planets as our earth.
22.8 STAGES IN EVOLUTION OF THE EARTH
- C.Chamberlin has attempted to describe and explain the evolution of different components of the earth. e.g. continents and ocean basins, folds and faults, volcanoes and earthquakes, mountains and plains, heat of the interior of the earth and its structure and the origin and evolution of its atmosphere through specific periods or stages.
- FIRST STAGE – ‘the period of planetesimal accession’ or ‘the period of acquisition of the present shape and size by the earth’.
- SECOND STAGE – “the period of dominant volcanism” or ‘the period of the evolution of the earth’s interior and the evolution of continents and ocean basins’.
- THIRD STAGE – ‘the actual geological period’ or ‘the period of the formation of the folds and faults, mountains and plateaux etc.
- These stages of the evolution of the earth are separated from each other only for the sake of convenience; otherwise these are so interlinked with each other that it is quite difficult to differentiate one stage from the other.
22.9 EVOLUTION OF THE EARTH
- The planet earth initially was a barren, rocky and hot object with a thin atmosphere of hydrogen and helium. This is far from the present day picture of the earth.
- Hence, there must have been some events–processes, which may have caused this change from rocky, barren and hot earth to a beautiful planet with ample amount of water and conducive atmosphere favouring the existence of life.
- Between the 4,600 million years and the present, led to the evolution of life on the surface of the planet.
- The earth has a layered
- From the outermost end of the atmosphere to the centre of the earth, the material that exists is not uniform. The atmospheric matter has the least density
- From the surface to deeper depths, the earth’s interior has different zones and each of these contains materials with different characteristics.
22.10 DEVELOPMENT OF LITHOSPHERE
- The earth was mostly in a volatile state during its primordial stage.
- Due to gradual increase in density the temperature inside has increased.
- As a result, the material inside started getting separated depending on their densities.
- This allowed heavier materials (like iron) to sink towards the centre of the earth and the lighter ones to move towards the surface.
- With passage of time it cooled further and solidified and condensed into a smaller size.
- This later led to the development of the outer surface in the form of a crust.
- During the formation of the moon, due to the giant impact, the earth was further heated up.
- It is through the process of differentiation that the earth forming material got separated into different layers.
- Starting from the surface to the central parts, we have layers like the crust, mantle, outer core and inner core.
- From the crust to the core, the density of the material increases.
22.11 EVOLUTION OF ATMOSPHERE AND HYDROSPHERE
- The present composition of earth’s atmosphere is chiefly contributed by nitrogen and oxygen.
- There are three stages in the evolution of the present atmosphere.
- FIRST STAGE – This marked by the loss of primordial atmosphere.
- SECOND STAGE – the hot interior of the earth contributed to the evolution of the atmosphere
- THIRD STAGE- the composition of the atmosphere was modified by the living world through the process of photosynthesis.
- The early atmosphere, with hydrogen and helium, is supposed to have been stripped off as a result of the solar winds.
- This happened not only in case of the earth, but also in all the terrestrial planets, which were supposed to have lost their primordial atmosphere through the impact of solar winds.
- During the cooling of the earth, gases and water vapour were released from the interior solid earth. This started the evolution of the present atmosphere.
- The early atmosphere largely contained water vapour, nitrogen, carbon dioxide, methane, ammonia and very little of free oxygen
- The process through which the gases were outpoured from the interior is called degassing.
- As the earth cooled, the water vapour released started getting condensed.
- The carbon dioxide in the atmosphere got dissolved in rainwater and the temperature further decreased causing more condensation and more rains
- The rainwater falling onto the surface got collected in the depressions to give rise to oceans.
- The earth’s oceans were formed within 500 million years from the formation of the earth.
- The oceans are as old as 4,000 million years and around 3,800 million years ago, life began to evolve.
- However, around 2,500-3,000 million years before the present, the process of photosynthesis got evolved. Life was confined to the oceans for a long time.
- Oceans began to have the contribution of oxygen through the process of photosynthesis.
- Eventually, oceans were saturated with oxygen, and 2,000 million years ago, oxygen began to flood the atmosphere.
22.12 ORIGIN OF LIFE
- The last phase in the evolution of the earth relates to the origin and evolution of life
- It is undoubtedly clear that the initial or even the atmosphere of the earth was not conducive for the development of life
- Modern scientists refer to the origin of life as a kind of chemical reaction, which first generated complex organic molecules and assembled them.
- This assemblage was such that they could duplicate themselves converting inanimate matter into living substance.
- The record of life that existed on this planet in different periods is found in rocks in the form of fossils.
- The microscopic structures closely related to the present form of blue algae have been found in geological formations that are much older than these were some 3,000 million years ago.
- It can be assumed that life began to evolve sometime 3,800 million years ago.
|Eons||Era||Period||Epoch||Age / Years Before Present||Life / Major Events|
|Cainozoic (From 65 million years to the present|
|Quaternary||Holocene Pleistocene||0 |
|Modern Man Homo Sapiens|
Rats - Mice
|65 – 144|
144 – 208
208 – 245
|Extinction of Dinosaurs Age of Dinosaurs
245 – 570
|245 – 286|
286 – 360
360 – 408
408 – 438
438 – 505
505 – 570
First trace of
life on land:
|Soft – bodied
form – Ocean
are rich in
|Origin of the
Origin of the
INTERIOR OF THE EARTH
- The configuration of the surface of the earth is largely a product of the processes operating in the interior of the earth.
- Exogenic as well as endogenic processes are constantly shaping the landscape.
- A proper understanding of the physiographic character of a region remains incomplete if the effects of endogenic processes are ignored.
- Human life is largely influenced by the physiography of the region.
- Therefore, it is necessary that one gets acquainted with the forces that influence landscape development.
The energy emanating from within the earth is the main force behind endogenic geomorphic processes. This energy is mostly generated by radioactivity, rotational and tidal friction and primordial heat from the origin of the earth.
The exogenic processes derive their energy from atmosphere determined by the ultimate energy from the sun and also the gradients created by tectonic factors.
23.2 SOURCES OF INFORMATION ABOUT THE INTERIOR
- The earth’s radius is 6,370 km.
- Most of our knowledge about the interior of the earth is largely based on estimates and inferences.
- Yet, a part of the information is obtained through direct observations and analysis of materials.
23.2.1 DIRECT SOURCES
- The most easily available solid earth material is surface rock or the rocks we get from mining areas.
- Gold mines in South Africa are as deep as 3 – 4 km. Going beyond this depth is not possible as it is very hot at this depth.
- Besides mining, scientists have taken up a number of projects to penetrate deeper depths to explore the conditions in the crustal portions.
- Scientists world over are working on two major projects such as “Deep Ocean Drilling Project” and “Integrated Ocean Drilling Project”.
- The deepest drill at Kola, in Arctic Ocean, has so far reached a depth of 12 km.
- This and many deep drilling projects have provided large volume of information through the analysis of materials collected at different depths.
- Volcanic eruption forms another source of obtaining direct information.
- As and when the molten material (magma) is thrown onto the surface of the earth, during volcanic eruption it becomes available for laboratory analysis.
- However, it is difficult to ascertain the depth of the source of such magma.
- The energy emanating from within the earth is the main force behind endogenic geomorphic processes. This energy is mostly generated by radioactivity, rotational and tidal friction and primordial heat from the origin of the earth.
- The exogenic processes derive their energy from atmosphere determined by the ultimate energy from the sun and also the gradients created by tectonic factors.
23.2.2 INDIRECT SOURCES OF EARTH’S INTERIOR
- Analysis of properties of matter indirectly provides information about the interior.
- Through the mining activity that temperature and pressure increase with the increasing distance from the surface towards the interior in deeper depths.
- Moreover, it is also known that the density of the material also increases with depth.
- It is possible to find the rate of change of these characteristics
- Knowing the total thickness of the earth, scientists have estimated the values of temperature, pressure and the density of materials at different depths.
- Another source of information are the meteors that at times reach the earth.
- However, it may be noted that the material that becomes available for analysis from meteors, is not from the interior of the earth.
- The material and the structure observed in the meteors are similar to that of the earth.
- They are solid bodies developed out of materials same as, or similar to, our planet.
- Hence, this becomes yet another source of information about the interior of the earth.
- The gravitation force (g) is not the same at different latitudes on the surface. It is greater near the poles and less at the equator.
- This is because of the distance from the centre at the equator being greater than that at the poles.
iii. GRAVITY ANOMALY
- The gravity values also differ according to the mass of material.
- The uneven distribution of mass of material within the earth influences this value.
- The reading of the gravity at different places is influenced by many other factors.
- These readings differ from the expected values. Such a difference is called gravity anomaly.
- Gravity anomalies give us information about the distribution of mass of the material in the crust of the earth.
23.2.3 OTHER INDIRECT SOURCES
- Magnetic surveys also provide information about the distribution of magnetic materials in the crustal portion, and thus, provide information about the distribution of materials in this part.
- Seismic activity is one of the most important sources of information about the interior of the earth.
23.3 STRUCTURE OF THE EARTH
23.3.1 THE CRUST
- It is the outermost solid part of the earth.
- It is brittle in nature.
- The thickness of the crust varies under the oceanic and continental areas.
- Oceanic crust is thinner as compared to the continental crust.
- The mean thickness of oceanic crust is 5 km whereas that of the continental is around 30 km.
- The continental crust is thicker in the areas of major mountain systems.
- It is as much as 70 km thick in the Himalayan region.
- It is made up of heavier rocks having density of 3 g/cm3. This type of rock found in the oceanic crust is basalt.
- The mean density of material in oceanic crust is 2.7 g/cm3.
23.3.2 THE MANTLE
- The portion of the interior beyond the crust is called the mantle.
- The mantle extends from Moho’s discontinuity to a depth of 2,900 km.
- The upper portion of the mantle is called asthenosphere. The word astheno means weak.
- It is considered to be extending upto 400 km.
- It is the main source of magma that finds its way to the surface during volcanic eruptions.
- It has a density higher than the crust’s density (3.4 g/cm3).
- The crust and the uppermost part of the mantle are called lithosphere. Its thickness ranges from 10-200 km.
- The lower mantle extends beyond the asthenosphere. It is in solid state.
23.3.3 THE CORE
- As indicated earlier, the earthquake wave velocities helped in understanding the existence of the core of the earth.
- The core-mantle boundary is located at the depth of 2,900 km.
- The outer core is in liquid state while the inner core is in solid state.
- The density of material at the mantle core boundary is around 5 g/cm3 and at the centre of the earth at 6,300 km, the density value is around 13 g/cm3.
- The core is made up of very heavy material mostly constituted by nickel and iron. It is sometimes referred to as the nife layer.
23.4 LAYERING SYSTEM OF EARTH
- The earth is made up of several concentric layers.
- The outer layer is the earth’s crust– the lithosphere, which comprises two distinct parts. The upper part consists of granitic rocks forms continents – SIAL and the lower part is a continuous zone of basaltic rocks forming the ocean floors – SIMA.
- It is located just below the outer sedimentary cover and is composed of granites and is dominated by Silica (SI) and Aluminium (AL).
- Average density of this layer is 2.9 and its thickness range from 50 to 300 km.
- This region is dominated by acid materials and silicates of potassium, sodium and aluminium.
- It is located just below SIAL layer and is composed of basalt and this layer is the source of magma and lava during volcanic eruptions .
- Silica (SI) and Magnesium (MA) are the dominant constituents.
- Average density between 2.9 – 4.7, whereas thickness varies from 1,000 km – 2,000 km and having abundance of basic matter and silicates of magnesium, calcium and iron are abundantly found.
- It is located below SIMA layer and composed of Nickel (Ni) and Ferrium (FE).
- This layer is made of heavy metals and is the reason for very high density of this layer.
