Rock Systems — Indian Geography

Q.1

WBCS Prelims 2022

Which state is known as the ‘Molassis basin’?

A. Bihar

B. Assam

C. Uttarakhand

D. Himachal Pradesh

Explanation

Why Correct: Assam contains the Molassis basin, a geological formation of soft sedimentary rocks in the Himalayan foothills.
Distractor Analysis: Bihar has the Gangetic alluvial plains. Uttarakhand contains the Siwalik Hills. Himachal Pradesh has the Dhauladhar and Pir Panjal ranges.
Takeaway: The Molassis basin is part of the Assam-Arakan basin, which holds significant hydrocarbon reserves.

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Q.2

WBCS Prelims 2018

In India Karst landforms are formed in

A. Jalpaiguri district, hills of Buxar and Jainti

B. Purnea district of Bihar

C. Jaisalmer region of Rajasthan

D. Mysore hills in Karnataka

Explanation

Why Correct: Karst topography forms in soluble carbonate rocks like limestone or dolomite through dissolution by groundwater. Jalpaiguri district in West Bengal contains the Buxa and Jainti hills with extensive limestone deposits that develop karst features such as sinkholes, caves, and underground drainage.
Distractor Analysis: Purnea district in Bihar lies in the Gangetic alluvial plain with no significant limestone formations. Jaisalmer region in Rajasthan has arid desert conditions with sandstone, which is non-soluble and does not form karst. Mysore hills in Karnataka consist mainly of granite and metamorphic rocks that are non-soluble and resistant to karstification.
Takeaway: Other prominent karst regions in India include the limestone areas of Meghalaya (Cherrapunji-Mawsynram belt), parts of Madhya Pradesh (Panna), and the Eastern Ghats in Andhra Pradesh.

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Q.3

WBCS Prelims 2014

Which one of the following pairs is not correct matched ?

A. Himalayas → Tertiary Fold Mountains

B. Deccan Trap → Volcanic Cone Eruption

C. Western Ghats → Palaeozoic Fold Mountains

D. Aravallis → Pre Cambrian Era Mountains

Explanation

Why Correct: Western Ghats are not Palaeozoic fold mountains; they formed as a faulted escarpment during the Cretaceous period through continental rifting, not Palaeozoic orogenic folding.
Distractor Analysis: Himalayas are Tertiary fold mountains created by the India-Eurasia collision. Deccan Traps are flood basalts from massive volcanic eruptions, not cone eruptions. Aravallis are Precambrian fold mountains, among Earth's oldest.
Takeaway: The Western Ghats are a UNESCO World Heritage site recognized for their exceptional biodiversity and geological history of continental drift.

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Q.4

WBCS Prelims 2013

Granites and gneisses of Dharwarian age are found in :

A. Siwalik Hills

B. Deccan Plateau

C. Chotonagpur Plateau

D. Kirthar Range

Explanation

Why Correct: The Chotanagpur Plateau contains the Dharwar Craton, which consists of ancient granites and gneisses dating back 2.5–1.8 billion years, making it one of India's oldest geological formations.
Distractor Analysis: The Siwalik Hills comprise young tertiary sedimentary rocks formed 16–2 million years ago. The Deccan Plateau consists of basaltic lava flows from the Cretaceous-Paleogene boundary around 66 million years ago. The Kirthar Range in Pakistan contains sedimentary rocks from the Eocene epoch about 56–34 million years old.
Takeaway: The Dharwar Craton extends across Karnataka and parts of eastern India, with the Singhbhum region of Jharkhand containing some of the world's oldest rocks at over 3.6 billion years.

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Q.5

WBCS Prelims 2013

Gondwana beds are known for their :

A. Iron deposits

B. Coal deposits

C. Bauxite

D. Petroleum

Explanation

Why Correct: Gondwana beds, dating to the Permian period around 250 million years ago, contain India's major coal reserves, including the Damodar, Son, and Mahanadi valley coalfields.
Distractor Analysis: Iron deposits are primarily associated with Precambrian rocks like the Dharwar and Singhbhum systems. Bauxite, an aluminum ore, forms from lateritic weathering in tropical regions, not Gondwana strata. Petroleum originates from marine sedimentary basins like Mumbai High, Cambay, and Assam.
Takeaway: Gondwana coal is bituminous and non-coking, making it suitable for thermal power generation but requiring blending for metallurgical use.

