The geology of India is very diverse because an Indian rock belongs to different geologic periods, dating as far back as the Eoarchean Era. Some of the rocks are very deformed and altered. Other deposits include recently deposited alluvium that has yet to undergo digenesis. Mineral deposits of great variety are found in the Indian subcontinent in huge quantity. Even India's fossil record is impressive in which stromatolites, invertebrates, vertebrates and plant fossils are included. India's geographical land area can be classified into Archaean System; Dharwar System, Cuddapah System; Vindhyan System; Gondwana System, Deccan Trap; Tertiary System; Quarternary System.
Rocks of the Archaean System
• These rocks have been termed as a result of the hot-molten earth becoming cold. These are the oldest and primary rocks.
• Their original form has been destroyed because of too much metamorphosis.
• There are no fossils found in them.
• Gneiss is formed because of the metamorphosis of the igneous rocks. The Bundelkhand gneiss is the oldest one.
• The rocks of the Archaean system are found mainly in Karnataka, Tamil Nadu, Andhra Pradesh, Madhya Pradesh, Orissa, Chotanagpur plateau in Jharkhand and the southern-eastern part of Rajasthan.
Rocks of Dharwar System
• These rocks have been formed as a result of the erosion and sedimentation of the rocks of the Archaean system.
• These are the oldest sedimentary rocks.
• No fossil is found here. It is so because either there was no origin of species during their formation or the forms of the fossils got destroyed with the passage of time.
• The Aravali mountain range which is the oldest fold mountain of the world has been made with these rocks.
• The rocks of this system are found in the districts of Dharwar and Shimoga in Karnataka.
• The rocks of this system are found in the southern Deccan region from Karnataka to the Kaveri valley, districts of Bellary, Shimoga, Sasar mountain range in Jabalpur and Nagpur and the Champaner mountain range in Gujarat.
• In north India the rocks of this system are found in the Himalayan ranges of Ladakh, Zaskar, Garhwal and Kumaon, and the long range of Assam plateau.
• The rocks of this system are economically very important. All prominent metallic minerals (iron, gold, manganese etc) are found in these rocks.
Rocks of Cuddapah System
• These rocks have been formed as a result of erosion and sedimentation of the rocks of Dharwar system. These are also sedimentary rocks.
• The rocks have been named after the district of Cuddapah in Andhra Pradesh where these are semi-circular in expansion.
• These are famous for sandstone, limestone, and marble, asbestos.
• The Cuddapah rocks are also found in Rajasthan.
Rocks of the Vindhyan System
• These have been formed after the cuddapah rocks by the deposition of silt of river valleys and shallow oceans. Thus, these rocks are also sedimentary rocks.
• The evidences of fossils of micro-organisms are found in this structure.
• These rocks are found in the Vindhyas, e.g. the Malwa plateau, the Semari range in the Son valley, Bundelkhand etc.
• This structure is famous for house-building rocks. The Sanchi Stupa, the Red Fort, the Jama Masjid etc are built with the red sandstone of this structure. Besides, limestone, china clay, dolomite etc are also found in this structure.
• The diamond mines of Golconda in Karnataka and Panna in Madhya Pradesh are found in this structure.
Rocks of Gondwana System
• The word Gondwana has originated from the Gond region of Madhya Pradesh.
• 98% of coal in India is found in this structure.
• These rocks have been formed between the Carboniferous and Jurassic periods.
• Several cracks were formed in the peninsular India during the Carboniferous period. Basin-like depressions were made because of the sinking of land among these cracks. Coal was formed by the burying down of the vegetation of that period. This coal is now found mainly in the river valleys of the Damodar, the Son, the Mahanadi, the Godavari, and the Wardha etc.
The Deccan Trap
• The volcanic action in the peninsular India began in the last period (Cretaceous period) of the Mesozoic era. Thus, the Deccan trap has been formed as a result of fissure eruption.
• This structure is made up of basalt and dolorite rocks.
• These rocks are very hard and their weathering has resulted in the formation of the black soil.
• This structure is found in the most parts of Maharashtra and some parts of Gujarat, Madhya Pradesh and Tamil Nadu.
Rocks of the Tertiary System
• These rocks have been formed between the Eocene and Pliocene periods.
• The Himalayan mountain range has developed as discussed below:
I. The Great Himalayas were formed during the Oligocene period'
II. The Lesser Himalayas were formed during the Miocene period.
III. Shiwaliks were formed during the Pliocene and Upper Pliocene periods.
• Mineral oil in Assam, Rajasthan and Gujarat is found in the structures of the Eocene and Oligocene period.
• The Tertiary epoch has been divided chronologically into four parts- (a) Eocene (b) Oligocene (c) Miocene (d) Pliocene
Rocks of the Quarternary System
• These rocks are found in the plains of the Indus and the Ganga.
The Quarternary epoch is divided chronologically into two parts- Pleistocene and Holocene periods.
• During the Upper and Middle Pleistocene periods, old alluvial soil was formed which is known as 'bangar'.
• The formation of the alluvial soil began at the end of the Pleistocene period and it is still going on in the present Holocene period. It is known as 'khadar'.
• The Kashmir valley was formed during the Pleistocene period.
• This valley was a lake in the beginning. The continuous deposition of soil gave rise to the present form (valley) which is known as 'kareva'.
• Deposition of the Pleistocene period is found in the Thar Desert. The 'Rann of Kachchh' was previously a part of the ocean. It was filled by the sedimentary deposits during the Pleistocene and Holocene period.