- The diameter of this zone is 6,880 km and presence of iron indicates the magnetic property of the earth’s interior.
- This property also indicates the rigidity of the earth 23.5.
- The word lithosphere is derived from the Greek word lithos, meaning rock.
- The lithosphere is the solid outer section of Earth, which includes Earth’s crust, as well as the underlying cool, dense, and rigid upper part of the upper mantle.
- The lithosphere extends from the surface of Earth to a depth of about 70–100 km.
- This relatively cool and rigid section of Earth is believed to “float” on top of the warmer, non- rigid, and partially melted material directly below.
- This motion of the lithospheric plates is known as plate tectonics, and is responsible for many of the movements seen on Earth’s surface today including earthquakes, certain types of volcanic activity, and continental drift.
- The asthenosphere (from Greek ‘ weak’ + “sphere”) is the highly viscous, mechanically weak and deforming region of the upper mantle of the Earth.
- It lies below the lithosphere, at depths between approximately 80 and 200 km below the surface.
- The asthenosphere is generally solid, although some of its regions could be melted (e.g., below mid- ocean ridges).
- The lower boundary of the asthenosphere is not well defined and thickness of the asthenosphere depends mainly on the temperature.
- Seismic waves pass relatively slowly through the asthenosphere compared to the overlying lithospheric mantle.
CONTINENTAL DRIFT THEORY
24.1 CONTINENTAL DRIFT – EARLY CONTRIBUTIONS
- While observing the shape of the coastline of the Atlantic Ocean, we shall observe the symmetry of the coastlines on either side of the ocean.
- Hence, many scientists proposed based on this similarity and considered the possibility of the two Americas, Europe and Africa, to be once joined together.
- Abraham Ortelius, a Dutch map maker, who first proposed such a possibility as early as 1596.
- Antonio Pellegrini drew a map showing the three continents together.
- Alfred Wegener, a German meteorologist who put forth a comprehensive argument in the form of “the continental drift theory” in 1912.
24.2 CONTINENTAL DRIFT THEORY
- This was regarding the distribution of the oceans and the continents proposed by Alfred Wegener.
- According to Wegener, all the continents formed a single continental mass, a mega ocean surrounded by the same. The super continent was named PANGAEA, which meant all earth.
- The mega-ocean was called PANTHALASSA, meaning all water.
- He argued that, around 200 million years ago, the super continent, Pangaea, began to split.
- Pangaea first broke into two large continental masses as Laurasia and Gondwanaland forming the northern and southern components respectively.
- Subsequently, Laurasia and Gondwanaland continued to break into various smaller continents that exist today.
- A variety of evidence was offered in support of the continental drift. Some of these are given below.
24.3 EVIDENCE IN SUPPORT OF THE CONTINENTAL DRIFT
1. THE MATCHING OF CONTINENTS (JIG-SAW-FIT)
- The shorelines of Africa and South America facing each other have a remarkable and unmistakable match.
- A map produced using a computer programme to find the best fit of the Atlantic margin was presented by Bullard in 1964 and it was proved to be quite perfect.
- The match was tried at 1,000- fathom line instead of the present shoreline.
2. ROCKS OF SAME AGE ACROSS THE OCEANS
- The radiometric dating methods developed in the recent period have facilitated correlating the rock formation from different continents across the vast ocean.
- The belt of ancient rocks of 2,000 million years from Brazil coast matches with those from western Africa.
- The earliest marine deposits along the coastline of South America and Africa are of the Jurassic age.
- It is the sedimentary rock formed out of deposits of glaciers.
- The Gondwana system of sediments from India is known to have its counter parts in six different landmasses of the Southern Hemisphere.
- At the base the system has thick tillite indicating extensive and prolonged glaciation.
- Counter parts of this succession are found in Africa, Falkland Island, Madagascar, Antarctica and Australia besides India.
- Overall resemblance of the Gondawana type sediments clearly demonstrates that theselandmasses had remarkably similar histories.
- The glacial tillite provides unambiguous evidence of palaeoclimates and also of drifting of continents.
4. PLACER DEPOSITS
- The occurrence of rich placer deposits of gold in the Ghana coast and the absolute absence of source rock in the region is an amazing fact.
- The gold bearing veins are in Brazil and it is obvious that the gold deposits of the Ghana are derived from the Brazil plateau when the two continents lay side by
5. DISTRIBUTION OF FOSSILS
- When identical species of plants and animals adapted to living on land or in fresh water are found on either side of the marine barriers, a problem arises regarding accounting for such distribution.
- The observations that Lemurs occur in India, Madagascar and Africa led some to consider a contiguous landmass “Lemuria” linking these three landmasses.
- Mesosaurus was a small reptile adapted to shallow brackish water.
- The skeletons of these are found only in two localities: the Southern Cape province of South Africa and Iraver formations of Brazil.
- The two localities presently are 4,800 km apart with an ocean in between them.
6. FORCE FOR DRIFTING
- Wegener suggested that the movement responsible for the drifting of the continents was caused by pole-fleeing force and tidal force.
- The polar-fleeing force relates to the rotation of the earth.
- You are aware of the fact that the earth is not a perfect sphere; it has a bulge at the equator.
- This bulge is due to the rotation of the earth.
- The second force that was suggested by Wegener—the tidal force—is due to the attraction of the moon and the sun that develops tides in oceanic waters. Wegener believed that these forces would become effective when applied over many million years.
- However, most of scholars considered these forces to be totally inadequate.
24.4 POST-DRIFT STUDIES
- It is interesting to note that for continental drift; most of the evidence was collected from the continental areas in the form of distribution of flora and fauna or deposits like tillite.
- A number of discoveries during the post-war period added new information to geological literature.
- Particularly, the information collected from the ocean floor mapping provided new dimensions for the study of distribution of oceans and continents.
24.5 CONVECTIONAL CURRENT THEORY
- Arthur Holmes in 1930s discussed the possibility of convection currents operating in the mantle portion.
- These currents are generated due to radioactive elements causing thermal differences in the mantle portion.
- Holmes argued that there exists a system of such currents in the entire mantle portion.
- This was an attempt to provide an explanation to the issue of force, on the basis of which contemporary scientists discarded the continental drift theory.
24.6 DISTRIBUTION OF EARTHQUAKES AND VOLCANOES
- While observing the distribution of seismic activity and volcanoes, we will notice a line of dots in the central parts of the Atlantic Ocean almost parallel to the coastlines.
- It further extends into the Indian Ocean.
- It bifurcates a little south of the Indian subcontinent with one branch moving into East Africa and the other meeting a similar line from Myanmar to New Guiana.
- We will notice that this line of dots coincides with the mid oceanic ridges.
- The shaded belt showing another area of concentration coincides with the Alpine-Himalayan system and the rim of the Pacific Ocean.
- In general, the foci of the earthquake in the areas of mid- oceanic ridges are at shallow depths whereas along the Alpine- Himalayan belt as well as the rim of the Pacific, the earthquakes are deep-seated ones.
- The map of volcanoes also shows a similar patterns.
- The rim of the Pacific is also called rim of fire due to the existence of active volcanoes in this area.
PACIFIC RING OF FIRE
The Ring of Fire is an area in the basin of the Pacific Ocean where a large number of earthquakes and volcanic eruptions occur. In a 40,000 km (25,000 mi) horseshoe shape, it is associated with a nearly continuous series of oceanic trenches, volcanic arcs, and volcanic belts and/or plate movements. It has 452 volcanoes (more than 75% of the world’s active and dormant volcanoes). The Ring of Fire is sometimes called the circum-Pacific belt. About 90% of the world’s earthquakes and 81% of the world’s largest earthquakes occur along the Ring of Fire. The next most seismically active region (5–6% of earthquakes and 17% of the world’s largest earthquakes) is the Alpide belt, which extends from Java to the northern Atlantic Ocean via the Himalayas and southern Europe.
24.7 CONCEPT OF SEA FLOOR SPREADING
- As mentioned above, the post- drift studies provided considerable information that was not available at the time Wegener put forth his concept of continental drift. Particularly, the mapping of the ocean floor and palaeomagnetic studies of rocks from oceanic regions revealed the following facts:
- It was 193ealized that all along the mid oceanic ridges, volcanic eruptions are common and they bring huge amounts of lava to the surface in this area.
- The rocks equidistant on either sides of the crest of mid-oceanic ridges show remarkable similarities in terms of period of formation, chemical compositions and magnetic properties. Rocks closer to the mid-oceanic ridges are normal polarity and are the youngest. The age of the rocks increases as one moves away from the crest.
- The ocean crust rocks are much younger than the continental rocks. The age of rocks in the oceanic crust is nowhere more than 200 million years old. Some of the continental rock formations are as old as 3,200 million years.
- The sediments on the ocean floor are unexpectedly very thin. Scientists were expecting, if the ocean floors were as old as the continent, to have a complete sequence of sediments for a period of much longer duration. However, nowhere was the sediment column found to be older than 200 million years.
- The deep trenches have deep-seated earthquake occurrences while in the mid oceanic ridge areas; the quake foci have shallow depths.
- These facts and a detailed analysis of magnetic properties of the rocks on either sides of the mid-oceanic ridge led Hess (1961) to propose his hypothesis, known as the “sea floor spreading”.
- Hess argued that constant eruptions at the crest of oceanic ridges cause the rupture of the oceanic crust and the new lava wedges into it, pushing the oceanic crust on either side. The ocean floor thus spreads.
- The younger age of the oceanic crust as well as the fact that the spreading of one ocean does not cause the shrinking of the other, made Hess concept termed Plate Tectonic.
- A tectonic plate (also called lithospheric plate) is a massive, irregularly-shaped slab of solid rock, generally composed of both continental and oceanic lithosphere.
- Plates move horizontally over the asthenosphere as rigid units. The lithosphere includes the crust and top mantle with its thickness range varying between 5-100 km in oceanic parts and about 200 km in the continental areas.
- A plate may be referred to as the continental plate or oceanic plate depending on which of the two occupy a larger portion of the plate.
- Pacific plate is largely an oceanic plate whereas the Eurasian plate may be called a continental plate.
- The theory of plate tectonics proposes that the earth’s lithosphere is divided into seven major and some minor Young Fold Mountain ridges, trenches, and/or faults surround these major plates.
- These plates have been constantly moving over the globe throughout the history of the earth.
- It is not the continent that moves as believed by Wegener. Continents are part of a plate and what moves is the plate.
- Moreover, it may be noted that all the plates, without exception, have moved in the geological past, and shall continue to move in the future period as well.
- Wegener had thought of all the continents to have initially existed as a super continent in the form of Pangaea.
- However, later discoveries reveal that the continental masses, resting on the plates, have been wandering all through the geological period, and Pangaea was a result of converging of different continental masses that were parts of one or the other plates.
- Scientists using the palaeomagnetic data have determined the positions held by each of the present continental landmass in different geological periods.
- Position of the Indian sub- continent (mostly Peninsular India) is traced with the help of the rocks analysed from the Nagpur area.
The major plates are as follows:
Some important minor plates are listed below:
25.2 PLATE BOUNDARIES
There are three types of plate boundaries:
25.2.1 DIVERGENT BOUNDARIES
- In Divergent boundaries, new crust is generated as the plates pull away from each other.
- The sites where the plates move away from each other are called spreading sites.
- The best-known example of divergent boundaries is the Mid- Atlantic Ridge.
- At this, the American Plate(s) is/are separated from the Eurasian and African Plates.
25.2.2 CONVERGENT BOUNDARIES
- Where the crust is destroyed as one plate dived under another.
- The location where sinking of a plate occurs is called a subduction zone.
- There are three ways in which convergence can occur.
