Geological structure of India

The study of geological structure helps us to understand the nature and composition of rocks which further determines the characteristics of the soils and the availability of mineral resources.

• Sedimentary or stratified rocks found in the ground, are formed due to the deposition of sediments, which produces fertile soil. The Gangetic Plains are a suitable example. On the contrary, the soil made from old crystalline rocks is infertile, but there is a plurality of metallic minerals.
• It helps in knowing about the vast mineral wealth of the underlying rock structure which are unevenly distributed.
• Both the oldest rocks of the world from the Pre-Cambrian period and the youngest rocks of the Holocene epoch are found in the geological structure of India.
• On the one hand, the ancient rocks of the Archean era are found in peninsular India and on the other hand, there is a plurality of the new layered rocks of the Quaternary era in the plains. Based on the variations in its geological structure and formations, India can be divided into three geological divisions.
• The Pangaea broke into two parts in the Carboniferous period due to the forces of gravity and buoyancy. The northern and southern parts were subsequently called as Laurasia and Gondwanaland, respectively, leaving Tethys sea in the middle.
• Molluscs, Corals and Sharks originated in the Silurian period, which is also known as the period of Vertebrates
• Gondwanaland further broke up into the peninsular India, Madagascar, Australia, Antarctica etc in the Jurassic Period.

Peninsular Plateau of the south

It is a part of the Gondwanaland and made up of the most ancient rocks of the world. It has never been submerged under sea since the Pre-Cambrian period. It is formed from igneous rocks of Archean era which are now transformed into Gneiss and Schist. The series of rocks found in the formation of rocks of Peninsular India are as follows:

1. Archean Rock

• These are oldest and primary rocks. The cooling and solidification of the upper crust of the earth's surface in the pre-Cambrian era resulted in formation of Archean rocks.
• About 86.7% part of the earth's historical formation has been done in the Pre- Cambrian era (Archean era).
• Due to the excessive transformation, it has lost its fundamental structure. Now these are Gneiss and Schist type of rocks.
• Two-third part of Indian Peninsula is made up of Archean rocks. Three fully defined types of Archean rocks found in peninsular India are-
• Bengal Gneiss 2. Bundelkhand Gneiss 3. Nilgiri Gneiss
• Archean rocks are the repository of India's mineral wealth. These rocks have abundance of metallic and nonmetallic minerals like Iron, Copper, Manganese, Asbestos, Dolomite, Zinc, Silver and Gold.
• These rocks are exposed all along the Himalayas in the extra peninsula forming the backbone of the mountain system.

2. Dharwar Rocks

• These are the oldest metamorphosised sedimentary rocks formed from the erosion and deposition of the Archean rocks. These are highly metamorphosised and are devoid of fossils. These rocks are found in Dharwar and Bellary districts of Karnataka, Aravalli range, Balaghat, Rewa, Chhotanagpur, etc. Iron-ore, copper and gold are the major minerals found in these rocks.
• Aravalli mountains are formed in this rock system, which are the oldest fold mountains of the world. These rocks are found in Dharwar and Shimoga districts of Karnataka. This has been found in both peninsular and extra peninsular region.

• In Dharwar and Bellary districts of Karnataka which spreads into Nilgiri and Madurai districts of Tamil Nadu.
• It has expanded to middle-east region of Chhotanagpur, Meghalaya Plateau and Mikir hills.
• These rocks reach upto the Aravallis of Delhi; Delhi ridge to Alwar and Himachal Pradesh.

Dharwar Rocks in India

• The Dharwar rocks in India are spread in different areas. These rocks are known by different names in different regions
• They are known as Dharwar from north Karnataka to Kaveri Valley, Sasar ranges in middle eastern Deccan region, Chilly ranges in Bhatinda and Balaghat, Goudite range in Hazaribagh, Kudorite range in Visakhapatnam, Champaner range in Gujarat and Aravalli ranges near Delhi.

Dharwar Rocks of extra peninsular India.

• Ladakh, Zaskar, Garhwal and Kumaon mountain ranges of the western Himalayas, in Spiti valley in Himachal Pradesh and Shillong range in Assam have rocks of Dharwar origin

3. Cuddappah Rock

• These rocks have been formed by the erosion and deposition of Dharwar rocks. They are less metamorphosised but still devoid of fossils. These rocks are found in Krishna valley, Nallamalai hills, Papadahani and Cheyyar valleys, Cuddappah rocks of Rajasthan is also known as Delhi Ridge. These are also sedimentary rocks
• These rocks have been named after the Cuddappah district in Andhra Pradesh where, they are spread in semicircular in shape
• It is famous for sandstone, limestone, marble and asbestos. Diamonds are also found in same rocks system

4. Vindhyan Rock

• It is named after Vindhyan mountain which act as water-divide between the Gangetic plains and the Deccan plateau
• It is famous for limestone, china clay, dolomite etc.
• These are sedimentary rocks formed by the deposition of silt of river valleys and shallow oceans. These rocks are spread from Chittorgarh of Rajasthan to Sasaram of Bihar.

5. Godwana Rock

• These are the rocks formed in upper-Carboniferous to Jurassic era, hence these are especially important for coal deposits.
• About 98% of the coal deposits of India are found in these rocks. These are stratified rocks in which the fossils of fishes and reptiles are found.
• The best examples of these rocks are found in the river valleys of Damodar, Mahanadi and Godavari and its tributaries.

