Continental Drift theory

CHAPTER-V

CONTINENTAL DRIFT AND PLATE TECTONICS

The possibility of drifting of continents was first suggested by the French scholar Antonio Snider in 1858, but was opposed.  In 1910 F.B.Taylor of America invoked the hypothesis of horizontal displacement of continents, with a view of explaining the distribution of mountain ranges, but Taylor received a scant attention.  German professor Alfred Wegener was the first to put forward this idea in the form of a theory in 1912,its English translation was made in 1924 since then it has attracted much attention and publicity, and a huge literature has grown around this theory.

     According to the Wegener, all the continental mass which he called ‘Pangaea’ was united.  This super continent was surrounded by a mega ocean called ‘Panthalassa’, meaning all water.  He argued that, around 200 million years ago, the super continent Pangaea began to split.  First it broke into two large continents called Laurasia and Gondwana forming the northern and southern components respectively.  These two blocks were separated by a long shallow inland sea called “Tethys”.  The super continent Pangaea started breaking and the present shape and relative position is the result of fragmentation of Pangaea by rifting and the drifting apart of the broken parts.  According to this theory the continents are made of lighter SIAL and are floating on the denser SIMA.  The drifting of Pangaea was made possible chiefly due to differential gravitational forces.  The continents drifted in two directions – one towards Equator and other towards West.  On account of equatorial drift Africa and Eurasia were pushed closer together and the Tethys marine deposits located in between the two raised up in the form of mighty fold mountains extending from the Pyrenees and the Alps, and the Atlas mountain of N.Africa to the extensive Himalayan ranges of Asia.  On account of this equator ward drift Peninsular India and Africa separated from Antarctica and Australia, and as a result of their further drift in the course of time a portion of panthalasa got converted into Indian ocean.  The reason for the equatorial movement was the gravitational attraction exerted by the earth’s equatorial buldge.  The other movement of the continents was towards the west, the main reason for this drift as given by Wegener was the tidal force of the moon and the sun on the continents.  North America and the South America got separated from Europe and Africa respectively and the Atlantic ocean came into existence.

Evidence in favour of the Drift theory

Jig-saw Fit evidence:  He was struck by the geographical similarity between the opposing coasts of the Atlantic Ocean.  The outlines of the coast on two sides of the Atlantic are such that they can be easily joined together and one appears to be a detached portion of the other.  The eastern coast of South America can be fitted into the western coast of Africa. Similarly the eastern coast of North America can be fitted against the western coast of the Europe.

Geological structure:  There is remarkable similarity in the geological structure of the lands located on the two coasts of the Atlantic Ocean.  One, the Appalachian mountains of the North America which come right upto the coast and then continue their trend across the North Atlantic Ocean in the fold mountains of South West Ireland, Wales and Central Europe.

Paleo-climatic evidence:  The distribution of the Permo-carboniferous glaciations presents a powerful proof of the fact that at one time these landmasses were assembled together, since the evidences of these glaciations are found in Brazil, Falkland, South Africa, Peninsular India as well as in Australia. It is difficult to explain the distribution of glacial on land and water.  In the opinion of Wegener, all these land masses were united together to form one super mass of land.

Paleontological evidences:  Fossil remains of land animals and plants and of fresh water species in distant lands provide good evidence as they are now separated by oceans.  These species could migrate freely across united continents but not across as intervening ocean.

Polar wandering:  Paleomagnetic studies have shown that there has been periodic change in the position of magnetic pole that it recorded in the rocks by way of permanent magnetism.  It shows the changing position of the earth’s poles in geological time scale.  This is known as polar wandering.  This clearly demonstrates that the continents have frequently moved and changed direction of their motion from time to time. 

Weathering, Mass wasting and Erosion

CHAPTER – IV

WEATHERING, MASS WASTING AND EROSION

Weathering is the disintegration and decomposition of rock in place.  It is a general term applied for a group of processes which act at or near the earth’s surface and reduce solid rock masses by physical disintegration or by chemical decomposition.  It is a process of the breakdown of rocks.

This disintegrated rock debris are called as regolith are subjected to gravity and tend to fall or slip down the slopes especially when aided by the lubricating action of water, though water is not the transporting agent in this case.  This process of gravitational transfer of mass of rock debris down slopes is called Mass wasting.

Erosion is essentially concerned with various ways in which the mobile agencies acquire and remove rock debris.  The principal erosional agents are running water, ground water, wind, glaciers, waves and currents.  Each of the agents does erosion in distinctive processes and gives rise to distinctive landforms.  Basically there are five common aspects

i.                     The acquisition of rock fragments

ii.                   Wearing away the surface through impact of rock materials in transit

iii.                  Breaking down the rock particles by mutual wear while in transit

iv.                 Transportation of the acquired rock debris by moving medium

v.                   Ultimately its deposition somewhere either in transit or at the end.

Factors controlling the weathering:

Broadly speaking four factors influence the rate of weathering – rock structure, climate, topography, and vegetation.