- These are: (i) between an oceanic and continental plate; (ii) between two oceanic plates; and (iii) between two continental plates
25.2.3 TRANSFORM BOUNDARIES
- Where the crust is neither produced nor destroyed as the plates slide horizontally past each other.
- Transform faults are the planes of separation generally perpendicular to the mid oceanic ridges.
- As the eruptions do not take all along the entire crest at the same time, there is a differential movement of a portion of the plate away from the axis of the earth.
- Also, the rotation of the earth has its effect on the separated blocks of the plate portions.
25.3 RATES OF PLATE MOVEMENT
- The strips of normal and reverse magnetic field that parallel the mid-oceanic ridges help scientists determine the rates of plate movement.
- These rates vary considerably.
- The Arctic Ridge has the slowest rate (less than 5 cm/yr), and the East Pacific Rise near Easter Island, in the South Pacific about 3,400 km west of Chile, has the fastest rate (more than 15 cm/yr).
25.4 FORCE FOR THE PLATE MOVEMENT
- At the time that Wegener proposed his theory of continental drift, most scientists believed that the earth was a solid, motionless body.
- However, concepts of sea floor spreading and the unified theory of plate tectonics have emphasised that both the surface of the earth and the interior are not static and motionless but are dynamic.
- The fact that the plates move is now a well-accepted fact.
- The mobile rock beneath the rigid plates is believed to be moving in a circular manner.
- The heated material rises to the surface, spreads and begins to cool, and then sinks back into deeper depths.
- This cycle is repeated over and over to generate what scientists call a convection cell or convective flow.
- Heat within the earth comes from two main sources: radioactive decay and residual heat. Arthur Holmes first considered this idea in the 1930s, which later influenced Harry Hess’ thinking about seafloor spreading.
- The slow movement of hot, softened mantle that lies below the rigid plates is the driving force behind the plate movement.
25.5 MOVEMENT OF THE INDIAN PLATE
- The Indian plate includes Peninsular India and the Australian continental portions.
- The subduction zone along the Himalayas forms the northern plate boundary in the form of continent— continent convergence.
- In the east, it extends through Rakinyoma Mountains of Myanmar towards the island arc along the Java Trench.
- The eastern margin is a spreading site lying to the east of Australia in the form of an oceanic ridge in South West Pacific and the Western margin follows Kirthar Mountain of Pakisthan.
- It further extends along the Makrana coast and joins the spreading site from the Red Searift south eastward along the Chagos Archipelago.
- The boundary between India and the Antarctic plate is also marked by oceanic ridge (divergent boundary) running in roughly West-East direction and merging into the spreading site, a little south of New Zealand.
- India was a large island situated off the Australian coast, in a vast ocean.
- The Tethys Sea separated it from the Asian continent till about 225 million years ago.
- India is supposed to have started its northward journey about 200 million years ago at the time when Pangaea broke.
- India collided with Asia about 40- 50 million years ago causing rapid uplift of the Himalayas. The positions of India since about 71 million years till the present.
- It also shows the position of the Indian subcontinent and the Eurasian plate.
- About 140 million years before the present, the subcontinent was located as south as 50°S latitude.
- The two major plates were separated by the Tethys Sea and the Tibetan block was closer to the Asiatic landmass.
- During the movement of the Indian plate towards the Asiatic plate, a major event that occurred was the outpouring of lava and formation of the Deccan Traps
- This started somewhere around 60 million years ago and continued for a long period of time.
- Note that the subcontinent was still close to the equator.
- From 40 million years ago and thereafter, the event of formation of the Himalayas took place.
- Scientists believe that the process is still continuing and the height of the Himalayas is rising even to this date.
MOUNTAIN BUILDING, ISLAND ARC FORMATION & TRENCH FORMATION
26.1 PLATE TECTONIC THEORY – MOUNTAIN FORMATION
- Plate tectonic theory is a comprehensive theory which offers explanations for various relief features and tectonic events viz. Mountain building, folding and faulting, continental drift, vulcanicity, seismic events (earthquakes) etc. It envisages the formation of mountains due to collision of plate boundaries.
- Three types of plate boundaries have been identified e.g. (i) destructive plate boundaries. (ii) Constructive plate boundaries and (iii) conservative plate
- Two plates moving together under the impact of thermal convective currents collide against each other and the plate boundary having relatively denser materials is subducted under the other plate boundary of relatively lighter material.
- This subduction of plate boundary causes lateral compressive force which ultimately squeezes and folds the sediments and materials of the margins of the plates and thus mountains are formed.
26.2 EARTH MOVEMENTS AND THE MAJOR LANDFORMS
- The face of the earth is constantly being reshaped by the agents of denudation-running water, rain, frost, sun, wind, glaciers and waves, so that our present landforms are very varied and diverse.
- Since the dawn of geological time, no less than nine orogenic or mountain building movements have taken place, folding and fracturing the earth’s crust.
- Some of them occurred in Pre- Cambrian times between 600- 3,500 million years ago.
- The three more recent orogenies are Caledonian, Hercynian and Alpine.
- The Caledonian, about 320 million years ago raised the mountains of Scandinavia and Scotland, and is represented in North America.
- These ancient mountains have been worn down and no longer exhibit the striking forms that they must once have had.
- In a later period, during the Hercynian earth movements about 240 million years ago, were formed such ranges as the Ural Mountains.
- The Pennine and Welsh Highlands in Britain, the Harz Mountains in Germany, the Appalachians in America as well as the high plateaux of Siberia and The mountains have also been reduced in size by the various sculpturing forces.
- We are now living in an era very close to the last of the major orogenic movement of the earth, the Alpine, about 30 million years Young fold mountain ranges were buckled up and overthrust on a gigantic scale.
- Being the most recently formed, these ranges, such as the Alps, Himalayas, Andes and Rockies are the loftiest and the most imposing.
- Their peaks are sometime several metres high.
- But the time will come when these lofty ranges will be lowered like those that existed before them.
- From the eroded materials, new rocks will be formed, later to be uplifted to form the next generation of mountain.
26.3 TYPES OF MOUNTAINS
- Mountains make up a large proportion of the earth’s surface. Based on their mode of formation, four main types of mountains can be distinguished
26.3.1 FOLD MOUNTAINS
- These mountains are by far the most widespread and also the most important.
- They are caused by large-scale earth movement when stresses, are set up in the earth’s crust.
- Such stresses may be due to the increased load of the overlying rocks, flow movements in the mantle, magmatic intrusions into the crust, or the expansion or contraction of some part of the earth.
- When such stresses are initiated, the rocks are subjected to compressive forces that produce wrinkling or folding along the lines of weakness.
- As illustrated, folding effectively shortens the earth’s crust, creating from the original level surface series of ‘waves’.
- The upfolded waves are called anticlines and the troughs or downfolds are synclines.
- The formation of up- and downfolds closely resembles that of the wrinkles of a table-cloth when it i pushed from either one or both sides of the table.
- In the great fold mountains of the world such as the Himalayas, Rockies, Andes and Alps, due the complexity of the compressional forces, the foIds developed much more complicated forms.
- When the crest of a fold is pushed too far, an overfold is formed.
- If it is pushed still further, it becomes a recumbent fold. In extreme cases, fractures may occur in the crust, so that the upper part of the recumbent fold slides forward over the lower part along a thrust plane, forming an overthrust fold.
- The over-riding portion of the thrust fold is termed a nappe.
- Since the rock strata have been elevated to great heights, sometimes measurable in miles, Fold Mountains may be called mountains of elevation
- The Fold Mountains are also closely associated with volcanic activity.
- They contain many active volcanoes, especially in the Circum-Pacific fold mountain system.
- When the earth’s crust bends folding occurs, but when it cracks, faulting takes
- Faulting may be caused by tension or compression, forces which lengthen or shorten the earth’s crust, causing a section of it to subside or to rise above the surrounding level.
- In earth movements generate tensional forces that tend to pull the crust apart, and faults are developed.
- If the block enclosed by the faults remains as it is or rises, and the land on either side subsides, the upstanding block becomes the horst or block mountain.
- The faulted edges are very steep, with scarp slopes and the summit is almost level, e.g. the Hunsruck Mountains, the Vosges and Black Forest of the Rhineland.
- Tension may also cause the central portion to be let down between two adjacent fault blocks forming a graben or rift valley, which will have steep walls.
- The East African Rift Valley system is 3,000 miles long, stretching from East Africa through the Red Sea to Syria.
- Compressional forces set up by earth movements may produce a thrust.
- A block may be raised or lowered in relation to surrounding areas, illustrates a rift valley formed in this way.
- In general large-scale block mountains and rift valleys are due to tension rather than compression.
- The faults may occur in series and be further complicated by tilting and other Denudation through the ages modifies faulted landforms.
26.3.3 VOLCANIC MOUNTAINS
- The subducted part of the plate after reaching a depth of 100 km or more in the mantle is liquefied and thus expands in volume because of conversion of the portion of plate into magma.
- This expansion of molten materials causes further rise in the mountain.
- These are, in fact, volcanoes which are built up from material ejected from fissures in the earth’ crust.
- The materials include molten lava, volcanic bombs, cinder, ashes, dust and liquid mud.
- They fall around the vent in successive layers, building up a characteristic volcanic clone.
- Volcanic mountains are often called mountains of accumulation.
- They are common in the Circum- Pacific belt and include such volcanic peaks as Fuji (Japan) Mt. Mayon (Philippines), Mt. Merapi (Sumatra), Mt. Agung (Bali) and Mt. Catopaxi (Ecuador).
26.3.4 RESIDUAL MOUNTAINS
- These are mountains evolved by denudation.
- Where the general level of the land has been lowered by the agents of denudation some very resistant areas may remain and these form residual mountains, e.g. Mt. Manodnock in U.S.A. Residual mountains may also evolve from plateaux which have been dissected by rivers into hills and valleys like the ones illustrated.
- Here the ridges and peaks are all very similar in height.
- Examples of dissected plateaux, where the down-cutting streamshave eroded the uplands into mountains of denudation, are the Highlands of Scotland, Scandinavia and the Deccan Plateau.
26.4 ISLAND ARC FORMATION
- When one tectonic plate meets another and sinks underneath it is known as the subduction phenomenon.
- There are many subduction zones in the Ring of Fire, and it is in these zones that island arcs can form.
- Subduction occurs when oceanic lithosphere meets continental lithosphere.
- The lithosphere under the oceans is denser and heavier than that under the continent.
- When the two run into each other, the oceanic lithosphere, therefore, sinks under the continent.
- When two oceanic plates meet, one will sink under the other.
- The oceanic lithosphere melts into the asthenosphere and turns into magma.
- It’s like a recycling of the rocks that make up the crust and lithosphere.
- When the oceanic rock sinks under continental lithosphere and melts into the magma of the asthenosphere, some of it may leak into the crust and bubble up to the surface.
- When magma bubbles up to the surface of the earth’s crust, we get volcanoes.
- As the volcanoes forming at a subduction zone erupt, they build up rock at the surface.
- Over time enough builds up to create a volcanic island that rises above the surface of the ocean. Because a subduction zone can create several volcanoes in a row, these types of islands tend to form in chains or clusters, which we call island arcs.
26.5 OCEAN TRENCHES
- Ocean trenches are a result of tectonic activity, which describes the movement of theEarth’s lithosphere.
- In particular, ocean trenches are a feature of convergent plate boundaries, where two or more tectonic plates meets.
- At many convergent plate boundaries, dense lithosphere melts or slides beneath less-dense lithosphere in a process called subduction, creating a trench.
- Ocean trenches occupy the deepest layer of the ocean, the hadalpelagic zone.
- The intense pressure, lack of sunlight, and frigid temperatures of the hadalpelagic zone make ocean trenches some of the most unique habitats on Earth.