6. Deccan Trap

• This was formed in Cretaceous period of the Mesozoic era. At the end of the Mesozoic era, an extensive volcano erupted through fissure and a vast area of about 10 lakh km got buried under the lava.
• In this region, the thickness of basaltic lava is 600 - 1,500 m and even at some places it is 3,000 m. This region is known as Deccan Trap. The Raj Mahal trap was formed even earlier in the Jurassic period.
• The Deccan Trap is found in most parts of Maharashtra, Gujarat and south-western M.P. and Tamil Nadu.
• These rocks are used in building and construction activities.Quartz, Bauxite and semi-precious stones are also found here.
• This structure is made up of Basalt and Dolomite rocks. These rocks are quite harsh.
• Black soil has been formed by the fragmentation of these rocks, which is known as Black Cotton Soil or Regur Soil.

Deccan Trap

7. Tertiary Rocks

• The tertiary rocks have been formed between the Eoceneera and the Pliocene era.
• The tertiary rocks are found mainly in the Himalayan region, In peninsular India, these are only limited to coastal areas.Petroleum is also found in this series of rocks.
• This series is found in a developed form in the Kumaon region of Uttarakhand where it is known as the Lilang series.
• They may also be found in the Salt Range, Potwar Plateau, Jammu and Punjab's outer Himalayan areas, Assam, Sind, and Baluchistan.

8. Quaternary Rocks

• The quaternary rocks include Pleistocene and the current Holocene age rocks. The expansion of the Pleistocene series is found in the upper valleys of Jhelum, in Kashmir, Ganga, Brahmaputra, Narmada, Tapi, Mahanadi, Godavari and Krishna.
• New deposits of deltaic soils are found at the mouth of rivers like Ganga and Sutlej along with peninsular rivers like Narmada, Tapi, Mahanadi, Godavari, Krishna and Kaveri.
• During Pleistocene period, the entire Kashmir Valley was a lake. Due to endogenetic forces, the lake drained through Baramullah Gorge leaving lacustrine deposits called Karewas. These rocks are useful for the cultivation of Zafran (a local variety of saffron), almonds and walnut.
• Rann of Kutch, a salt marsh in the Thar desert was previously a part of the ocean. The sedimentary deposits of Pleistocene and Holocene period filled this area.

Importance of the Peninsular Plateau

• Geographically, Deccan Plateau has been a region of vertical activity, therefore several falls and rapids are formed here, which are the major source for the generation of hydro electricity
• There are several natural depressions found in the plateau region, which act as water reservoirs and supplies an important means of irrigation. By the weathering and erosion of the Deccan plateau, the fertile Black soil has been formed which is very important for the cultivation of cotton
• Laterite soil has been formed in the rainfed highland plains regions of the Western Ghats. These tracts are important for the cultivation of spices, tea, coffee, etc
• Red-soil is found in the remaining parts of the peninsular plateau, which is important for the cultivation of coarse grains, rice, tobacco and vegetables.
• In the heavy rainfall regions of the Western Ghats, evergreen forests are found. Teak, Deodar, Mahogany, Sandalwood, Bamboo, etc. are commercially important trees of these forests
• The Chhotanagpur region is known as the Ruhr of India, due to the presence of rich deposits of minerals.

Great Mountain Ranges of the North

• The evolution of the mountain ranges of the Himalayas is the result of a long geological history. It has rocks from both the Pre-Cambrian and the Eocene periods.
• Regarding the evolution of Himalayas, the Geosynclinal Theory and the Plate-Tectonics Theory which was based on the concept of Sea Floor Spreading of Harry Hess are most accepted. (Geosynclines are the elongated, narrow and shallow water-bodies).
• According to Geosynclinal Theory, about 70 million years ago there was a geosyncline, called Tethys Sea, in the place of the Himalayas and it seperated the Angaraland in the north from Gondwanaland in the south.
• The sediments from these two headlands were deposited into the Tethys Sea. These sediments gradually got raised due to the compression forces of the two landmasses.
• The compression of the sediments formed the fold mountain ranges of Kunlun, Himalaya and Karakoram. The median mass, less affected by the folding, is known as the Tibetan Plateau.
• The formation of Himalayas is best explained by the Plate Tectonic Theory of Harry Hess. According to this theory, about 70 million years ago, some plates in the south moved in the north-eastern direction towards the Eurasian plate.
• About 20-30 million years ago, these two landmasses came very close, due to which the sediments in the Tethys Sea got folded and Himalayas started evolving. About 10 million years ago, all the ranges of Himalayas got fully developed.
• The Greater Himalayas were formed in the Eocene and Oligocene periods of the Cenozoic era. The Lesser Himalayas were formed due to folding of the Ootwar sediments in the Miocene period.The Shiwaliks were formed in the Pliocene period due to the deposition of sediments down hills from the Greater and the Lesser Himalayas.
• The evolution process continued in the Pleistocene and Holocene periods of the Neozoic or Quaternary era. In fact, Himalayas are the young fold mountains, which are still under the evolutionary process.
• The earthquakes continuously changing the course of all Himalayan rivers and the lake deposits (called Karewas) found in the Pir Panjal Range at a height of 1,500-1,850 m indicates the ongoing process of evolution of the himalayas