Types of Weathering:

Weathering is of two types – Physical or mechanical and Chemical weathering.  In the physical weathering the rocks are disintegrated by temperature changes, frost action and organism.  While in chemical weathering the rock minerals are decomposed, dissolved and loosened by water, oxygen and carbon dioxide of the atmosphere and soil water and by organisms and the products of their decay.  The physical, chemical and biological agents co-operate with one another actively and both mechanical breaking up or disintegration and chemical decomposition of rocks proceed simultaneously in nature.

The physical or mechanical weathering takes place in four ways. 

i.                     Frost action and crystal growth

ii.                   Thermal expansion and contraction by temperature changes

iii.                  Organic activity

iv.                 Expansion by unloading.

Physical weathering:

Frost action and crystal growth:  This type is found in cold climatic areas.  When water fills the pores, cracks and crevices in rocks and then freezes it expands in volume and exerts a bursting pressure thereby the rocks are ruptured, fragmented and wedged apart.  It is the most effective weathering process in the areas where there is repeated freezing and thawing takes place.

                Closely related to the formation of ice crystals in rocks is the disintegration of rocks by the growth of salt crystals.  The salt crystals form in dry climates as a result of capillary action of water containing salts.  During the long dry period as the water rises to the surface and evaporates, tiny crystal of salt are left behind in the porous outer zone of the sandstone.  The force generated by these crystals lead to grain-by-grain breaking if the sandstone which disintegrates into sand.

Thermal expansion and contraction by temperature changes:  when the rock surfaces are exposed to marked diurnal changes of temperature as in the hot deserts, the alternate heating and cooling results in alternate expansion and contraction which exerts a powerful disruptive force on the rocks.  In the hot dry regions the difference between the day and night temperatures often exceeds 30 degrees centigrade.  The intense heat of the day causes the thin surface layer of the rock to expand and to pull it away from the cooler layer within.  This process of peeling of flakes and curved shells of rock just as in onion is called Exfoliation.

Organic activity:  This type referrers to the action of plants and animals.  As the plant roots grow, they wedge the rocks apart and cause the widening of joints and other fractures.  Organic activity is however, is of great importance in chemical than in physical weathering. Dead organisms produce acids as they decay and thus promote chemical weathering.  Earthworms, ants, termites and other burrowing animals move material to or near the surface where they are more readily subjected to chemical weathering processes.

Expansion by unloading:  unloading occurs when large igneous bodies are exposed through the erosional removal of overlying rock the resultant reduction in pressure.  Igneous and metamorphic rocks formed at great depths are in a compressed state because of the continuing pressure of the overlying rock.  On being exposed to the surface they expand slightly in volume.  Expansion or dilation accompanying the unloading causes thick shells of rock to break free from the parent mass below.  The process which produces thin onion like layers or concentric large scale fractures is called Exfoliation.

Chemical weathering:

In general, chemical weathering is probably more important than physical weathering, this is particularly in the warm and humid climates of the equatorial, tropical, sub-tropical zones, where heat and moisture are abundant.  Water is the main agent of chemical weathering.  Although water in a pure form is almost inactive, but when mixed with oxygen or carbon dioxide it becomes an active chemical agent.  Oxygen, carbon dioxide as well as watervapour are present in the atmosphere in abundant near the earth’s surface. Under this influence the rocks decompose, decay and break into smaller particle size and new secondary minerals are formed.  The chief chemical weathering processes are

i.                     Solution

ii.                   Oxidation

iii.                  Hydration and hydrolysis

iv.                 Carbonation

Solution:  it is a simple process in which the rock salt, gypsum and other minerals get dissolved in the water.  Limestone is not soluble in pure water but gets easily dissolved in rain water which contains carbon dioxide.  In fact atmospheric carbon dioxide is dissolved in all surface waters of the land, including rain water, soil water and river water.

Oxidation:  The presence of the dissolved oxygen in water in contact with mineral surfaces leads to oxidation.  Since some dissolved oxygen is always present in rainwater, surface water as well as sub-surface water oxidation is a universal phenomenon.  The effects of oxidation are most apparent in rocks containing Iron.  Iron rusts or oxidizes under the influence of moisture, as the dissolved oxygen changes the ferrous iron in mineral compounds to the more oxidized ferric state.  When iron combines with oxygen, the original mineral structure is destroyed and the mineral components are free to participate in other chemical reactions.

Hydration and Hydrolysis:  The process of hydration involves the absorption of water.  Most rock- forming minerals absorb rain water.  This not only increases their volume but also produces chemical changes resulting in the formation of new minerals which are relatively softer and more voluminous.  By the process of hydration a hydrite is converted to gypsum and haematite to limonite.  Both these reactions are however reversible upon application of heat, which indicates that there has been no fundamental chemical change.  In hydrolysis, on the other hand, there is a chemical change, and both the mineral and water molecules decompose and react to form new compounds. The significant result of hydration and hydrolysis is that the new minerals formed are more easily attacked by the chemical and physical weathering processes.  The hydrolysis of exposed granite surfaces results in grain-by-grain break-up of the rock, creating many interesting boulder and pinnacle forms by rounding of angular joint blocks.