26.6 MID ATLANTIC RIDGE FORMATION
- The North American and Eurasian Plates are moving away from each other along the line of the Mid Atlantic Ridge.
- The Ridge extends into the South Atlantic Ocean between the South American and African Plates.
- The ocean ridge rises between 2 to 3 km above the ocean floor, and has a rift valley at its crest marking the location at which the two plates are moving apart.
- The Mid Atlantic Ridge, like other ocean ridge systems, has developed as a consequence of the divergent motion between the Eurasian and North American, and African and South American Plates.
- As the mantle rises towards the surface below the ridge the pressure is lowered (decompression) and the hot rock starts to partially melt.
- This produces basaltic volcanoes when an eruption occurs above the surface and characteristic basalt “pillow lava” in underwater eruptions. In this way, as the plates move further apart new ocean lithosphere is formed at the ridge and the ocean basin gets water.
- This process is known as “sea floor spreading” and results in a symmetrical alignment of the rocks of the ocean floor which get older with distance from the ridge crest.
EARTHQUAKES AND VOLCANOES
- The study of seismic waves provides a complete picture of the layered interior.
- An earthquake in simple words is shaking of the earth. It is a natural event. It is caused due to release of energy, which generates waves that travel in all direction.
- The release of energy occurs along a Fault (A fault is a sharp break in the crustal rocks).
- Rocks along a fault tend to move in opposite direction.
- As the overlying rock strata press them, the friction locks them together.
- However, their tendency to move apart at some point of time overcomes the
- As a result, the blocks get deformed and eventually, they slide past one another abruptly.
- This causes a release of energy, and the energy waves travel in all directions.
- The point where the energy is released is called the focus of an earthquake, alternatively, it is called the hypocentre.
- The energy waves travelling in different directions reach the The point on the surface, nearest to the focus, is called epicentre. It is the first one to experience the waves. It is a point directly above the focus.
27.2 EARTHQUAKES – ORIGIN BASED ON PLATE TECTONICS
- As per theory of the plate tectonics the crust or the earth is composed of solid and moving plates having either continental crust or oceanic crust or even both continental- oceanic crusts.
- The earth’s crust consists of 6 major plates (Eurasian plate, American plate, African plate, Indian plate, Pacific plate and Antarctic plate) and 20 minor plates. These plates are constantly moving in relation to each other due to thermal convective currents originating deep within the earth.
- Thus, all the tectonic events take place along the boundaries of these moving plates.
27.3 TYPES OF PLATE BOUNDARIES
- constructive plates boundaries,
- destructive plate boundaries and
- conservative plate
- Constructive plate boundaries represent the trailing ends of divergent plates which move in opposite directions from the mid- oceanic ridges,
- Destructive plate boundaries are those where two convergent plates collide against each other and the heavier plate boundary is subducted below the relatively lighter plate boundary.
- Conservative plate boundaries are those where two plates slip past each other without any collision.
27.4 MAGNITUDE OF EARTHQUAKES ON DIFFERENT PLATE BOUNDARIES
- Major tectonic events associated with these plate boundaries are ruptures and faults along the constructive plate boundaries, faulting and folding along the destructive plate boundaries and transform faults along the conservative plate boundaries.
- All sorts of disequilibrium are caused due to different types of plate motions and consequently earthquakes of varying magnitudes are caused
- Moderate earthquakes are caused along the constructive plate boundaries because the rate of rupture of the crust andconsequent movement of plates away from the mid-oceanic ridges is rather slow and the rate of upwelling of lavas due to fissure flow is also slow.
- Consequently, shallow focus earthquakes are caused along the constructive plate boundaries or say along the mid- oceanic ridges.
- Earthquakes of high magnitude and deep focus are caused along the convergent or destructive plate boundaries because of collision of two convergent plates and consequent subduction of one plate boundaries.
- Here mountain building, faulting and violent volcanic eruptions (central explosive type of eruptions) cause severe and disastrous earthquakes having the focus at the depth upto 700 km.
27.5 DISTRIBUTION OF EARTHQUAKE
- The earthquakes of the Mid- Continental Belt along the Alpine- Himalayan Chains are caused due to collision of Eurasian plates and African and Indian plates.
- The earthquakes of the western marginal areas of North and South Americas are caused because of subduction of Pacific plate beneath the American plate and the resultant tectonic forces whereas the earthquakes of the eastern margins of Asia are originated because of thesubduction of Pacific plate under Asiatic plate.
- Similarly, the subduction of African plate below European plate and the subduction of Indian plate under Asiatic plate cause earthquakes of the mid- continental belt.
- Creation of transform faults along the conservative plate boundaries explains the occurrence of severe earthquakes of California (USA).
- Here one part of California moves north-eastward while the other part moves south-westward along the fault plane and thus is formed transform fault which causes earthquakes.
27.6 EARTHQUAKE WAVES
- All natural earthquakes take place in the lithosphere.
- It is sufficient to note here that the lithosphere refers to the portion of depth up to 200 km from the surface of the earth.
- An instrument called ‘seismograph’ records the waves reaching the surface.
- Earthquake waves are basically of two types — body waves and surface waves.
27.6.1 BODY WAVES
- Body waves are generated due to the release of energy at the focus and move in all directions travelling through the body ofthe earth. Hence, they are known as body waves.
27.6.2 SURFACE WAVES
- The body waves interact with the surface rocks and generate new set of waves called surface waves. These waves move along the surface.
- The velocity of waves changes as they travel through materials with different densities.
- The denser the material, the higher is the velocity.
- Their direction also changes as they reflect or refract when coming across materials with different densities.
- The surface waves are the last to report on seismograph.
- These waves are more destructive.
- They cause displacement of rocks, and hence, the collapse of structures occurs.
TYPES OF BODY WAVES
- There are two types of body waves. They are called P and S- waves.
- P-waves move faster and are the first to arrive at the These are also called ‘primary waves’.
- The P-waves are similar to sound waves.
- This characteristic of the S-waves is quite important, as it has helped scientists to understand the structure of the interior of the earth.They travel through gaseous, liquid and solid.
- S-waves arrive at the surface with some time lag. These are called secondary waves.
- An important fact about S-waves is that they can travel only through solid materials.
- This characteristic of the S-waves is quite important, as it has helped scientists to understand the structure of the interior of the earth.
Long Period waves or Lwaves:
- These waves generally affect only the surface of the earth and die out a smaller depth. T. These waves cover longest distances of all the seismic waves. Though their speed is lower than P and S waves but these are most violent and destructive.
27.7 PROPAGATION OF EARTHQUAKE WAVES
- Different types of earthquake waves travel in different manners.
- As they move or propagate, they cause vibration in the body of the rocks through which they pass.
- P-waves vibrate parallel to the direction of the wave. This exerts pressure on the material in the direction of the propagation.
- As a result, it creates density differences in the material leading to stretching and squeezing of the material.
- Other three waves vibrate perpendicular to the direction of propagation.
- The direction of vibrations of S- waves is perpendicular to the wave direction in the vertical plane.
- Hence, they create troughs and crests in the material through which they pass.
27.8 EMERGENCE OF SHADOW ZONE
- Earthquake waves get recorded in seismographs located at far off locations.
- However, there exist some specific areas where the waves are not reported. Such a zone is called the ‘shadow zone’
- The study of different events reveals that for each earthquake,there exists an altogether different shadow zone.
- It was observed that seismographs located at any distance within 105° from the epicentre, recorded the arrival of both P and S-waves.
- However, the seismographs located beyond 145° from epicentre; record the arrival of P- waves, but not that of S-waves.
- Thus, a zone between 105° and 145° from epicentre was identified as the shadow zone for both the types of waves.
- The entire zone beyond 105° does not receive S-waves.
- The shadow zone of S-wave is much larger than that of the P- waves.
- The shadow zone of P-waves appears as a band around the earth between 105° and 145° away from the epicenter.
- The shadow zone of S-waves is not only larger in extent but it is also a little over 40 per cent of the earth surface.
27.9 TYPES OF EARTHQUAKES
- The most common ones are the tectonic earthquakes. These are generated due to sliding of rocks along a fault plane.
- A special class of tectonic earthquake is sometimes recognised as volcanic earthquake. However, these are confined to areas of active volcanoes.
- In the areas of intense mining activity, sometimes the roofs of underground mines collapse causing minor tremors. These are called collapse earthquake.
- Ground shaking may also occur due to the explosion of chemical or nuclear devices. Such tremors are called explosion earthquakes
- The earthquakes that occur in the areas of large reservoirs are referred to as reservoir induced earthquakes.
27.10 MEASURING EARTHQUAKES
- The earthquake events are scaled either according to the magnitude or intensity of the shock.
- The magnitude scale is known as the Richter
- The magnitude relates to the energy released during the quake.
- The magnitude is expressed in absolute numbers, 0-10.
- The intensity scale is named after Mercalli, an Italian seismologist.
- The intensity scale takes into account the visible damage caused by the event.
- The range of intensity scale is from 1-12.
27.11 VULCANICITY AND VOLCANOES
- The term vulcanicity covers all those processes in which molten rock material or magma rises into the crust or is poured out on its surface, there to solidify as a crystalline or semi crystalline rocks.
- A volcano is a rupture in the crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface.
27.11.1 Distribution of Volcanoes
- World’s active volcanoes are found along the constructive plate margins or divergent plate margins (along the mid-oceanic ridges where two plates move in opposite directions) whereas 80 per cent volcanoes are associated with the destructive or convergent plateboundaries (where two plates collide).
- Besides, some volcanoes are also found in intraplate regions g. volcanoes of the Hawii Island, fault zones of East Africa etc.
- Two plates move in opposite directions from the mid-oceanic ridges due to thermal convective currents which are originated in the mantle below the crust (plates). This splitting and lateral spreading of plates creates fractures and faults (transform faults) which cause pressure release and lowering of melting point and thus materials of upper mantle lying below the mid- oceanic ridges are melted and move upward as magmas under the impact of enormous volume of accumulated gases and vapour. This rise of magmas along the mid-oceanic ridges (constructive or divergent plate boundaries) causes fissure eruptions of volcanoes and there is constant upwelling of lavas.
27.11.2 CLASSIFICATION ON THE BASIS OF PERIODICITY OF ERUPTIONS
Volcanoes are divided into 3 types on the basis of period of eruption and interval period between two eruptions of a volcano e.g. (i) active volcanoes, (ii) dormant volcanoes and (iii) extinct volcanoes
i. ACTIVE VOLCANOES
- Active volcanoes are those which constantly eject volcanic lavas, gases, ashes and fragmental material.
- It is estimated that there are about more than 500 volcanoes in the world. Etna and Stromboli of the Mediterranean Sea are the most significant examples of this category.
- Most of the active volcanoes are found along the mid-oceanic ridges representing divergent plate margins (constructive plate margins) and convergent plate margins (destructive plate margins represented by eastern and western margins of the Pacific Ocean).
ii. DORMANT VOLCANOES
- Dormant volcanoes are those which become quiet after their eruptions for some time and there are no indications for future eruptions but suddenly they erupt very violently and cause enormous damage to human health and wealth.
iii. EXTINCT VOLCANOES
- The volcanoes are considered extinct when there are no indications of future
- They become explosive if somehow water gets into the vent; otherwise, they are characterised by low- explosivity.The upcoming lava moves in the form of a fountain and throws out the cone at the top of the vent and develops into cinderThe crater is filled up with water and Jakes are formed. It may be pointed out that no volcano can be declared permanently dead as no one knows,what is happening below the ground surface.
27.11.13 CLASSIFICATION OF VOLCANOES BASED ON NATURE OF ERUPTION
- Volcanoes are classified on the basis of nature of eruption and the form developed at the surface.
