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

Detail

1. How Indian rocks are grouped by age

• Indian rock systems are groups of rocks that formed in 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.
• The first four are together called the Precambrian or Purana rocks. "Precambrian" means older than about 541 million years.

• The last four are covered in the next section (Rock systems II).

• Each system is linked to its own minerals and landforms. This is why knowing a rock's age helps you predict which ores are found in it.

• Shield: a large, ancient block of hard crystalline rock that has stayed stable for a very long time. The Peninsula is such a shield. The north is young, folded (Himalaya) and covered with alluvium (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 up [7].
• Craton: the old, stable core of a continent. The Peninsula is a mosaic (a patchwork) of several Archaean cratons [9][10]:
• Dharwar (Karnataka and nearby areas)
• Bhandara/Bastar (central India)
• Singhbhum (Jharkhand–Odisha)
• Southern Granulite terrain (Tamil Nadu, Kerala)
• Bundelkhand (Uttar Pradesh–Madhya Pradesh)

• Mewar (Rajasthan)

• Proterozoic: the part of the Precambrian from about 2.5 billion to 542 million years ago. During this time the Cuddapah and Vindhyan rocks were laid down on top of the older basement [7].

System	Age	Main rocks	Metamorphism	Fossils	Key minerals
Archaean	older than about 2.5 billion years	granite, gneiss, schist, charnockite	high	none (azoic)	mica, building stone; basement for later ores
Dharwar	about 2.5–1.8 billion years	schist, quartzite, BIF	very high	none	iron, manganese, gold, copper, uranium
Cuddapah	Proterozoic	sandstone, shale, limestone, quartzite	little	very few	limestone, barytes, asbestos, uranium
Vindhyan	Proterozoic (after Cuddapah)	sandstone, shale, limestone	almost none	rare (stromatolites)	red sandstone, limestone, diamonds

2. Archaean system: India's oldest rocks

• Age: older than about 2.5 billion years.
• How they formed:
• The early Earth was very hot, and its outer layer was molten.
• As this outer layer cooled, the first solid crust formed.

• The result was granites and gneisses, with some schists.

• Gneiss: a metamorphic rock with light and dark bands, formed when heat and pressure squeeze granite-like rock.

• Schist: a metamorphic rock that splits into thin, shiny flakes because its mica crystals are lined up.
• Azoic (no fossils): life had not yet formed hard parts, so these rocks hold no fossils.
• Where found: the Archaean rocks underlie much of the Peninsular Plateau:
• 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

• Dharwar craton: 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 [8][11].

• Greenstone belt: a long strip of old volcanic and sedimentary rocks, turned green by metamorphism. It often holds gold.

• Charnockite: a hard, dark granite-like rock that contains a mineral called hypersthene.

• It is named after Job Charnock, the founder of Kolkata. His tombstone in Kolkata is cut from this rock.
• It forms the hills around Chennai and the Nilgiris.

3. Dharwar system: the first sedimentary rocks

• Age: about 2.5–1.8 billion years.
• How they formed:
• The Archaean rocks were worn down by weathering and erosion.
• The broken pieces settled in seas and basins and became India's first sedimentary rocks.

• Later, heat and pressure changed them strongly, so they are now highly metamorphosed.

• Main rocks: schists, quartzites and BIFs.

• BIF (banded iron formation): a rock made of 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.

• Type area: the system was first studied around Dharwar and Shimoga (Shivamogga) in Karnataka. It is also found in the Aravallis and on the Chota Nagpur Plateau.

• The most important system for the economy. It holds:
• Iron ore: Bababudan and Kudremukh (Karnataka), Bailadila (Chhattisgarh), Noamundi (Jharkhand)
• Manganese, copper and uranium

• Gold at Kolar and Hutti (see below)

• Kolar Gold Fields (KGF), Kolar district, Karnataka: now closed.

• The mines were run by Bharat Gold Mines Limited (BGML). BGML was set up in April 1972 [12].

• Mining stopped making money. All production stopped from October 2000, and the company was formally closed on 1 March 2001 [12].

• Hutti, Raichur district, Karnataka: still working.

• Hutti Gold Mines Limited (HGML) runs mines at Hutti and Hira-Buddini in Raichur district [5].
• The gold lies in the greenstone belts of the Eastern Dharwar Craton: the Hutti, Kushtagi and Deodurg–Raichur schist belts [5][6].

• Gold is also found in the Chitradurga, Dharwad, Gadag, Gulbarga, Hassan, Haveri, Kolar and Tumkur districts of Karnataka (IMY 2012) [5].

• Aravallis:

• They are among the oldest fold mountains in the world, about 2.5 billion years old (NCERT).
• They belong to this Precambrian group, made up of the Aravalli and Delhi supergroups.

• They are rich in minerals. Rajasthan's non-ferrous minerals (copper, lead, zinc) come from these old peninsular rocks.

