Indian rock systems

Also called: Geological formations of India · Topic: Minerals, Rocks and the Rock Cycle · NCERT: Beyond NCERT

Meaning

Indian rock systems are groups of rocks that formed during the same long period of Earth's history. Geologists list them by age, from oldest to youngest: Archaean → Dharwar → Cuddapah → Vindhyan → Gondwana → Deccan Traps → Tertiary → Quaternary.

They matter because each system has its own minerals and landforms. If you know how old a rock is, you can predict which ores may be found in it. This explains why India's metals, coal and building stone are found where they are.

Explanation

How the systems are grouped

• Order by age: Archaean → Dharwar → Cuddapah → Vindhyan → Gondwana → Deccan Traps → Tertiary → Quaternary.
• Precambrian or Purana rocks: the first four systems (Archaean, Dharwar, Cuddapah and Vindhyan).
• "Precambrian" means older than about 541 million years.

• Together they make up the Peninsular shield. A shield is a large, ancient block of hard crystalline rock that has stayed stable for a very long time.

• Younger systems: Gondwana, Deccan Traps, Tertiary and Quaternary.

• The Gondwana rocks hold most of India's coal.

• The north of India is young. It has folded mountains (the Himalaya) and is covered with alluvium (the Indo-Gangetic plains).

• The Indian Platform is a Precambrian continental shield. It is one of the four shields around which the Asian continent was built [6].

• Craton means the old, stable core of a continent. The Peninsula is a patchwork of several Archaean cratons: Dharwar, Bhandara/Bastar, Singhbhum, Southern Granulite, Bundelkhand and Mewar [8][9].
• Proterozoic is the part of the Precambrian from about 2.5 billion to 542 million years ago. The Cuddapah and Vindhyan rocks were laid down on the older basement during this time [6].

The four Precambrian systems: how each formed

• Archaean (older than about 2.5 billion years)
• The early Earth was very hot, and its outer layer was molten.
• As the outer layer cooled, the first solid crust formed. It was made of granite and gneiss (a banded metamorphic rock made when heat and pressure squeeze granite-like rock), with some schist (a metamorphic rock that splits into thin, shiny flakes).
• These rocks are azoic, which means they have no fossils. Life had not yet formed hard parts.

• Charnockite is a hard, dark granite-like rock that contains hypersthene. It is named after Job Charnock, the founder of Kolkata.

• Dharwar (about 2.5–1.8 billion years)

• Weathering and erosion wore down the Archaean rocks.
• The broken pieces settled in seas and basins. They became India's first sedimentary rocks.
• Later, heat and pressure changed them strongly, so they are now highly metamorphosed and have no fossils.

• The main rocks are schist, quartzite and BIF.

  • BIF (banded iron formation) has thin, alternating layers of iron oxide and silica.
  • It formed when iron dissolved in the early oceans reacted with oxygen.
  • BIFs are the main source of high-grade iron ore.

• Cuddapah (Proterozoic, younger than Dharwar)

• Its base is the Eparchaean unconformity. An unconformity is a surface where a long gap in time separates older, eroded rocks below from younger layers above.
• In India, "Archaean" means the rocks that lie below this unconformity [6].

• The rocks are sandstone, shale, limestone and quartzite. They show little metamorphism, so many of the original layers can still be seen.

• Vindhyan (Proterozoic, after Cuddapah)

• It is a very thick pile of sandstone, shale and limestone that is largely unmetamorphosed. The rocks still look much as they did when they were laid down.

• Fossils are rare. The main traces of early life are stromatolites: layered mounds built up by mats of blue-green algae (cyanobacteria) in shallow water.

• The Cuddapah and Vindhyan basins are less deformed basins inside the old cratons. They formed in the Mesoproterozoic (the middle part of the Proterozoic), along the edges of ancient plates [9].

Why rock type decides the mineral

System	Main rocks	Metamorphism	Fossils	Key minerals
Archaean	granite, gneiss, schist, charnockite	high	none (azoic)	mica, building stone; the basement for later ores
Dharwar	schist, quartzite, BIF	very high	none	iron, manganese, gold, copper, uranium
Cuddapah	sandstone, shale, limestone, quartzite	little	very few	limestone, barytes, asbestos, uranium
Vindhyan	sandstone, shale, limestone	almost none	rare (stromatolites)	red sandstone, limestone, diamonds

• Strong heat and pressure over a long time gives metal ores.
• The Dharwar rocks were squeezed and heated for billions of years.

• This concentrated metals into ore bodies.

• Sediments that changed very little give non-metallic materials.

• The Cuddapah and Vindhyan rocks give cement limestone, building stone and barytes.
• They also hold uranium and diamonds.

Where it occurs

• Archaean:
• Karnataka, Tamil Nadu and Andhra Pradesh
• Odisha and the Chota Nagpur Plateau (Jharkhand)
• Bundelkhand, where the rock is called the Bundelkhand gneiss
• parts of Rajasthan
• Charnockite forms the hills around Chennai and the Nilgiris.

• The Dharwar craton is one of the oldest and largest Archaean cratons in the world. It is a granite–greenstone terrain with a history going back about 3.4–3.5 billion years [7][10].