Major types of volcanoes are as follows:
- Barring the basalt flows, the shield volcanoes are the largest of all the volcanoes on the earth.
- The Hawaiian volcanoes are the most famous examples. These volcanoes are mostly made up of basalt, a type of lava that is very fluid when erupted.
- For this reason, these volcanoes are not sleep.
- They become explosive if somehow water gets into the vent; otherwise, they are characterised by low-explosivity.
- The upcoming lava moves in the form of a fountain and throws out the cone at the top of the vent and develops into cinder cone.
- These volcanoes are characterised by eruptions of cooler and moreviscous lavas than basalt. These volcanoes often result in explosive eruptions. Along with lava, large quantities of pyroclastic material and ashes find their way to the ground. This material accumulates in the vicinity of the vent openings leading to formation of layers, and this makes the mounts appear as composite volcanoes.
- These are the most explosive of the earth’s volcanoes. They are usually so explosive that when they erupt they tend to collapse on themselves rather than building any tall structure. The collapsed depressions are called calderas.
- Their explosiveness indicates that the magma chamber supplying the lava is not only huge but is also in close vicinity.
FLOOD BASALT PROVINCES
- These volcanoes outpour highly fluid lava that flows for long distance.
- Some parts of the world are covered by thousands of km of thick basalt lava flows.
- There can be a series of flows with some flows attaining thickness of more than 50 m. Individual flows may extend for hundreds of Km.
- The Deccan Traps from India, presently covering most of the Maharashtra plateau, are a much larger flood basalt province.
- It is believed that initially the trap formations covered a much larger area than the present.
MID-OCEAN RIDGE VOLCANOES
- These volcanoes occur in the oceanic areas. There is a system of mid-ocean ridges more than 70,000 km long that stretches through all the ocean basins.
- The central portion of this ridge experiences frequent eruptions.
- The materials of the crust or lithosphere are generally called as rocks.
- The word lithosphere is derived from the word ‘lithos’ meaning Rock.
- The earth’s crust is composed of rocks.
- A rock is an aggregate of one or more mineral.
- Rock may be hard or soft and in varied colours. For example, granite is hard, soapstone is soft. Similarly, Gabbro is black and quartzite is milky.
- Rocks do not have definite composition of mineral.
- Feldspar and quartz are the most common minerals found in rocks.
- Petrology is science of rocks. A petrologist studies rocks in all their aspects viz., mineral composition, texture, structure, origin, occurrence, alteration and relationship with other rocks.
28.2 FORMATION OF ROCKS
- More than one element of the earth’s crust is organized to form compounds which are known as minerals and minerals are organized to form rocks.
28.3 CLASSIFICATION OF ROCKS
- There are many different kinds of rocks which are grouped under three families on the basis of their mode of formation.
- Igneous Rocks — solidified from
- magma and lava;
- Sedimentary Rocks — the result of deposition of fragments of rocks by exogenous processes;
- Metamorphic Rocks — formed out of existing rocks undergoing recrystallisation.
28.3.1 IGNEOUS ROCKS
- As igneous rocks form out of magma and lava from the interior of the earth, they are known as primary rock
- The igneous rocks (Ignis – in Latin means ‘Fire’) are formed when magma cools and Otherwise, When magma in its upward movement cools and turns into solid form it is called igneous rock.
- The process of cooling and solidification can happen in the earth’s crust or on the surface of the earth.
- Igneous rocks are mostly associated with volcanoes, hence called as volcanic rock.
- Igneous rocks do not contain fossils and are less affected by chemical weathering.
- Igneous rocks are classified based on texture.
- Texture depends upon size and arrangement of grains or other physical conditions of the materials.
- If molten material is cooled slowly at great depths, mineral grains may be very large.
- Sudden cooling (at the surface) results in small and smooth grains.
- Intermediate conditions of cooling would result in intermediate sizes of grains making up igneous rocks.
- Granite, gabbro, pegmatite, basalt, volcanic breccia and tuff are some of the examples of igneous rocks.
IMPORTANT CLASSIFICATIONS OF IGNEOUS ROCKS
Based on the amount of silica (Si O2)
- Acidic igneous rocks – more silica content – g. Granite
- Basic igneous rocks – less silica content – e.g. Gabro
Based on mode of occurrence
- Intrusive igneous rocks – When the rising magma during a volcanic activity do not reach the earth’s surface rather they are cooled and solidified are below the surface of the earth.
- Extrusive igneous rocks – The igneous rocks formed due to cooling and solidification of hot and molten lava at the surface of the earth.
DIFFERENT SHAPES FORMED BY MAGMA
- BATHOLITH – long, irregular and undulating forms of solidified magma. Usually, dome shaped,
- LACOLITH – formed by injection of magma along bedding planes of horizontally bedded sedimentary
- SILL – Parallel to bedding planes (horizontal) and formed due to injection and solidification of magma between sedimentary
- DYKES – Perpendicular to the beds of sedimentary rocks (vertical).
28.3.2 SEDIMENTARY ROCKS
- The word ‘sedimentary’ is derived from the Latin word sedimentum, which means settling.
- Sedimentary rocks are formed of sediments derived from the older rocks, plants and animal remains and hence these rocks contain fossil of plants and animals.
- These rocks are found over the largest surface area of the earth.
- above 75% of the surface area of the earth). But it is only 5% in composition of the crust.
- Rocks (igneous, sedimentary and metamorphic) of the earth’s surface are exposed to denudational agents, and are broken up into various sizes of fragments.
- Such fragments are transported by different exogenous agencies and deposited.
- These deposits through compaction turn into rocks.
- This process is called lithification.
- In many sedimentary rocks, the layers of deposits retain their characteristics even after lithification.
- Hence, there are number of layers of varying thickness in sedimentary rocks like sandstone, shale etc.
CLASSIFICATION OF SEDIMENTARY ROCKS
- Depending upon the mode of formation, sedimentary rocks are classified into three major groups:
- mechanically formed — sandstone, conglomerate, clay rock, limestone, shale,loes.
- organically formed— geyserite, chalk, limestone, coal, dolomites, peats, etc.
- chemically formed — chert, limestone, gypsum, salt rock, halite, potash etc.
28.3.3 METAMORPHIC ROCKS
- The word metamorphic means ‘change of form’.
- These rocks form under the action of pressure, volume and temperature (PVT) changes.
- Metamorphism occurs when rocks are forced down to lower levels by tectonic processes or when molten magma rising through the crust comes in contact with the crustal rocks or the underlying rocks are subjected to great amounts of pressure by overlying rocks.
- Metamorphism is a process by which already consolidated rocks undergo recrystallization and reorganisation of materials within original rocks.
- Mechanical disruption and reorganisation of the original minerals within rocks due to breaking and crushing without any appreciable chemical changes is called dynamic metamorphism.
- The materials of rocks chemically alter and recrystallise due to thermal metamorphism.
- There are two types of thermal metamorphism — contact metamorphism and regional metamorphism.
- In contact metamorphism the rocks come in contact with hot intruding magma and lava and the rock materials recrystallise under high temperatures.
- Quite often new materials form out of magma or lava are added to the rocks.
- In regional metamorphism, rocks undergo recrystallisation due to deformation caused by tectonic shearing together with high temperature or pressure or both.
- In the process of metamorphism in some rocks grains or minerals get arranged in layers or lines.
- Such an arrangement of minerals or grains in metamorphic rocks is called foliation or lineation.
- Sometimes minerals or materials of different groups are arranged into alternating thin to thick layers appearing in light and dark shades.
- Such a structure in metamorphic rocks is called banding and rocks displaying banding are called banded rocks.
- Metamorphic rocks are classified into two major groups — foliated rocks and non-foliated rocks.
- Gneissoid, granite, syenite, slate, schist, marble, quartzite etc. are some examples of metamorphic rocks.
IMPORTANT METAMORPHIC ROCKS
1. MARBLES formed from LIMESTONE
2. SCHISTS formed from SHALE ROCKS
3. SLATE formed from SHALES and other ARGILLACEOUS ROCKS
4. GNEISS formed from CONGLOMERATES & GRANITES
5. QUARTZITE formed from SANDSTONES
28.4 ROCK CYCLE
- Rocks do not remain in their original form for long but may undergo transformation.
- Rock cycle is a continuous process through which old rocks are transformed into new ones.
- Igneous rocks are primary rocks and other rocks (sedimentary and metamorphic) formed from these primary rocks.
- Igneous rocks can be changed into metamorphic rocks.
- The fragments derived out of igneous and metamorphic rocks form into sedimentary rocks. Sedimentary rocks themselves can turn into fragments and the fragments can be a source for formation of sedimentary rocks.
- The crustal rocks (igneous, metamorphic and sedimentary) once formed may be carried down into the mantle (interior of the earth) through subduction process (parts or whole of crustal plates going down under another plate in zones of plate convergence) and the same melt down due to increase in temperature in the interior and turn into molten magma, the original source for igneous.
- Weathering is action of elements of weather and climate over earth materials.
- There are a number of processes within weathering which act either individually or together to affect the earth materials in order to reduce them to fragmental state.
- v Weathering is defined as mechanical disintegration and chemical decomposition of rocks through the actions of various elements of weather and climate.
- As very little or no motion of materials takes place in weathering, it is an in-situ or on- site process.
29.2 CHEMICAL WEATHERING PROCESSES
- A group of weathering processes viz; solution, carbonation, hydration, oxidation and reduction act on the rocks to decompose, dissolve or reduce them to a fine clastic state through chemical reactions by oxygen, surface and/or soil water and other acids.
- Water and air (oxygen and carbon dioxide) along with heat must be present to speed up all chemical reactions.
- Over and above, the carbon dioxide present in the air, decomposition of plants and animals increases the quantity of carbon dioxide underground.
- When something is dissolved in water or acids, the water or acid with dissolved contents is called solution.
- This process involves removal of solids in solution and depends upon solubility of a mineral (like nitrates, sulphates, and potassium ) in water or weak acids.
- So, these minerals are easily leached out without leaving any residue in rainy climates and accumulate in dry regions.
- Minerals like calcium carbonate and calcium magnesium bicarbonate present in limestones are soluble in water containing carbonic acid (formed with the addition of carbon dioxide in water), and are carried away in water as solution.
- Carbon dioxide produced by decaying organic matter along with soil water greatly aids in this reaction.
- Carbonation is the reaction of carbonate and bicarbonate withminerals and is a common process helping the breaking down of feldspars and carbonate minerals.
- Carbon dioxide from the atmosphere and soil air is absorbed by water, to form carbonic acid that acts as a weak acid.
- Calcium carbonates and magnesium carbonates are dissolved in carbonic acid and are removed in a solution without leaving any residue resulting in cave information.
- Hydration is the chemical addition of water. Minerals take up water and expand; this expansion causes an increase in the volume of the material itself or rock.
- Calcium sulphate takes in water and turns to gypsum, which is more unstable than calcium sulphate.
- This process is reversible and long, continued repetition of this process causes fatigue in the rocks and may lead to their disintegration.
- Many clay minerals swell and contract during wetting and drying and a repetition of this process results in cracking of overlying material.
- Salts in pore spaces undergo rapid and repeated hydration and help in rock fracturing.
OXIDATION AND REDUCTION
- In weathering, oxidation means a combination of a mineral with oxygen to form oxides or hydroxides.
- In the process of oxidation rock breakdown occurs due to the disturbance caused by addition of oxygen.
- Red colour of iron upon oxidation turns to brown or yellow.
- When oxidised minerals are placed in an environment where oxygen is absent, reduction takes place.
- Such conditions exist usually below the water table, in areas of stagnant water and waterlogged ground.
- Red colour of iron upon reduction turns to greenish or bluish grey. These weathering processes are interrelated.
- Hydration, carbonation and oxidation go hand in hand and hasten the weathering process.
