Deccan Traps

Geography glossary

Topic: Minerals, Rocks and the Rock Cycle · NCERT: Class 10, Ch 1 "Resources and Development"; Class 11, Ch 3 "Interior of the Earth"; Class 11, Ch 4 "Distribution of Oceans and Continents"

Meaning

The Deccan Traps are a huge flood basalt province (a region where very runny basalt lava poured out again and again from long cracks and spread in flat sheets over a vast area). They cover most of the Maharashtra plateau and parts of Gujarat, Madhya Pradesh, Karnataka and Telangana. They formed about 66 million years ago (66–60 million years), when the Indian plate passed over the Reunion hotspot.

They matter because they made India's black (regur) soil and the cotton belt. They also show how India drifted north across the Indian Ocean, and they formed at the same time as the mass extinction that wiped out the dinosaurs.

Explanation

How it formed (cause → process → result)

  • Cause: the Indian plate passed over the Reunion hotspot
  • A hotspot is a fixed spot where a plume of very hot mantle rock rises and melts the crust above it.

  • As the Indian plate moved north, it passed over this hotspot.

  • Process: magma rose through cracks

  • Molten rock pushed up through long cracks in the crust.
  • Magma filled these cracks and hardened into dykes. A dyke is a wall-like sheet of igneous rock that cuts across the rock layers.

  • The dykes worked as feeders. They carried lava up to the surface.

  • Result: fissure eruptions built a plateau

  • Runny basalt lava came out in fissure eruptions (lava flowing quietly out of long cracks, not from a cone-shaped volcano).
  • Each flow spread out flat and cooled. Over time, layer piled on layer.
  • This built a thick, flat-topped lava plateau.

Why it is called "trap"

  • Trap comes from the Swedish word trappa, meaning "stairs".
  • Each lava flow cooled into a flat layer. Some layers are soft and some are hard.
  • Rain and rivers wear away the soft layers faster than the hard ones.
  • Result: hill slopes look like step-like terraces. You can see them clearly in the Western Ghats of Maharashtra (the Sahyadri).

Age and place in India's rock systems

  • Age: about 66 million years (66–60 million years). This is the end of the Cretaceous period and the start of the Palaeogene.
  • Order from oldest to youngest: Gondwana → Deccan Traps → Tertiary → Quaternary.
  • Rock type: basalt, a dark, hard igneous rock (rock made when lava or magma cools).
  • Link to extinction: the eruptions happened at the same time as the end-Cretaceous mass extinction, which killed off the dinosaurs. Scientists discuss them as one possible cause.

What it gives us (resources)

  • Black soil (regur / black cotton soil)
  • Basalt weathers (breaks down slowly through rain and heat) into clay-rich black soil.
  • It holds moisture well.
  • It is rich in iron, lime and magnesia but poor in phosphorus.
  • It swells when wet and cracks when dry, so it turns itself over. That is why it is called "self-ploughing".

  • It is best for cotton.

  • Building stone: basalt is hard and is widely quarried for buildings and roads.

  • Few metallic minerals
  • Basalt is young volcanic rock. It does not have the old ore bodies found in Archaean and Dharwar rocks.
  • Bauxite caps form on some weathered basalt plateaus in hot, wet climates.

Where it occurs

  • India:
  • Maharashtra plateau. This is the core area, covering most of the state.
  • Gujarat: Saurashtra and Kachchh.
  • Madhya Pradesh: the Malwa plateau.
  • Karnataka and Telangana (parts).

  • Western Ghats of Maharashtra (Sahyadri): the clearest step-like trap scenery.

  • Cotton belt on Deccan soil: Maharashtra, Gujarat and Madhya Pradesh.

  • Drought-prone basalt regions: Marathwada and Vidarbha (Maharashtra).
  • The hotspot trail in the Indian Ocean:
  • India kept moving north while the hotspot stayed in one place.
  • The hotspot left a line of volcanic rock: Lakshadweep → Maldives → Chagos → Reunion Island.
  • A volcano on Reunion Island is still active today.

Don't confuse with

  • Gondwana system: these are older sedimentary rocks (about 300–150 million years old) laid down in rift basins, and they hold about 98% of India's coal. The Deccan Traps are younger volcanic basalt and hold almost no coal.
  • Central (cone) volcano: this builds a cone-shaped mountain from one vent, often with explosive eruptions. The Deccan Traps came from quiet fissure eruptions out of long cracks, which built a flat plateau.
  • Archaean and Dharwar rocks: these are very old rocks rich in metallic ores. Deccan basalt is young and poor in metals, apart from some bauxite caps.
  • Alluvial soil (Quaternary): this is laid down by rivers on the northern plains (bhangar and khadar). Regur forms where it stands, from weathered basalt.

Prelims Hooks

  • Trap comes from the Swedish word trappa ("stairs"). It refers to the step-like terraces of the Sahyadri in Maharashtra.
  • The Deccan Traps are flood basalt. The lava came through dykes in fissure eruptions, over the Reunion hotspot, about 66 million years ago.
  • Hotspot trail: Lakshadweep → Maldives → Chagos → Reunion Island, which still has an active volcano.
  • Trap question: the Deccan Traps are not Gondwana rocks. Gondwana means coal. Deccan means basalt and black soil.
  • Regur soil is rich in iron, lime and magnesia, poor in phosphorus, "self-ploughing", and best for cotton.
  • Extent: Maharashtra plateau, Saurashtra and Kachchh (Gujarat), Malwa plateau (MP), and parts of Karnataka and Telangana.

Mains Points

  • Deccan Traps and farming (GS-I, GS-III):
  • Basalt weathers into moisture-holding regur soil. This soil supports the cotton belt of Maharashtra, Gujarat and MP.
  • But hard basalt stores little groundwater, and much of the plateau lies in the rain-shadow (the dry side of the Western Ghats, away from the moist wind).

  • So Marathwada and Vidarbha often face drought and farm distress. Water conservation and careful irrigation planning are key responses.

  • Rock type decides resources:

  • Deccan basalt gives soil and building stone, but almost no metals or coal.

  • So industries based on minerals are concentrated in the older Peninsular rocks (Archaean, Dharwar and Gondwana) of the east, not on the Deccan lava plateau.

  • A record of plate tectonics:

  • The Deccan Traps and the Reunion hotspot trail show that India moved north over a fixed hotspot.
  • Together with the Talchir tillite and Glossopteris (from Gondwanaland) and the Indus–Tsangpo ophiolites (from the collision with Eurasia), they trace India's journey. This makes them useful in answers on continental drift and how India's landforms evolved.

Related concepts

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Sources

  1. 1Class 10, Ch 1 "Resources and Development"; Class 11, Ch 3 "Interior of the Earth"; Class 11, Ch 4 "Distribution of Oceans and Continents" (primary)