From the book ”The Western Ghats” by Mohan Pai (2005).
MILLIONS OF YEARS AGO.
A nineteenth century Austrian geologist Edward Suess named it the Gondwana. Although the early events of Gondwanaland are lost in the haze of a distant past, many details are being put together to form a plausible explanation of what might have taken place.
Continents have been coming together and breaking apart again for the whole history of the earth. Before Gondwana, Pangaea(meaning ‘all lands’) made up of Gondwana in the south and Laurasia in the north. Laurasia (made up of North America, Europe and Asia) broke away from Pangaea at about 200 to 150 million years ago (m.y.a.) and the remaining part of Pangaea became Gondwana made up of continents that are now Australia, India, South America, Africa and Antarctica. During the Carboniferous and Permian period (300 – 260 m.y.a) much of Gondwana was covered by ice. During those times we have little evidence of animals actually living there. Once the ice melted in the late Permian (240 m.y.a.) and the Triassic (225 m.y.a.), animals were able to colonise most of Gondwana which stayed warm until 40 m.y.a., when part of it now known as Antarctica froze over again. The break up of Gondwana began during the time that dinosaurs were the dominant land animals (in the Cretaceous – 120 m.y.a.), but it was such a slow process that they were unlikely to be affected by it.
To understand land forms it is necessary to know about the earth’s composition. Deep within the earth at a depth range of 100 to 350 km from the surface, lies a viscous layer of the earth, the asthenosphere, which has fluid-like properties. Above this layer rests the rigid lithosphere which includes the crust. The lithosphere is ‘floating’ on the asthenosphere. The lithosphere is not a continuous shell, but is divided in plates(like a jigsaw puzzle). It comprises of seven major and several minor distinct plates. The plates are in constant motion relative to each other and this motion is the cause of most of the phenomena in the earth’s crust. At some places they are moving away from each other. Usually one plate bends down the other at such boundaries and the process is called subduction. The interior of the plate is stable and the edges are usually the site of intense deformation. Thus Plate Tectonics is a near-complete theory that explains most of the phenomena occurring in the earth’s crust. In fact it is one theory that has changed people’s perception of all the disciplines of Earth Science. The theory of Plate Tectonics did not emerge overnight. It evolved gradually since the beginning of the 20th century as many hypothesis were laid down. The important ones are Continental Drift, Apparent Wandering and Sea-floor Spreading.
Continents break up and move apart at about the same speed that one’s finger nails grow – about 6 cms per year. A large chunk from Gondwana broke loose from its moorings in the Antarctica. This chunk fractured to form S. America and Africa to its west and Australia to its east. The central portion kept moving northwards. About 70 m.y.a. Madagascar is said to have separated leaving the Indian Plate to continue its migration in a north-easterly direction. This part became a temporary island before becoming part of the Indian subcontinent. In the beginning there is a stable “shield” area called the Deccan Plateau that occupies the triangular peninsula area of India(and which includes Sri Lanka). Most geologists regard this area as having formed a part of Gondwanaland.
As it moved up in this drift of about 10,000 km from approx. 30 degrees south latitude, the Indian Plate passed over a deep seated volcanic hot-spot in the region of today’s Reunion islands. The heat beneath generated basaltic magma which rose into lithosphere causing an uplift by crustal arching. It also tilted the Indian Plate in the easterly direction. The major rivers of the peninsular India would later flow eastward because of this tilt. A northerly tilt is also postulated to explain the expanse of older and deeper rocks in the Southern Western Ghats.
When the peninsular shield bumped into the Asian landmass around 45 m.y.a. after having undergone many uplifts and modi-fications, the Indian Plate lifted up the Asian Plate by subduction to form the elevated Tibetan plateau. The eastern portion of the Tethys sea caught up between the two moving landmasses was obliterated and its sediments raised up to form the Himalayas. These mountains not only define the subcontinent and separate it from the adjacent landmass; they also form the second major land form region of South Asia. The mountains that are formed are of the folded type, usually consisting of alternating ridges and valleys, all parallel to one another (though some complexity inevitably results) and generally trending in long bands parallel to the colliding areas.
The Deccan Volcanic Episode
The Deccan Plateau ranks as the world’s fourth largest (volumetrically) subcontinental outpouring of plateau basalt lavas extruded at unstable margins during active tectonic plate movements. The Deccan Traps occupying one-sixth of Indian landmass, consists of more than 2000 meters deep flat lying basalt lava flows and covers an area of over 5,00,000 sq. km. in west-central India. Estimates of the original area covered by the lava flows are as high as 1.5 million sq. km.
The volume of the basalt is estimated to be 5,12,000 cubic km. Trapp is a Swedish term for a terraced plateau (Ghats – steps or terraces). The term “Deccan Trap” was first used by Col. W. H. Sykes and the term “Trap” is used in the lithological sense of the rock basalt.
Since the Western Ghats are a result of a domal uplift, the underlying rocks are ancient – around 2000 million year old. The oldest of these rocks are found in the Nilgiris and the High Ranges of the southern Western Ghats.
The uplifted crust of the earth bears a central axial region of weakness coinciding with the track of upliftment. Peninsular India broke along its line of weakness and the western segment drifted westward into the sea(a process known as faulting), giving rise to the present day hill chain of the Western Ghats and the west coast. The exposed face of the eastern unsubmerged plate was lifted up to form the scarp of the Western Ghats. This happened during the Eocene (between 45 and 65 m.y.a.), even before India became part of the Asian mainland. By the time the peninsular India ended its northward drift and collided with the Asian mainland, the Western Ghats were very much in place.
At this time the marked eastward tilt permanently changed the pattern of drainage. The western faulting led to ‘river capture’ and diversion of easterly drainage to the west in many instances. The river Sharavathy and Kali in Karnataka are classical examples of westerly diversion of drainage due to uplift and faulting. The Shara-vathy with its spectacular waterfalls and deep gorges has receded about 28 km due to headward erosion. The Western Ghats thus represent tectonically active region with high rates of uplift, high summit altitudes, steep slopes, deep gorges and large potential for erosion and correspondingly high sedimentation yields.
Northern Sahyadris, Selbari Range
The northern half of the Ghats from Tapti valley down to Goa is formed of horizontal beds of massive Deccan Trapp. The resistant character of the Trapp has accentuated the relief and the Ghats appear in the form of a steeply-cut wall facing the Arabian Sea. The scarp of the Ghats in this region presents a magnificent vertical profile of over 1000 m of successive layers. The view from ‘Arthur’s Seat’ just outside Mahabaleshwar is very impressive.
Bedthi River Gorge, Uttara Kannada
In Kodagu and south Mysore the Ghats expose Charnokites. Some of the highest peaks in the Ghats are to be found in this Charnokitic region. In the Nilgiris the average elevation is over 6000 ft., the highest point Dodabetta being 2637 m.(8,650 ft.) high. This aspect of the Western Ghats, which hitherto showed scarp-like face only to the west, changes south of Mysore. The hill masses, although continuous with the Ghats, appear more like tableland – lifted abruptly to elevations of over 2,440 m.(8000 ft.), presenting steep sided precipices on all sides.
South of the Palghat Gap are the complex group of hills forming the Anaimalai, Palni, the Varushanad, and Cardamom Hills mainly of Charnokites. The rivers follow straight courses and the drainage pattern is rectilinear. As we proceed south, in Kerala, the Ghats recede inland leaving a fertile coastal plain.