29.3 PHYSICAL WEATHERING PROCESSES
- Physical or mechanical weathering processes depend on some applied forces.
The applied forces could be:
- Gravitational forces such as overburden pressure, load and shearing stress;
- Expansion forces due to temperature changes, crystal growth or animal activity;
- Water pressures controlled by wetting and drying cycles.
- Many of these forces are applied both at the surface and within different earth materials leading to rock fracture.
- Most of the physical weathering processes are caused by thermal expansion and pressure release.
- These processes are small and slow but can cause great damage to the rocks because of continued fatigue the rocks suffer due to repetition of contraction and expansion.
UNLOADING AND EXPANSION
- Removal of overlying rock load because of continued erosion causes vertical pressure release with the result that the upper layers of the rock expand producing disintegration of rock masses.
- Fractures will develop roughly parallel to the ground surface.
- Large, smooth rounded domes called exfoliation domes result due to this process.
TEMPERATURE CHANGES AND EXPANSION
- Various minerals in rocks possess their own limits of expansion and contraction.
- Because of diurnal changes in the temperatures, this internal movement among the mineral grains of the superficial layers of rocks takes place regularly.
- This process is most effective in dry climates and high elevations where diurnal temperature changes are drastic.
- In rocks like granites, smooth surfaced and rounded small to big boulders called tors form due to such exfoliation..
FREEZING, THAWING AND FROST WEDGING
- Frost weathering occurs due to growth of ice within pores and cracks of rocks during repeated cycles of freezing and melting.
- This process is most effective at high elevations in mid-latitudes where freezing and melting is often repeated.
- Glacial areas are subject to frost wedging In this process, the rate of freezing is important.
- Rapid freezing of water causes its sudden expansion and high pressure.
- The resulting expansion affects joints, cracks and small inter granular fractures to become wider and wider till the rock breaks apart.
- Salts in rocks expand due to thermal action, hydration and crystallisation. Many salts like calcium, sodium, magnesium potassium and barium have a tendency to expand.
- Salt crystals in near-surface pores cause splitting of individual grains within rocks, which eventually fall off. This process of falling off of individual grains may result in granular disintegration or granular foliation.
- Salt crystallisation is most effective of all salt-weathering processes.
- Sodium chloride and gypsum crystals in desert areas heave up overlying layers of materials and with the result polygonal cracks develop all over the heaved surface.
- With salt crystal growth, chalk breaks down most readily, followed by limestone, sandstone, shale, gneiss and granite etc.
29.4 BIOLOGICAL ACTIVITY AND WEATHERING
- Biological weathering is contribution to or removal of minerals and ions from the weathering environment and physical changes due to growth or movement of organisms..
- Burrowing and wedging by organisms like earthworms, termites, rodents etc., help in exposing the new surfaces to chemical attack and assists in the penetration of moisture and air.
- Human beings by disturbing vegetation, ploughing and cultivating soils, also help in mixing and creating new contacts between air, water and minerals in the earth material.
- Decaying plant and animal matter help in the production of humic, carbonic and other acids which enhance decay and solubility of some elements.
- Algae utilise mineral nutrients for growth and help in concentration of iron and manganese oxides.
- Plant roots exert a tremendous pressure on the earth materials mechanically breaking them apart.
29.5 SIGNIFICANCE OF WEATHERING
- Weathering processes are responsible for breaking down the rocks into smaller fragments and preparing the way for formation of not only regolith and soils, but also erosion and mass movements.
- Biomes and biodiversity is basically a result of forests (vegetation) and forests depend upon the depth of weathering mantles.
- Weathering aids mass wasting, erosion and reduction of relief and changes in landforms are a consequence of erosion.
- Weathering of rocks and deposits helps in the enrichment and concentrations of certain valuable ores of iron, manganese, aluminium, copper etc., which are of great importance for the national economy.
- Weathering is also an important process in the formation of soils.
29.6 MASS MOVEMENTS
- Mass movements transfer the mass of rock debris down the slopes under the direct influence of gravity.
- The movements of mass may range from slow to rapid, affecting shallow to deep columns of materials and include creep, flow, slide and fall. .
- Mass movements are very active over weathered slopes rather than over un-weathered materials.
- That means mass movements do not come under erosion though there is a shift (aided by gravity) of materials from one place to another.
- Weak unconsolidated materials, thinly bedded rocks, faults, steeply dipping beds, vertical cliffs or steep slopes, abundant precipitation and torrential rains and scarcity of vegetation etc., favour mass movements.
- They are:
- 1. removal of support from below to materials above through natural or artificial means
- 2. Increase in gradient and height of slopes;
- 3. Overloading through addition of materials naturally or by artificial filling;
- 4. Overloading due to heavy rainfall, saturation and lubrication of slope materials;
- 5. Removal of material or load from over the original slope surfaces;
- 6. Occurrence of earthquakes, explosions or machinery;
- 7. Excessive natural seepage;
- 8. heavy drawdown of water from lakes, reservoirs and rivers leading to slow outflow of water from under the slopes or river banks;
- 9. indiscriminate removal of natural vegetation.
- Heave (heaving up of soils due to frost growth and other causes), flow and slide are the three forms of
- Mass movements can be grouped under three major classes:
- slow movements;
- rapid movements;
29.6.1 SLOW MOVEMENTS
1. CREEP can occur on moderately steep, soil covered slopes.
- Movement of materials is extremely slow and imperceptible except through extended observation. Materials involved can be soil or rock derbies.
- Depending upon the type of material involved, several types of creep viz., soil creep, talus creep, rock creep, rock-glacier creep , can be identified.
2. SOLIFLUCTION which involves slow downslope flowing soil mass or fine grained rock debris saturated or lubricated with water.
- This process is quite common in moist temperate areas where surface melting of deeply frozen ground and long continued rain respectively, occur frequently.
- When the upper portions get saturated and when the lower parts are impervious to water percolation, flowing occurs in the upper parts.
29.6.2 RAPID MOVEMENTS
- These movements are mostly prevalent in humid climatic regions and occur over gentle to steep slopes.
- Movement of water-saturated clayey or silty earth materials down low-angle terraces or hillsides is known as earthflow.
- Quite often, the materials slump making step like terraces and leaving arcuate scarps at their heads and an accumulation bulge at the toe.
- When slopes are steeper, even the bedrock especially of soft sedimentary rocks like shale or deeply weathered igneous rock may slide downslope.
- In the absence of vegetation cover and with heavy rainfall, thick layers of weathered materials get saturated with water and either slowly or rapidly flow down along definite channels.
- When the mudflows emerge out of channels onto the piedmont or plains, they can be very destructive engulfing roads, bridges and houses.
- Avalanche is more characteristic of humid regions with or without vegetation cover and occurs in narrow tracks on steep slopes.
- This debris avalanche can be much faster than the mudflow.
- Debris avalanche is similar to snow avalanche.
- These are known as relatively rapid and perceptible movement.
- The materials involved are relatively dry.
- The size and shape of the detached mass depends on the nature of discontinuities in the rock, the degree of weathering and the steepness of the slope.
1. SLUMP is slipping of one or several units of rock debris with a backward rotation with respect to the slope over which the movement takes place.
2.DEBRIS SLIDE – Rapid rolling or sliding of earth debris without backward rotation of mass is known as debris slide.
- Debris fall is nearly a free fall of earth debris from a vertical or overhanging face.
3. ROCKSLIDE – Sliding of individual rock masses down bedding, joint or fault surfaces is rockslide.
- Over steep slopes, rock sliding is very fast and destructive.
- Slides occur as planar failures along discontinuities like bedding planes that dip steeply.
4. ROCK FALL -Rock fall is free falling of rock blocks over any steep slope keeping itself away from the
- Rock falls occur from the superficial layers of the rock face, an occurrence that distinguishes it from rockslide which affects materials up to a substantial depth.
LANDFORMS AND THEIR EVOLUTION
30.1 LANDFORMS AND LANDSCAPES
- Small to medium tracts or parcel of the earth’s surface are known a landforms.
- Several related landforms combine together to make landscapes.
- Each landform has its own characteristic features such as shape, size, materials and is a result of the action of certain geomorphic processes and agent of change.
- Running water, ground water, glaciers, wave and winds are five agents of change which bring about changes on the surface of the earth and give birth to various types of landforms.
- These agents of change work through the processes of erosion, transportation and deposition.
- They cause erosion of rock materials by virtue of their force and transport the eroded material to some other place. These two processes are followed by deposition.
30.2 RUNNING WATER OR RIVER
- When the rainfall occurs, water flows down the slope of the land and remove rock material in the direction of flow.
- The gullies further deepen, widen and lengthen and unite to form a network of valleys.
- At early stage down cutting dominates during which irregularities are removed.
- In the middle stage, streams cut their beds slower and lateral erosion become dominant.
- At a later stage the valley sides are reduced to lower and lower slopes.
- It is clear from the above discussion that the work of a river can be divided into three stages viz. youth, mature and old.
- In the youthful stage, streams are few with poor integration which flow in shallow V-shaped valleys over the original slopes.
- Down cutting and headward erosion are more prominent. Stream divides or water sheds are broad. Gorges, Canyon, V-shaped valley, waterfall, rapids etc are main features in this stage.
- There are no flood plains.
- Meanders, if present develop over the upland represented by the stream divides. River capture or stream piracy is a major event of this stage.
- Streams are plenty with good integration.
- Down cutting gradually give way to lateral cutting.
- Stream divides start becoming narrow and V-shaped valleys become broad.
- Trunk streams are broad enough to have wider flood plains within which streams may flow in meanders confined within the valley.
- Deposition starts along with erosion. River meandering, ox-bow lakes and flood plains are the main features of this stage. Waterfalls and rapids disappear.
- Almost all the features of youthful, stage disappear and the river reaches its old stage.
- The river does not do any erosional work and deposition is the main work done by the river.
- The valley becomes still broader and the stream divides become still narrower.
- The river flows in peneplain.
- Delta is the most prominent feature of this stage.
- Sometimes flood plains and natural levees are also formed.
30.3 EROSIONAL LANDFORMS
- Erosional work by the river is a dynamic process which is involved in the removal of therock material from the area over which it flows.
- The erosion work of the river can either be physical or chemical.
- The river does the erosional work by the energy of its water which is known as its physical or mechanical erosion.
- The water also dissolves many rock minerals in its valley which is called chemical erosion.
- The water adopts different processes of erosion under different sets of conditions.
1. HYDRAULIC ACTION
- The Pressure and drag of flowing water exerted upon grains projecting from the bed and banks results in hydraulic action.
- Weak bedrock and various forms of regolith are easily carved out by hydraulic action.
- Mechanical wearing of the rocks is called abrasion.
- It occurs when rock particles carried in the current strike against the exposed bedrocks of the channels.
- Small particles are further reduced by crushing and grinding when caught between larger cobbles and boulders.
- Chemical reactions between ions carried and solution in river water and exposed minerals surfaces result in the form of erosional which is called solution.
30.4 TYPES OF EROSION
- Following three types of erosion by the river are normally recognised:
(i) HEADWARD EROSION
- This is the process of cutting back upstream of a valley above its original source by rain-wash, gullying and spring-sapping.
- Thus, the source of the stream gradually recedes and ultimately may notch the ridge which forms the original watershed. This may ultimately to lead to river capture.
(ii) VERTICAL EROSION
- When the river cuts the rock material in a vertical direction e., when the river is engaged in down cutting, it is called vertical erosion. This leads to deepening of the river valley.
(III) LATERAL EROSION
- Lateral erosion takes place when the river cuts the sides of its valley.
- Lateral erosion results in widening of the river.
