Geological structure primarily pertains to the arrangement and deposition of rocks within the Earth's crust, influenced by earth movements or their absence. This concept also extends to the morphological features of rocks, exemplified by the Gondwana structure.

Geological Time Scale:

The chronological dating of geological formations (geological strata) and life is based on their time and place of origin, evolution, and extinction. 

Giovanni Ardunia first developed the Geological Time Scale in 1760, while the standardised version emerged during the International Geological Congress in 1881, Italy. T.S. Holland advocated the Indian Geological Time Scale.

Geological History of India:

The geological history of India involves the analysis of its geological structure and rock systems in relation to their geographical locations. Physiographic divisions, namely Peninsular India and Extra Peninsular India, serve as reference points for understanding geological formations.

Major Events in India's Geological History

Peninsular India's association with the ancient landmass since the Earth's Crust formation.

The upheaval of the Himalayas during the tertiary period.

Aggradational formation of the Indo-Gangetic plain during the Pleistocene period, an ongoing process characterised by sedimentation in the floodplains of rivers and the lower Gangetic plain.

Due to the intricate and diverse geological history of India, the Geological Survey of India has classified the rock systems of the country into four major divisions:

1. Archaean Rock System:
2. Purana Rock System:
3. Dravidian Rock System:
4. Aryan Rock System:

These four major rock systems collectively offer a comprehensive understanding of India's geological evolution, covering a vast expanse of time and contributing to the country's rich geological heritage.

Archaean Rock System (Precambrian Rocks)

The Archaean Rock System encompasses rocks formed before the Cambrian system, dating back approximately 4 billion years. This system includes:

Archaean Gneisses and Schists [4 Billion Years]:

Gneiss exhibits varying mineral compositions, ranging from granite to gabbro.

Schists are predominantly crystalline and may contain minerals like mica, talc, hornblende, and chlorite.

• Oldest rocks formed around 4 billion years ago during the pre-Cambrian era.
• Resulted from the solidification of molten magma on a highly heated Earth's surface.
• Referred to as the 'Basement Complex' due to their age, forming the base for subsequent layers.
• Azoic or unfossiliferous, foliated, thoroughly crystalline, and includes plutonic intrusions (volcanic rocks found deep inside).

Dharwar System [4 – 1 Billion Years]:

• Formation period ranges from 4 billion years ago to 1 billion years ago.
• Comprises highly metamorphosed sedimentary rock-system, resulting from the metamorphosis of sediments of Archaean gneisses and schists.
• Found abundantly in the Dharwad district of Karnataka.
• Economically significant for hosting valuable minerals such as high-grade iron-ore, manganese, copper, lead, gold, etc.

Purana Rock System (1400 – 600 Million Years) includes two divisions

Cuddapah System:

• Deposits of unfossiliferous clay, slates, sandstones, and limestones in synclinal basins.
• Prominently observed in the Cuddapah district of Andhra Pradesh.
• Rich in ores of iron, manganese, copper, cobalt, nickel, and large deposits of cement-grade limestones.

Vindhyan System (1300-600 million years):

• Named after the Vindhya mountains, it comprises ancient sedimentary rocks (4000 m thick) superimposed on the Archaean base.
• Mostly unfossiliferous and covers a large area of the Deccan trap.
• Diamond-bearing regions, yielding Panna and Golconda diamonds.
• Devoid of metalliferous minerals but provides durable stones, ornamental stones, limestone, and pure glass-making sand.

Dravidian Rock System (Palaeozoic) formed about 600 – 300 million years ago

Found in the Extra Peninsular region (Himalayas and Ganga plain) and is abundant in fossils. The Carboniferous rocks (350 million years) within the Dravidian system are particularly noteworthy:

Carboniferous Rocks (350 Million Years):

• Mainly composed of limestone, shale, and quartzite.
• Mount Everest is composed of Upper Carboniferous limestones.
• Marks the beginning of coal formation, with most of the coal found in India not belonging to the Carboniferous period.
• The term "Carboniferous" denotes coal-bearing, and high-quality coal from regions like the Great Lakes Region-USA, U.K., and Ruhr region is Carboniferous coal.