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Q.6

WBCS Prelims 2011

Which of the following has the oldest rock in India ?

A. Himalayas

B. Indo-Gangetic Plain

C. The Aravallis

D. Siwaliks

Explanation

Why Correct: The Aravalli Range contains rocks dating back over 2.5 billion years to the Precambrian era, making them the oldest geological formations in India.
Distractor Analysis: The Himalayas are relatively young fold mountains formed around 50 million years ago. The Indo-Gangetic Plain consists of recent alluvial deposits less than 2 million years old. The Siwaliks are the youngest outer foothills of the Himalayas, formed in the last 5-10 million years.
Takeaway: Guru Shikhar (1,722 m) is the highest peak of the Aravalli Range, located in Rajasthan, and serves as a key landmark for this ancient mountain system.

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Q.7

WBCS Prelim 2007

Charnockite is

A. Nilgiri gneiss

B. A Gemstone

C. Found in Jharkhand

D. A glacier

Explanation

Why Correct: Charnockite is a type of granitic rock named after Job Charnock, the founder of Kolkata, and is specifically associated with the Nilgiri Hills where it forms part of the Precambrian basement.
Distractor Analysis: Charnockite is not a gemstone but a coarse-grained igneous rock. While charnockite occurs in several Indian states including Jharkhand, its defining geological identity is as Nilgiri gneiss. A glacier is a moving mass of ice, completely unrelated to this rock type.
Takeaway: Charnockite is characterized by the presence of orthopyroxene minerals and is part of the granulite facies of metamorphic rocks, often studied in Indian Precambrian geology.

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Q.8

WBCS Prelims 2005

Tipam sandstones are found in

A. Assam

B. Jharkhand

C. Rajasthan

D. West Bengal

Explanation

Why Correct: Tipam sandstones are a geological formation belonging to the Miocene age, primarily exposed in the northeastern states, especially Assam and parts of Arunachal Pradesh.
Distractor Analysis: Jharkhand contains Gondwana and Vindhyan sedimentary rocks. Rajasthan features the Aravalli ranges and desert sandstones. West Bengal has Tertiary sediments in the Bengal Basin but not the specific Tipam formation.
Takeaway: The Tipam Group includes the Tipam Sandstone Formation and the overlying Girujan Clay Formation, both part of the Surma Basin's stratigraphy in Northeast India.

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Q.9

WBCS Prelims 2005

The Deccan Lava region was formed during

A. Cretaceous period

B. Jurassic period

C. Triassic period

D. Cambrian period

Explanation

Why Correct: The Deccan Traps formed during the Late Cretaceous period (66–100 million years ago) through massive volcanic eruptions that covered 500,000 sq km of western India.
Distractor Analysis: Jurassic period (201–145 million years ago) saw the breakup of Pangaea and dinosaur diversification. Triassic period (252–201 million years ago) marks the beginning of the Mesozoic era and recovery from the Permian-Triassic extinction. Cambrian period (541–485 million years ago) is the first period of the Paleozoic era, known for the Cambrian explosion of marine life.
Takeaway: The Deccan Traps eruptions coincided with the Cretaceous-Paleogene extinction event that eliminated non-avian dinosaurs, with volcanic activity potentially contributing to the mass extinction.

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Q.10

WBCS Prelims 2000

Most fossils are found in

A. Granite

B. Sedimentary rocks

C. Black soil

D. Lave flow

Explanation

Why Correct: Sedimentary rocks form through gradual deposition and compaction of sediments, which preserves organic remains under low-temperature, low-pressure conditions ideal for fossilization.
Distractor Analysis: Granite is an igneous rock formed from cooled magma at high temperatures that destroys organic material. Black soil (regur) develops from volcanic basalt in tropical climates and lacks the depositional environment for fossil preservation. Lava flow refers to molten rock that solidifies into igneous rock, incinerating any organic matter it encounters.
Takeaway: The three main rock types are igneous (formed from magma), sedimentary (formed from sediments), and metamorphic (formed from existing rocks under heat/pressure).

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