30.5 DEPOSITIONAL WORK OF RIVER
- The moment the transporting power of the river decreases due to decrease in the velocity or volume of its water, the river starts depositing its load.
- Most of its load is deposited along the banks in the lower valley or at its mouth.
- Nearly one-fourth reaches the sea.
- The first to be deposited are big boulders which are followed by pebbles sand and clay.
- In highlands when the river passes through a bed of hard rocks, the main action of the river is downcutting.
- This results in a narrow and steep-sided valley which is known as The Sutlej, the Indus, the Brahmaputra, the Gandak, and the Kosi rivers have cut deep gorges in the Himalayas.
- This is just a magnified form of a gorge.
- In areas of arid climate with little rainfall, the valley sides fail to be widened at all and the river cuts deeper only in its floor.
- A canyon is a variant of gorge, still there is some distinction between the two.
- A gorge has very steep straight sides while a canyon is characterised by steep step-like side slopes.
- A gorge is almost equal in width at its top than at its bottom.
- Canyons commonly form in horizontal based sedimentary rocks and gorges form in hard rocks.
3. V-SHAPED VALLEY
- Most of the world’s rivers have formed V-shaped valleys.
- Such valleys are normally formed in areas of sufficient rainfall where the rocks are not very hard.
- Down cutting and side or lateral cutting are done simultaneously by the river and a V-shaped valleys is formed.
- When the river water falls down almost vertically from a sufficient height along the course of the river, it forms a waterfall, Hard and soft rocks are found at several places in the course of the river.
- The soft rocks are eroded easily and quickly and the river bed is lower at the place of soft rock.
- Thus, the water falls from a
- Sometimes there is a band of a hard-rock along the path of a river which makes it jump over or fall downwards.
- This leads to the formation of rapids at places where the hard valley bottom offers greater resistance to the erosion than the strips above and below it.
- A series of rapids is called a cascade.
6. POT HOLE AND PLUNGE POOL
- Whenever a soft rock comes in the way of the river, it is eroded and scattered all around.
- Eddies are formed and water whirls round and produces depressions by plucking the sediment up.
- This makes the depression deep and cylindrical.
- Sometimes these depressions look like discs and are known as pot holes. Stone pieces also enter the pot holes along with water and act as grinders.
- The pot holes have a diameter varying from a few centimetres, to many metres.
- The depth of a pot hole is greater than its diameter.
- Pot holes of over 7 metres depth have been observed. When the pot holes grow in size they are called plunge pools.
7. INCISED OR ENTRENCHED MEANDERS
- These are very deep and wide meanders cut in hard rocks by the action of a river.
- Such meanders develop over original land surface in the initial stages of development of streams and get intrenched into rocks due to erosion or uplift of land.
- They widen and deepen in the course of time.
8. RIVER TERRACES
- River terraces are terraces found on both sides of the river valley and represent old valley floor or phases plain levels.
- They may be bedrock surfaces without any alluvial cover or alluvial terraces consisting of stream deposits.
- The river terraces may occur at the same elevation on either side of the rivers in which case they are called paired terraces.
- The terraces may result due to (i) receding water after a peak flow; (ii) change in hydrological regime due to climatic changes; (iii) tectonic uplift of land; (iv) sea level changes in case of rivers closer to the sea.
30.6 LANDFORMS FORMED BY RIVER:
- The river forms a large variety of landforms through its erosion, transportation and deposition throughout its source course from its source to The course of the river can be divided into following three parts:
- Upper Course
- Middle Course
- Lower Course
1. Landforms by the Upper Course River.
- The upper of the mountain course begins at the source of the river in hills or the mountain areas.
- The river tumbles down the steep slopes and as a result of its highvelocity, its erosion and transporting power are at the maximum.
(i) ALLUVIAL CONES AND ALLUVIAL FANS
- Velocity of river water is suddenly reduced due to reduction in slope when it enters the plains after leaving the mountains after leaving the mountain.
- Hence, the rock material brought down by the river from highlands is deposited at the foothills. This is generally coarser material composed of sand, gravel and boulders.
- In due course of time, the deposited material projects like a cone from the foothill and is known as alluvial cone.
- The river crosses alluvial cones’ by dividing itself in many channels and forms alluvial fans.
- Alluvial fan is semi-circular in form and points upstream.
- It is broader than alluvial cone but its height is less than the alluvial cone.
- Almost all the rivers coming out of the Himalayas, Rockies and Andes mountains form alluvial fans.
(ii) RIVER MEANDERING AND OXBOW LAKE
- A river normally follows curved path rather than going straight.
- To and fro movement of the river channel across its plain results in developing S-shaped meanders, common to all rivers of large size.
- The speed of the river water slows down and the river does both the works of erosion as well as deposition.
LANDFORMS IN THE LOWER COURSE OF THE RIVER
- The load-bearing capacity of the river is drastically reduced in its lower course due to sluggish flowof water as a result of gentle slope of land.
- Therefore, the work of river in this course is entirely that of deposition and it does not do any erosion at all.
(i) BRAIDED STREAM
- The lower river plain is characterised by an excess deposit of the load on the floor of the channel because of the reduction in the carrying capacity of the slow-moving stream.
- The stream which thus gets divided into a network of channels, forming bars of sand and islands,
(ii) NATURAL LEVEES AND FLOOD PLAINS
- The current of the river is slowest at the sides and bottom because of retardation due to friction, in the lower course of the river the slightest slackening of speed at once results in deposition so thatsilt accumulates at the bottom and the sides of the river.
- After a long time, the bed of the river is raised and it flows between raised banks along a bed which is at a higher level than the plains on either side, Such raised banks or embankments are known as natural levees.
- Most of the sediment of the river is deposited there after the flood is over. In this way, a vast plain is formed which is known as flood plain.
- The flood plains receive fresh soil every year and are very fertile. Rivers Huang Ho, Mississippi, Ganga, Indus, Nile, etc. have formed extensive flood plains.
- When the river enters the sea or a lake, it deposits the entire load at its mouth giving rise to the formation of a delta. It is a triangular feature with its apex pointing upstream and is marked as a fan-shaped area of fine alluvium.
- The mud deposited along the banks because of slow carrying capacity of water becomes an obstacle and divides the river into numerous branches called distributaries.
- The development of the network of such channels and sub-channels goes on extending the delta area. The river has no alternative but to discharge its water sluggishly only through these distributaries.
- The size, shape and the rate of growth of the delta is variable as it depends upon the interplay of a number of factors.
- The Ganga-Brahmaputra Delta, 1,25,000 sq. km. in area, is one of the biggest deltas of the world. Other well-known deltas are those of rivers Nile, Mississippi, Po, Volga and the Yangtze.
- Part of the surface water percolates in the ground and is termed as groundwater.
- All types of rocks do not hold the same quantity of underground water. The water-holding capacity of a rock depends upon the pore spaces which are known as its porosity.
- Permeability is another property of a rock which means its capacity to allow water to pass through it.
- The pore spaces or openings if connected with each
EROSIONAL WORK OF UNDERGROUND WATER
- Underground water flows slowly and its physical erosional power is negligibly small. However, underground water is an important agent of erosion.
- Erosion by underground water includes four different activities : corrosion, attrition, solution and hydraulic action.
- When underground water carrying carbon dioxide enter cracks and joints in a limestone area, it dissolves the surrounding rock and widens the cracks and joints.
- Thus, long furrows with almost vertical walls are formed which are known as lapies in French.
2. SINK HOLES
- A sink hole is a funnel-shaped depression which has an average depth of three to nine metres.
- Sink holes are developed by enlargement of the cracks found in such rocks, as a result of continuous solvent action of the rainwater.
- The surface rock gradually subsidise creating deep and wide sinks on the hillsides and valley floor indicating the intense activity of underground water.
- There are many areas of limestone, dolomite and gypsum where sink holes are found in large numbers. The best example is that of the limestone plateau of Kentucky in the United States of America, where over 60,000 sink holes are found. In India, these are observed along the southern edge of Meghalaya’s limestone strata.
3. SWALLOW HOLES
- Swallow holes are cylindrical in shape lying underneath the sink hole at some depth.
- The surface streams en sink suddenly disappear underground through them.
- It is so, because these are linked with underground caves in rocks through vertical shafts.
- When a sink hole is enlarged due to solution of the rocks by underground water, it becomes a doline. Dolines are funnel-shaped at the surface and cylindrical below it.
- The walls of adjacent dolines collapse due to solution by underground water and they coalesce into a bigger hole known as Uvalas.
- Uvalas are bigger in size than dolines.
- They have vertical walls, closed basin and an oval shape.
- Sometimes uvalas are filled with water obtained from heavy rainfall.
- They suffer large scale erosion and many uvalas combine to form a sort of cave which is known as ponor. Sometime a river falls into it and form bigger ponor.
7. CAVES AND CAVERNS
- In certain areas, there are hard insoluble rocks at the surface and soluble limestone below it.
- The underground water dissolves the limestone from below while the upper rock remains intact like a roof. Thus, a cave is formed.
- A large limestone cave formed by solution due to
- A part of the roof of a cavern collapses, but part of its remains intact and looks like a bridge. Since this bridge is made by nature, it is called natural bridges.
30.8 DEPOSITIONAL WORK OF UNDERGROUND WATER
- The underground water dissolves a large quantity of minerals but the same is deposited elsewhere later on. Following features are formed due to depositional work of underground water:
- The water, containing limes one in solution, seeps through the roof of the caverns in the form of a continuous chain of drops.
- A portion of the drops hangs on the roof and on the evaporation of water, a small deposit of limestone is left behind contributing to the formation of a stalactite.
- A stalactite grows downwards from the roof. Its thickness is maximum near the roof and it thins out downwards.
- The remaining portion of the drop falls to the floor of the cavern.
- This also evaporates, leaving behind a small deposit of limestone aiding the formation of stalagmite, thicker and flatter, rising upwards from the floor.
3. CAVERN PILLARS OR COLUMNS
- Sometimes a stalactite from above and stalagmite from below develop towards each other and combine together to form a This is known as cavern pillar or column.
4. GLACIAL PROCESSES
- Like river, a glacier does the work of erosion, transportation and deposition on the surface of the earth and forms a variety of landforms through these actions.
30.9 LANDFORMS CREATED BY THE EROSIONAL WORK OF A GLACIER
- It has been observed that there is unequal movement of ice in a glacier.
- The top of ice moves much faster than its bottom and ice in the centre also moves faster than ice along the sides of glacier.
- The cracks marked on the glacier because of the splitting up of ice as a result of its unequal movement are known as crevasses.
- When the upper part of a valley glacier comes out of the ice fieldalong a steep slope, it develops a big crack known as bergschrund.
- It is a steep-walled rock basin on the side of a mountain resembling the shape of a vast amphitheatre or an armchair.
- It is open at one end, has a flat bottom and very steep slopes on tree sides.
- Often water accumulates in a cirque forming a lake. Such a lake is called cirque or tarn lake.
- When several cirques erode cutting a mountain back towards a common height from different sides, a pyramidal peak is formed.
- It is known as horn because it resembles that shape. The Metter-horn peak of Swiss Alps is its best example.
5. COL OR PASS
- When two cirques from opposite sides of a hill develop and meet each other, col or a pass is Many passes in the Alps were formed by glacial action. Some of the world’s passes are used for transportation.
6. COMB RIDGE OR ARETE
- When the cirques from two sides of a mountain ridge are enlarged and they extend towards eachother, the ridge top is eroded and takes the shape of a comb.
- Hence, it is known as comb ridge or arete.
7. GLACIAL TROUGH AND TROUGH LAKE
- Plucking of bedrocks by the overriding glacier leads to the formation of glacial trough, a channel of the valley glacier. The glacial trough, if filled up by water gives rise to trough lake.
- The glacial troughs formed near the sea get filled in by the sea water giving rise to flords.