• Dharwar:

• Type area: Dharwar and Shimoga (Shivamogga) in Karnataka. The system is also found in the Aravallis and on the Chota Nagpur Plateau.
• Iron ore: Bababudan and Kudremukh (Karnataka), Bailadila (Chhattisgarh), Noamundi (Jharkhand).
• Hutti and Hira-Buddini gold mines (Raichur district, Karnataka): still working and run by HGML [4]. The gold lies in the Hutti, Kushtagi and Deodurg–Raichur schist belts of the Eastern Dharwar Craton [4][5].
• Kolar Gold Fields (Kolar district, Karnataka): run by BGML. All production stopped from October 2000, and the company was formally closed on 1 March 2001 [11].

• Aravallis (Rajasthan): among the oldest fold mountains in the world, about 2.5 billion years old. They are made up of the Aravalli and Delhi supergroups. They are the source of Rajasthan's copper, lead and zinc.

• Cuddapah:

• the crescent-shaped Cuddapah basin in Andhra Pradesh (named after Cuddapah, now Kadapa)
• the Chhattisgarh basin
• the Kaladgi basin (northern Karnataka)

• Tummalapalle uranium, YSR (Kadapa) district, Andhra Pradesh:

  • It lies in the south-western part of the Cuddapah basin. UCIL commissioned the mine and processing plant on 20 April 2012. It was UCIL's first project outside Jharkhand [2].
  • It holds about 60,000 tonnes of uranium and is designed to mine 3,000 tonnes of ore a day [2].
  • The uranium sits in dolostone of the Vempalle Formation. It occurs along a strike length of about 160 km, from Reddypalle to Maddimadugu [3].

• Vindhyan:

• It stretches from Sasaram (Bihar) to Chittorgarh (Rajasthan) and crosses the Son valley and the Malwa plateau.
• Panna diamond belt (Madhya Pradesh): the diamonds come from the Majhgawan kimberlite pipe. Kimberlite is a volcanic rock that rises from the mantle through narrow, pipe-shaped vents and carries diamonds to the surface.
  • The pipe's kimberlite nature was recognised in 1930.
  • NMDC runs Majhgawan as a fully mechanised mine [12].
• Stromatolite Park, Bhojunda (Chittorgarh district): the GSI has declared it a National Geological Monument [1].

Don't confuse with

• Archaean vs Dharwar: both are fossil-free and metamorphosed. The difference is origin. Archaean rocks are the first crust, formed when molten rock cooled. Dharwar rocks began as sediments worn from the Archaean rocks and were later metamorphosed.
• Dharwar vs Cuddapah/Vindhyan: Dharwar rocks are very highly metamorphosed and hold metal ores. Cuddapah rocks show little metamorphism and Vindhyan rocks almost none. Both give limestone, building stone, uranium (Cuddapah) and diamonds (Vindhyan).
• Precambrian systems vs Gondwana: Precambrian rocks give metals and mica. The younger Gondwana rocks give most of India's coal.
• Peninsular shield vs northern India: the Peninsula is an old, stable crystalline shield. The north is young: folded mountains (Himalaya) and alluvial plains.

Prelims Hooks

• Oldest to youngest: Archaean → Dharwar → Cuddapah → Vindhyan → Gondwana → Deccan Traps → Tertiary → Quaternary.
• Least to most metamorphosed: Vindhyan < Cuddapah < Dharwar. Archaean rocks are azoic, and Dharwar rocks are also fossil-free.
• Charnockite is named after Job Charnock. It forms the hills around Chennai and the Nilgiris.
• Gold: Hutti and Hira-Buddini (Raichur, Karnataka) are working mines run by HGML [4]. Kolar was closed in 2001 [11].
• Tummalapalle (Kadapa, Andhra Pradesh): uranium in the Cuddapah basin. UCIL commissioned it in 2012 [2].
• Vindhyan: stretches from Sasaram to Chittorgarh. It includes the Panna diamonds (Majhgawan pipe, NMDC) [12] and the Bhojunda Stromatolite Park, a GSI National Geological Monument [1].

Mains Points

• Why the Peninsula is rich in minerals (GS-I/III):
• The old, stable Archaean–Dharwar shield was heated, squeezed and pushed together for billions of years. This concentrated metals into ore bodies.
• As a result, iron, manganese, gold, copper, uranium and mica are clustered in Karnataka, Odisha, Jharkhand, Chhattisgarh and Rajasthan.

• This decides where industry is located, as in the Chota Nagpur–Odisha–Chhattisgarh steel belt.

• Resource depletion and new finds (GS-III):

• Kolar closed in 2001 when mining stopped making money [11]. Deep, low-grade Dharwar gold now costs more to mine.

• New uranium finds in the Cuddapah basin, such as Tummalapalle [2][3], support India's nuclear power plans and energy security.

• Geo-heritage conservation:

• Precambrian sites preserve the record of early life and early crust, for example the Bhojunda stromatolites [1].
• Quarrying is common in the Vindhyan and Aravalli belts, so these sites need legal protection.

Related concepts

• Archaean system
• Dharwar system
• Cuddapah system
• Vindhyan system

Read more

• India's rock systems I: Archaean to Vindhyan (the Precambrian shield)