Aryan Rock System (Upper Carboniferous to the Recent)

• Gondwana System derived its name from the Gonds, the most primitive people of Telangana and Andhra Pradesh. 
• It includes deposits laid down in synclinal troughs on an ancient plateau surface. Notable for containing nearly 98% of India's coal reserves, Gondwana coal is younger than Carboniferous coal.
•  It also has rich deposits of iron ore, copper, uranium, antimony, and serves as a source of building materials.

The Jurassic System involves marine transgressions, giving rise to shallow water deposits in Rajasthan and Kachchh. 

Deccan Trap, resulting from volcanic outbursts, covers a vast area in Peninsular India, with basaltic lava flows creating distinct geological features. 

It has been divided into three groups - the Upper Trap, Middle Trap, and Lower Trap.

The Tertiary System (Eocene to Pliocene) spans about 60 to 7 million years ago and is crucial in India's geological history, marking the birth of the Himalayas and the formation of India's present shape.

FAQs: Geological Structure, Time Scale, and History of India

1. What does the term "Geological Structure" encompass?

Ans. Geological structure primarily pertains to the arrangement and deposition of rocks within the Earth's crust, influenced by earth movements or their absence. This concept also extends to the morphological features of rocks, exemplified by the Gondwana structure.

2. What is the Geological Time Scale, and who developed it?

Ans. The Geological Time Scale involves the chronological dating of geological formations (geological strata) and life based on their time and place of origin, evolution, and extinction. Giovanni Ardunia first developed the Geological Time Scale in 1760, and the standardised version emerged during the International Geological Congress in 1881, Italy. T.S. Holland advocated the Indian Geological Time Scale.

3. How is the geological history of India analysed, and what are the major events in this history?

Ans. The geological history of India involves the analysis of its geological structure and rock systems in relation to their geographical locations. Major events include Peninsular India's association with the ancient landmass since the Earth's Crust formation, the upheaval of the Himalayas during the tertiary period, and the ongoing aggradational formation of the Indo-Gangetic plain during the Pleistocene period.

4. What are the major rock systems classified by the Geological Survey of India?

Ans. The Geological Survey of India has classified India's rock systems into four major divisions: Archaean Rock System, Purana Rock System, Dravidian Rock System, and Aryan Rock System.

5. Tell us about the Archaean Rock System.

Ans. The Archaean Rock System encompasses rocks formed before the Cambrian system, dating back approximately 4 billion years. It includes Archaean Gneisses and Schists, the oldest rocks formed around 4 billion years ago during the pre-Cambrian era.

6. What characterizes the Dharwar System within the Archaean Rock System?

Ans. The Dharwar System, dating from 4 billion years ago to 1 billion years ago, comprises highly metamorphosed sedimentary rock-system found abundantly in the Dharwad district of Karnataka. It is economically significant for hosting valuable minerals.

7. Provide an overview of the Purana Rock System.

Ans. The Purana Rock System, dating from 1400 to 600 million years ago, includes the Cuddapah System with deposits of unfossiliferous clay, slates, sandstones, and limestones. The Vindhyan System, named after the Vindhya mountains, comprises ancient sedimentary rocks.

8. What is notable about the Carboniferous Rocks within the Dravidian Rock System?

Ans. The Carboniferous Rocks (350 Million Years) in the Dravidian Rock System, mainly composed of limestone, shale, and quartzite, mark the beginning of coal formation. Most of India's coal, however, does not belong to the Carboniferous period.

9. characterises the Aryan Rock System?

Ans. The Aryan Rock System spans from the Upper Carboniferous to the Recent, and it includes the Gondwana System, which contains nearly 98% of India's coal reserves. Gondwana coal is younger than Carboniferous coal and also has rich deposits of iron ore, copper, uranium, and antimony.

10. Tell us about the Deccan Trap and its significance.

Ans. The Deccan Trap resulted from volcanic outbursts covering a vast area in Peninsular India. It has been divided into three groups: Upper Trap, Middle Trap, and Lower Trap. Weathering of these rocks has given rise to black cotton soil known as 'regur.'

11. What is the significance of the Tertiary System in India's geological history?

Ans. The Tertiary System, spanning from Eocene to Pliocene about 60 to 7 million years ago, is crucial in India's geological history as it marks the birth of the Himalayas and the formation of India's present shape.