4 Geography · 4.2.1 Location, extent and geological structure
4.2.1.3 Geological structure of India: Archaean to Quaternary formations
India's rocks span more than 2.5 billion years, from the ancient Archaean core of the Peninsula to the young alluvium of the Quaternary plains, and are grouped into four great systems.
- APPSC Group 1
- Advanced level
- 7 min read
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Core facts
- India has three geological divisions:
- the Peninsular Block
- the Himalaya and other peninsular mountains
- the Indo-Ganga-Brahmaputra Plain
- Four major rock groups, oldest to youngest:
| Group | Age | Key systems |
|---|---|---|
| Archaean | Oldest (before about 2.5 billion years) | Gneisses and schists, Dharwar |
| Purana | Precambrian | Cuddapah, Vindhyan |
| Dravidian | Palaeozoic (Cambrian to Carboniferous) | Marine rocks, mostly in the Himalaya |
| Aryan | Upper Carboniferous to present | Gondwana, Deccan Trap, Tertiary, Quaternary |
- Archaean rocks are the oldest and cover large parts of the Peninsula. They include granite and gneiss. They are rich in minerals but have no fossils.
- Dharwar system (named after Dharwad in Karnataka) holds gold, iron ore, manganese and copper. It is the oldest sedimentary-metamorphic series with minerals.
- Cuddapah system is in Andhra Pradesh (the Cuddapah/Kadapa basin).
- Vindhyan system lies in Rajasthan, Madhya Pradesh and Bihar; it gives sandstone and limestone.
- Gondwana system holds most of India's coal.
- Deccan Trap is basalt from lava flows (fissure eruptions) at the end of the Cretaceous.
- Quaternary deposits form the northern plains' alluvium.
Explained
Peninsular Block
- The oldest and most stable part, made mainly of igneous and metamorphic rocks. It was part of Gondwanaland, which broke apart. The Indian plate then drifted north.
- Its northern boundary runs from Kachchh along the western flank of the Aravali near Delhi, then roughly parallel to the Yamuna and Ganga up to the Rajmahal hills.
- Faults matter. The Malda fault separates the north-eastern parts (Meghalaya and Karbi Anglong plateaus) from the Chotanagpur plateau. The Bhima fault is noted for repeated seismic activity.
The four groups in more detail
- Archaean: includes Bengal, Bundelkhand and Nilgiri gneisses. The Dharwar rocks are metamorphosed sediments laid on the gneiss. The Aravali range, among the oldest fold mountains of the world, is worn down from very ancient folding.
- Purana: Cuddapah and Vindhyan basins are mostly unmetamorphosed sediments (quartzite, slate, limestone, sandstone). The Vindhyan beds are a major source of building stone.
- Dravidian: marine Palaeozoic rocks, found mainly in the Himalayas, such as in Spiti and Kashmir.
- Aryan group:
- Gondwana system (Upper Carboniferous to Jurassic): river and lake deposits in basins along the Damodar, Mahanadi, Godavari and Wardha valleys. Holds most of India's coal, and also fossil plants.
- Deccan Trap (Late Cretaceous to Eocene): basalt covering the north-western Deccan; weathers into black (regur) soil.
- Tertiary system: associated with the upheaval of the Himalaya; includes the Shiwaliks. Oil in Assam and Gujarat is found in Tertiary rocks.
- Quaternary (Pleistocene and Holocene): the alluvium of the plains, divided into older Bhangar and newer Khadar.
Special rock names
- Charnockite is named after Job Charnock, the founder of Calcutta, whose tombstone is made of it.
- Khondalite is named after the Khond tribe of Odisha.
Deeper
- Three-block structure. The Peninsular Block is the stable shield. The Himalaya is the zone of young folding. The Indo-Ganga-Brahmaputra Plain was originally a geosynclinal depression between the two, filled by sediments from Himalayan and peninsular rivers. Alluvium is 1,000 to 2,000 m deep (NCERT).
- Plate tectonic story. About 140 to 150 million years ago India began separating from Gondwanaland and drifted north. Its passage over a hot spot is linked to the Deccan eruptions (about 66 million years ago). The collision with Eurasia, about 50 to 55 million years ago, raised the Himalaya.
- Mineral-geology link. Archaean and Dharwar belts hold metallic minerals (iron, manganese, gold, copper). Gondwana basins hold coal. Vindhyan and Cuddapah beds hold limestone, diamond-bearing zones and building stone. Tertiary basins hold petroleum. So the geology of a region predicts its mineral wealth.
- Relict mountains. Aravali, Nallamala, Javadi, Veliconda, Palkonda and Mahendragiri are old, worn-down (relict or residual) mountains of the Peninsula, unlike the young Himalayan fold mountains.
AP angle
- The Cuddapah basin in Andhra Pradesh (Kadapa and Kurnool region) gives its name to the Cuddapah system, a crescent-shaped basin of Precambrian sediments. It includes the Nallamala hills.
- Rocks of the Eastern Ghats around Visakhapatnam are charnockite and khondalite.
- The Cuddapah rocks are linked with minerals such as barytes (Mangampeta, Kadapa), limestone and asbestos (Pulivendula), and the diamond-bearing Banaganapalle conglomerates of the Kurnool group. The diamond-bearing kimberlite pipes of Wajrakarur (Anantapur) lie west of the basin, in older rocks.
- Gondwana rocks lie along the Godavari valley.
Common traps
- ✗ "The coastal plains are one of the three geological divisions." ✓ The three are the Peninsular Block, the Himalaya with other peninsular mountains, and the Indo-Ganga-Brahmaputra Plain. Coastal plains are a physiographic division.
- ✗ "Gondwana rocks are the oldest." ✓ Archaean is the oldest; Gondwana is much younger (Upper Carboniferous to Jurassic).
- ✗ "The Deccan Trap is sedimentary." ✓ It is igneous (basalt).
- ✗ "Cuddapah system is in Karnataka." ✓ It is in Andhra Pradesh; Dharwar is the Karnataka one.
- ✗ "Charnockite is named after a tribe." ✓ It is named after Job Charnock; khondalite is the tribal name.
Quick revision
- Three divisions: Peninsular Block, Himalaya and peninsular mountains, Indo-Ganga-Brahmaputra Plain.
- Order of age: Archaean, Purana, Dravidian, Aryan.
- Dharwar: gold, iron, manganese.
- Cuddapah basin: Andhra Pradesh.
- Gondwana: coal.
- Deccan Trap: basalt; black soil.
- Tertiary: Himalayan uplift, oil.
- Quaternary: alluvium (Bhangar, Khadar).
Go deeper: Mains writing (beyond what APPSC Group 1 usually asks)
Mains writing Mains writing
- Frame: geology is the base of physical geography. Structure decides relief, drainage, soils, minerals and hazards.
- Analysis points:
- A stable shield means few earthquakes in the Peninsula, but reservoir-induced and fault-related tremors do occur (Koyna, Latur 1993).
- The young Himalaya is tectonically active, so earthquake and landslide risk are high.
- Mineral belts follow geological belts, so industrial regions grew where geology gave coal and iron together (Chotanagpur).
- Way forward: use geological mapping for hazard zoning, mineral exploration and groundwater planning.
- Example: the Deccan lava gave black soil that suits cotton, shaping Maharashtra's agriculture.
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