Archaean system

Also called: Archaean rocks, Archaean gneisses and granites, Gneiss and granite of the Peninsula · Topic: Minerals, Rocks and the Rock Cycle · NCERT: Class 11, Ch 2 "Structure and Physiography"

Meaning

The Archaean system is India's oldest group of rocks, more than about 2.5 billion years old. These rocks are mainly granites and gneisses, with some schists and charnockite, and they formed when the Earth's first crust cooled and hardened. They matter because they form the basement of the Peninsular Plateau, the old and stable core of India. All the later rock systems, and many of India's ores, sit on top of them.

Explanation

How the Archaean rocks formed (cause → process → result)

• Cause: the early Earth was very hot.

• Its outer layer was molten (melted rock).

• Process: the outer layer slowly cooled.

• As it cooled, the first solid crust formed.

• Over time, heat and pressure squeezed and changed much of this rock.

• Result: granites, gneisses and some schists.

• Gneiss: a metamorphic rock with light and dark bands. It forms 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.
• Metamorphism (a rock being changed by heat and pressure without melting) is high in these rocks.

Main features

• Age: older than about 2.5 billion years. This makes them the oldest system in India.
• Azoic (no fossils): life had not yet grown hard parts like shells or bones, so these rocks hold no fossils.
• Where they sit in the order of Indian rock systems (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.

• How Indian geologists draw the line: "Archaean" in India means the rocks that lie below the Eparchaean unconformity [2].

• An unconformity is a surface where a long gap in time separates older, worn-down rocks below from younger layers above.

• This unconformity marks the base of the Cuddapah rocks.

• Minerals: mica and building stone. Most of all, the Archaean rocks are the basement (the bottom layer) on which later ore-bearing rocks were laid down.

Shield, cratons and special rock types

• 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 Indian Platform is a Precambrian continental shield. It is one of the four shields around which the Asian continent was built up [2].

• Craton: the old, stable core of a continent. The Peninsula is a mosaic (a patchwork) of several Archaean cratons [4][5]:

• 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)

• 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 [3][6].

• Greenstone belt: a long strip of old volcanic and sedimentary rocks that metamorphism has turned green. 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.

Where it occurs

• Peninsular Plateau: the Archaean rocks lie under much of it.
• Karnataka, Tamil Nadu and Andhra Pradesh
• Odisha and the Chota Nagpur Plateau (Jharkhand)
• Bundelkhand (Uttar Pradesh–Madhya Pradesh), where the rock is called the Bundelkhand gneiss

• parts of Rajasthan (Mewar craton)

• Charnockite landforms: the hills around Chennai and the Nilgiris.

• Dharwar craton (Karnataka): a granite–greenstone terrain about 3.4–3.5 billion years old [3][6].
• Other Archaean cratons: Bhandara/Bastar (central India), Singhbhum (Jharkhand–Odisha), Southern Granulite terrain (Tamil Nadu, Kerala), Bundelkhand and Mewar [4][5].
• World context: the Indian Platform is one of the four Precambrian shields around which Asia was built up [2].
• Where they are absent: the young, folded Himalaya and the alluvium-covered Indo-Gangetic plains in the north.

Don't confuse with

• Dharwar system: Archaean rocks formed from cooling crust (igneous origin, later metamorphosed). Dharwar rocks are India's first sedimentary rocks, made from worn-down Archaean material. Both are fossil-free and highly metamorphosed. The Dharwar holds the main metallic ores (iron, manganese, gold).
• Precambrian / Purana group: this is the wider group (Archaean + Dharwar + Cuddapah + Vindhyan). The Archaean is only its oldest part, not the whole group.
• Cuddapah system: it lies above the Eparchaean unconformity and shows little metamorphism. The Archaean lies below it and is highly metamorphosed [2].
• Charnockite vs ordinary granite: charnockite is a dark granite-like rock marked by hypersthene. It is a special Archaean rock of the Chennai hills and the Nilgiris, not the common granite of the plateau.

Prelims Hooks

• Oldest rock system of India: Archaean (older than about 2.5 billion years). Order: Archaean → Dharwar → Cuddapah → Vindhyan.
• Archaean rocks are azoic. Trap: Dharwar rocks are also fossil-free, but they are sedimentary in origin.
• Charnockite: named after Job Charnock. It forms the hills around Chennai and the Nilgiris, and it contains hypersthene.
• Bundelkhand gneiss is an Archaean rock of the Uttar Pradesh–Madhya Pradesh region.
• In India, "Archaean" means the rocks below the Eparchaean unconformity, which is the base of the Cuddapah [2].
• Dharwar craton: a granite–greenstone terrain about 3.4–3.5 billion years old, one of the oldest and largest in the world [3][6].

Mains Points

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

• It shapes where industry is located, for example the steel belt of Chota Nagpur–Odisha–Chhattisgarh.

• Physiography and resources (GS-I):

• The hard Archaean granites and gneisses form the stable, worn-down Peninsular Plateau and its hills (Chennai hills, Nilgiris).

• They also give mica and building stone, and they make up the basement of the Peninsula, in contrast to the young folded Himalaya and the alluvial plains.

• Geo-heritage value:

• The Archaean cratons, such as the Dharwar craton going back about 3.4–3.5 billion years [3][6], record the Earth's earliest crust.
• Such Precambrian sites need protection from heavy quarrying so that this record is kept for science and education.

Related concepts

• Indian rock systems
• Dharwar system
• Cuddapah system
• Vindhyan system

Read more

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