Carbonation:  Carbonation and hydrolysis are closely linked.  Rain dissolves some carbon dioxide as it falls through the atmosphere, and additional amounts released by decaying organic matter are acquired as the water percolates through the soil.  Carbonic acid particularly attacks minerals which contain iron, calcium, magnesium, sodium or potassium.  These elements are soluble in carbonic acid and the minerals and the rocks containing them start decaying under its influence.  

volcanoes

Volcanoes: The volcanic phenomenon is a majestic natural phenomenon which we can neither prevent nor regulate.  But the ejection is not alike in all the cases.  On the basis of frequency of eruption, there are active, dormant and extinct volcanoes.  The volcanoes which erupt frequently as compared to other are active, best example is Mt. Stramboli.  The dormant or sleeping volcanoes are one which eruption has occurred in the past and every possible chance of eruption is there in the future, the best example is Mt. Vesuvius.  On the other hand Extinct is one where once the eruption had occurred and the possibility of eruption again is ruled out, example is Mt. Kilimanjaro.  Many a time a volcano thought to be extinct may suddenly become active. It happened in case of Vesuvius and Krakatao.

Volcanic types:  Volcanic eruptions are divided into two principal cases based on the modes of eruption

i.                     Central eruption

ii.                   Fissure eruption

CENTRAL ERUPTION:  Eruption is confined to a pipe-like vent and after the eruption, cone and crater structure is developed.  Example is Mt. Cotapoxi of Ecuador. In the central eruption the nature and intensity of the eruption show great variation according to the pressure of gases and viscosity of the lava.  When the lava is acidic meaning more silica content, therefore more viscous, the eruption is explosive.  On the contrary, when the lava is basic meaning, less content of silica, the eruption is peaceful and absence of explosion.

     Volcanoes of the central eruption may be sub divided into five types based on the nature and intensity

i.                     Hawaiian type

ii.                   Strambolian type

iii.                  Vulcanian type

iv.                 Vesuvian type

v.                   Pelean type

Hawaiian type : This type shows less explosion and eruption is peaceful.  Lava is thin basaltic variety.  Pit like crater or lateral cracks develops as lava rivers.  When the wind is strong the lava pieces are stretched into long shiny threads known as ‘Pele’s hair’ in the Hawaiian islands.  Ex. Hawaiian islands, Columbia plateau and Iceland.

Strambolian type : The basaltic lava is not quite as thin as in the Hawaiian type, the gases come either continuously or interruptedly with moderate explosive action.  The explosion is relatively mild, liquid lava fragmental materials are also ejected.  No cloud of black smoke can be seen either in Hawaiian or strambolian type.  This type is majorly found in the stramboli island in the Mediterranean sea.  The lava fountain activity of stramboli, reflected at night as a red glow on the down side of a towering steam plume has caused the volcano to be known as “ light house of the Mediterranean”.

Vulcanian type :  This type has been named after the volcano located in the Lipri islands, north of Sicily in the Mediterranean sea.  Vulcanian activity is explosive.  In this lava is so thick and viscous that it is unable to remain in liquid condition after coming in contact with air and solidifies and seals the mouth of the crater in between two eruptions.  This blocks the passage of the gases from inside, and after sometime when the gases have collected in adequate quantities and their pressure is intense, they force through the solidified vent with explosion.  Masses of black clouds filled with ash and dust rise to great heights and give the appearance of huge cauliflower from a distance.

Vesuvian type : In this type there is a violent explosion due to the intensity of the gases and the lava comes out with great force.  Lava comes out from the lateral cracks, the gases keep on accumulating in the main vent.  When the pressure has eased a little bit on the top on account of the ejection of lava from the lateral cracks, then under the influence of intense gases the lava comes out rapidly with a explosion.  The gases forming cauliflower like clouds.  The clouds rise to great heights and look very shining and bright, causing downpour of ash.

Pelean type : This type of Volcanic eruption is Violently explosively and the lava is highly viscous.  At the time of eruption the dense lava solidifies and closes the mouth of the crater and a dome is formed there.  After sometime, the powerful gases trapped inside either break through this obstacle or come out along the hill slope, and then exceedingly dense mass of hot, highly gas charged lava mixed with fragmental materials and ash flows down the slope like a avalanche.  These have been called “Nuees ardentes”.  This nuees ardentes is extremely dense but highly mobile on account of gases and moves rapidly down the hill slope almost without friction.  It is the most terrifying form of explosive volcanic activity.  It is however, soundless, as in spite of the fact that it has the velocity of winds in a hurricane.

FISSURE ERUPTION:  Lava of basic composition is less viscous and hence less explosive than central eruption.  There is a voluminous lava flow which spreads out over large areas like a thin sheet because of its low viscosity, covering the pre-existing topography.  Such outflows of basaltic lava builds large lava domes, shield volcanoes also lava plains and plateaus.  Basalt emerges from numerous fissures rather than a single pipe like vent, forming a huge plateau or the plain covering larger areas. Examples for this type are Deccan plateau of India, Columbia plateau of USA.