9. U shaped Valley
- Mountain glaciers cannot dig a new valley but deepen, straighten as well as widen the pre-existing valley by eliminating irregularities during its passage.
- Such a trough has steep sides and wider floor. This enables us to see over a longer distance in a glaciated valley.
- The original V-shaped valley becoming narrower towards its head is turned into a
10. HANGING VALLEY
- A steep slope develops at the confluence of the tributary glacier with the main glaciers.
- After the snow melts, the water of the tributary glacier makes a waterfall when it enters the main valley and it seems that the tributary valley is hanging when seen from the floor of main This is known as hanging valley.
30.10 DEPOSITIONAL WORK OF A GLACIER MORAINE
- When the glacier melts, it starts depositing its sediments known as moraines.
- Moraines consist of the heterogeneous rock material of unsorted nature.
- It is a mixture of fine sediments called glacier flour, angular stones and boulders of different sizes and shapes ranging from
Moraines are of the following types.
- (i)LATERAL MORAINES Material deposited on either Side of the glacier known as lateral moraine.
- (ii) MEDIAL MORAINES When two glaciers join, their lateral moraines also join near their confluence and are called medial moraines. Many alpine pastures in the Himalayas like Margs of Kashmir occupy the sites morainic deposits of old valleys.
- (iii) TERMINAL MORAINES Material dropped at the end of a valley glacier is in the form of ridge called terminal moraine. Each time a glacier retreats, a fresh terminal moraine is left at a short distance behind the first one.
- Drumlins are peculiar type of low round hillocks which resemble the shape of an inverted boat or a half egg splitlengthwise.
- Drumlins are smooth and elongated hillocks of boulder clay, with its long axis parallel to the direction of the moving ice which deposited it.
- Drumlins may measure up to 1 km in length and 30 m or so in height.
30.11 FLUVIO-GLACIAL DEPOSITS
- Glacial deposition produces a variety of forms as a result of not only glacial but fluvio-glacial action subsequent to the melting of ice in the lower parts of glaciated region.
1. KETTLE HOLES
- Usually pebbles and other fragments are found lying overthe glacier. When the glacier melts, pebbles and fragments subside and form a small depression.
- A large number of kettle holes are found in the prairies of North America.
- When glaciers melt in summer, the water flows on the surface of the ice or seeps down along the margins or even moves through holes in the ice.
- These waters accumulate beneath the glacier and flow like streams in a channel beneath the ice.
- Very coarse materials like boulder and blocks along with some minor fractions of rock debris carried into this stream settle in the valley of ice beneath the glacier and after the ice melts can be found as a sinuous ridge called esker.
- It is a sort of mound formed along the ice front.
- A kame is formed by the sub-glacial streams and consists of assorted material-gravel and sands.
- Many ridges are formed at the glacier margin and are known as Kame terraces.
4. OUTWASH PLAIN
- It is so named because the material has been washed out of the morainic deposit. Since it spreads over the valley bottom from side to side, it is also given the name of ‘valley trains’.
WAVES AND CURRENTS
- Sea waves and currents are most active along the sea coast and bring about drastic changes When waves break, the water is thrown with great force onto the shore.
- Also there is great force onto the shore. Also there is great churning of sediments on the sea bottom. Constant impact of breaking waves brings about considerable changes on the coasts.
- Storm waves and tsunami waves can cause far-reaching changes in a short period of time than normal breaking waves.
30.12 EROSIONAL LANDFORMS
1. SEA CLIFF
- A scarp face of the coast facing the sea is known as a sea cliff.
- In the beginning, the sea waves cut a groove in the rock at sea level which is known as a notch. This notch keeps on widening with the passage of time.
- It thus undermines the overhanging rock until it falls into sea and forms a cliff.
2. WAVE CUT PLATFORM
- As the cliff retreates further by the erosional work of the waves, a wave cut platform is produced. This is also known as Terrace.
3. SEA CAVES
- Sea waves can erode soft rocks faster than the rocks.
- At several places rocks are traversed by joints, faults or bands of weak rocks.
- Sea waves easily erode these weak points leading to the formation of sea caves.
4. MARINE ARCHES OR NATURAL BRIDGES
- A marine arch or a natural bridge is formed when the sea waves working from opposite directions are able to cut through the caves.
- When the roof of a marine arch collapses, a portion of the arch keeps standing in the sea like a pillar. This is known as
6. BLOW HOLES OR SPOUTING HORNS
- If a hole developed in the roof of a sea cave, it is known as blow hole or spouting horn.
- When the sea water enters the sea caves, the air of the cave is pressed up by sea water and the air passes through the hole with a noise.
- Spouting horn is the name given because of the voice the blow holes make.
30.13 DEPOSITIONAL LANDFORMS
- A beach is formed due to the deposition of sand, gravel and pebbles on the shore between low-tide level and the coastline.
- A beach grows in size when waves are less active but it may be completely destroyed by strong waves in a storm.
- Most of the beaches are made up of sand sized materials.
- Beaches called shingle beaches contain excessively small pebbles and even cobbles.
2. SAND DUNES
- Just behind the beach, the sands lifted and winnowed from over the beach surfaces will be depositedas sand dunes.
- Sand dunes forming long ridges parallel to the coastline are very common along low sedimentary coasts.
- Spit is a ridge or embankment of sediments deposited by the waves and attached to the land at one end and projecting in the open sea at the other.
- It is formed when the waves deposit the rock-waste tangentially to the headland.
- A bar is a sort of embankment of sand and gravel built by the depositional work of sea waves.
- It normally extends between two adjoining headlands of hard rocks or it runs roughly parallel to the shoreline.
- A bar formed in the off-shore zone lying approximately parallel to the coast is called off-shore bar.
- An off-shore bar which is exposed due to further addition of sand is known as barrier bar.
5. LAGOON OR HAFF
- When bars further extend, the sea water is partially enclosed between the coast and the bar which is known as a lagoon lake or haff.
- A lagoon generally maintains connection with the open sea through a narrow gap between the bar and the headlands.
- The Chilka and the Pulicat lakes along the East Coast and Vembanad on the Kerala Coast in the west are examples of lagoons in India.
- Work of wind is more prominent in desert areas where soil particles are loose due to lack of moisture and vegetation.
- A plenty of loose material is provided to be picked up by the blowing wind. Topography created by the erosional and depositional wind is called Aeolian topography.
30.14.1 EROSIONAL LANDFORMS
1. PEDIMENTS AND PEDIPLAINS
- Pediments are gently inclined rocky floors close to the mountains at their foot with or without thin cover or debris.
- These are formed as a result of erosion of mountain front through a combination of lateral erosion by streams and sheet flooring.
- Once the pediments are formed, the steep wash slope and free face retreat backwards through parallel retreat of slopes and the pediments extend backwards at the expense of mountain front.
- Gradually the mountain gets reduced leaving an inselberg which is a remnant of the mountain.
- In this way, high relief in the desert areas is reduced to low featureless plains called pediplains.
- Playa is an inland drainage basin with shallow saline lake of fluctuating volume, encircled usually by mud sheets.
- Water remains for a short period only due to high rate of evaporation in dry climate of the desert. Quite often the playas contain deposits salt and such playas are called alkali flats.
3. DEFLATION BASIN
- Many depressions are formed by deflation action of wind. These depressions are called deflation basins.
4. BLOW OUT
- In several desert areas which lack vegetal cover, strong winds form eddies and blows away the loose and soft rocks.
- Consequently a saucer-shaped depression is formed which is known as blowout.
- The maximum erosion of an upstanding mass of rock occurs slightly above its ground level, where friction close to the ground is absent and sand content in air is yet high.
- The intensity of cutting into the rock decreases, both upwards as well as downwards from this level of maximum erosion by wind.
- Due to greater erosion of the lower portion, the rock
30.14.2 DEPOSITIONAL LANDFORMS
- When the velocity of the wind decreases, it starts dropping its load.
- Some of the important depositional features due to wind action are the sand dunes and loess deposits.
1. SAND DUNES
- A sand dune is the mo important feature formed by the depositional work of wind.
- It is a mound, a hill or a ridge of sand with a crest or a definite summit.
- When the carrying capacity of sand laden wind is reduced due to reduction of its speed, it deposits its material whenever some obstacle comes in its way.
- A dune has a long and gentle windward slope and a much steeper leeward slope.
2. BARCHAN OR BARKHAN
- Barchan is the typical sand dune with a crescent-shaped front, having two horns or wings towards the leeward slope.
- It has a convex windward side.
- As the extremities of this dune move more rapidly than the middle portion, it gives it such a characteristic shape.
- A constant wind direction and a limited supply of sand are two essential conditions for the formation of Barkhan.
- Parabolic dunes are reverse barchans which are formed when sandy surfaces are partially covered with vegetation.
- Seif is similar to barchan with the only difference that it has only one wing or point. It is formed when there is shift in wind conditions.
- The long wing of seifs grows very long and high.
- Longitudinal dunes are formed when supply of sand is poor and wind direction is constant.
- They have considerable length but have low height.
- Transverse dunes are in direction perpendicular to the wind directions.
- These dunes are formed when the wind direction is constant and the source of sand is an elongated feature at right angles to the wind.
UPSC PREVIOUS YEAR QUESTIONS
1. Which of the following phenomena might have influenced the evolution of organisms?
- 1.Continental drift
- 2.Glacial cycles
Select the correct answer using the codes given below:
- (a) 1 only
- (b)2 only
- (c)Both 1 and 2
- (d)Neither 1 nor 2
2. A person stood alone in a desert on a dark night and wanted to reach his village which was situated 5km east of the point where he was standing. He had no instruments to find the direction but he located the polestar, the most convenient way now to reach his village is to walk in the
- Direction facing the polestar
- Direction opposite to the polestar
- Direction keeping the polestar to his left
- Direction keeping the polestar to his right
3. Which of the following is/are cited by the scientists as evidence/evidences for the continued expansion of universe?
- Detection of microwaves in space
- Observation of redshirt phenomenon in space
- Movement of asteroids in space
- Occurrence of supernova explosions in space code.
4. Electrically charged particles from space travelling at speeds of several hundred km/sec can severely harm living beings if they reach the surface of the earth. What prevents them from reaching the surface of the earth?
- The Earth’s magnetic field diverts them toward its poles
- Ozone layer around the Earth reflects them back to outer space
- Moister in the upper layers of atmosphere prevents them from reaching the surface of the Earth
- None of the statements (a), (b) and (c) is
5. What is the difference between asteroids and comets?
- Asteroids are small rocky planetoids, while comets are formed of frozen gases held together by rocky and metallic material.
- Asteroids are found mostly between the orbits of Jupiter and Mars, while comets are found mostly between Venus and Mercury
- Comets show a perceptible glowing tail, white asteroids do not.
Which of the statements given above is/are correct?
- 1 and 2
- 1 and 3
- 3 only
- 1, 2 and 3
6. Which one of the following sets of elements was primarily responsible for the origin of life on the Earth?
- (a)Hydrogen, Oxygen, Sodium
- (b)Carbon, Hydrogen, Nitrogen
- (c)Oxygen, Calcium, Phosphorous
- (d)Carbon, Hydrogen, Potassium.
7. Consider the following :
- Electromagnetic radiation
- Geothermal energy
- Gravitational force
- Plate movements
- Rotation of the earth
- Revolution of the earth
Which of the above are responsible for bringing dynamic changes on the surface of the earth?
- (a)1, 2, 3, and 4 only
- (b)1, 3, 5 and 6 only
- (c)2, 4, 5, and 6 only
- (d) 1, 2, 3, 4, 5, and 6
1. (C) 2. (C) 3. (A) 4. (A) 5. (B) 6. (B) 7 (D)