• Peninsular rocks hold most of India's coal, metallic minerals and mica. The coal comes from the younger Gondwana rocks. The metals and mica come from these Precambrian rocks.

4. Cuddapah system: sediments in a crescent-shaped basin

• Age: Proterozoic, younger than the Dharwar.
• Eparchaean unconformity: the base of the Cuddapah rocks.
• 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 [7].

• Main rocks: sandstone, shale, limestone and quartzite. They show little metamorphism, so many original layers can still be seen.

• Where found:
• the crescent-shaped Cuddapah basin in Andhra Pradesh, named after Cuddapah (now Kadapa)
• the Chhattisgarh basin

• the Kaladgi basin (northern Karnataka)

• 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 [10].

• Minerals: limestone, building stone, barytes, asbestos and uranium.
• Tummalapalle uranium:
• Location: YSR (Kadapa) district, Andhra Pradesh. It lies in the south-western part of the Cuddapah basin [3].
• Operator: Uranium Corporation of India Ltd (UCIL). The mine and processing plant were commissioned on 20 April 2012. This was UCIL's first project outside Jharkhand [3].
• Size: about 60,000 tonnes of uranium (2012). Further exploration showed it could become one of the most significant uranium deposits in the world [3].
• Capacity: designed to mine 3,000 tonnes of ore a day [3].
• Host rock: dolostone of the Vempalle Formation. The uranium occurs off and on along a strike length of about 160 km, from Reddypalle in the north-west to Maddimadugu in the south-east [4].

5. Vindhyan system: India's great sandstone belt

• Age: Proterozoic, after the Cuddapah.
• Main rocks: a very thick pile of sandstones, shales and limestones that are largely unmetamorphosed. They still look much as they did when they were laid down.
• Extent: from Sasaram (Bihar) in the east to Chittorgarh (Rajasthan) in the west. The belt crosses the Son valley and the Malwa plateau.
• Uses:
• Red sandstone is used for building.

• Limestone is used for cement.

• Panna diamond belt (Madhya Pradesh):

• Kimberlite: a volcanic rock that rises from deep in the mantle through narrow, pipe-shaped vents. It carries diamonds up to the surface.
• The diamonds come from the Majhgawan pipe. Its kimberlite nature was recognised in 1930 [13].

• Majhgawan is a fully mechanised mine run by NMDC (National Mineral Development Corporation) [13].

• Fossils are rare. The main traces of early life are stromatolites.

• Stromatolite: a layered mound built up over time by mats of blue-green algae (cyanobacteria) in shallow water. It is one of the oldest signs of life on Earth.

• Stromatolite Park, Bhojunda (Chittorgarh district, Rajasthan): declared a National Geological Monument by the Geological Survey of India (GSI) so that it is protected and maintained [2].

Prelims Hooks

• Oldest to youngest: Archaean → Dharwar → Cuddapah → Vindhyan → Gondwana → Deccan Traps → Tertiary → Quaternary.
• Archaean rocks are azoic. Watch for the trap: Dharwar rocks are sedimentary in origin but are also fossil-free and highly metamorphosed.
• Charnockite is named after Job Charnock. It forms the hills around Chennai and the Nilgiris.
• Dharwar is the most important system for metals. It holds iron at Bababudan, Kudremukh, Bailadila and Noamundi, and gold at Hutti (working) and Kolar (closed in 2001 [12]).
• Hutti and Hira-Buddini gold mines are in Raichur district, Karnataka. HGML runs them [5].
• Tummalapalle (Kadapa, Andhra Pradesh): uranium in the Cuddapah basin. UCIL commissioned it in 2012 [3].
• Vindhyan extent: from Sasaram (Bihar) to Chittorgarh (Rajasthan). It includes the Panna diamond belt (Majhgawan kimberlite pipe, run by NMDC) [13].
• Bhojunda Stromatolite Park (Chittorgarh) is a GSI National Geological Monument [2].
• Aravallis (about 2.5 billion years old) are among the world's oldest fold mountains. They belong to the Precambrian group.
• Least to most metamorphosed: Vindhyan < Cuddapah < Dharwar.

Mains Points

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

• This shapes where industry is located: the steel belt of Chota Nagpur–Odisha–Chhattisgarh.

• Rock type decides the resource:

• The highly metamorphosed Dharwar rocks give metallic ores.

• The barely changed Cuddapah and Vindhyan sediments give non-metallic materials: limestone for cement, building stone, barytes. They also hold uranium (Tummalapalle [3][4]) and diamonds (Panna [13]).

• Resource depletion and revival:

• Kolar closed in 2001 when mining became uneconomical [12]. Deep, low-grade Dharwar gold now costs more to mine.

• New uranium finds in the Cuddapah basin support India's nuclear power plans and its energy security (GS-III).

• Geo-heritage conservation:

• Precambrian sites hold the record of early life and early crust, for example the Bhojunda stromatolites [2].
• These sites need legal protection against quarrying, which is common in the Vindhyan and Aravalli belts.