Tertiary system

Geography glossary

Topic: Minerals, Rocks and the Rock Cycle · NCERT: Beyond NCERT

Meaning

The Tertiary system is the group of rocks in India formed about 66 to 2.6 million years ago, when the Indian plate hit the Eurasian plate and the Himalaya rose. These rocks are mainly young sediments, such as the Siwalik beds and the rocks of the coastal and north-east basins.

They matter because almost all of India's petroleum, its lignite and the younger, high-sulphur coal of the north-east come from these rocks.

Explanation

How it formed (cause → process → result)

  • Cause: two plates collided
  • The Indian plate moved north and hit the Eurasian plate.

  • The ocean between them closed, and the land was pushed up into the Himalaya.

  • Process: rivers and seas laid down sediment

  • Rivers washed sand, clay and boulders down from the rising mountains.
  • This material collected at the foot of the mountains and in low basins along the coasts and in the north-east.

  • Tiny sea creatures and plant matter were buried in these layers.

  • Result: young, soft rock layers

  • The Siwaliks formed. They are the outermost and lowest range of the Himalaya. They are made of loose sand, clay and boulder beds called conglomerate (rock made of rounded pebbles and boulders stuck together).

  • The buried organic matter slowly turned into oil, gas, lignite and coal.

  • The time span is also called the Tertiary period. In the newer geological time scale, it is split into the Palaeogene and Neogene.

Petroleum: how oil is trapped

  • Oil forms from tiny marine organisms buried in sediments.
  • Oil is light, so it moves upward through porous rock (rock with tiny holes that let fluids pass) until something stops it.
  • Anticline: an upward, arch-shaped fold in rock layers.
  • Oil and gas rise to the top of the arch.

  • A cap of non-porous rock above them holds them in place.

  • Fault trap: a fault is a break in the rock along which the two sides have moved.

  • The movement puts porous rock next to non-porous rock.

  • The oil is blocked and gathers there.

  • The plate collision folded and broke the Tertiary layers, which created many of these traps.

Lignite and north-east coal

  • Lignite: a low-grade brown coal.
  • It is soft, holds a lot of moisture and has little carbon.

  • It is younger than bituminous coal, so it has been buried for less time and turned into coal less fully.

  • North-east (Tertiary) coal: younger coal, about 55 million years old.

  • It has a high sulphur content [1].
  • So it causes pollution when burned or mined and is less useful in industry.

Ophiolites: ocean floor pushed onto land

  • Ophiolite: a slice of old oceanic crust and upper mantle. It was pushed up onto land where two plates collided and the ocean between them closed.
  • These rocks mark the lines where the Indian plate met other plates during the same collision that raised the Himalaya.

Where it occurs

Siwaliks

  • The outermost, lowest range of the Himalaya, along the foot of the mountains.

Oilfields

Region Oilfields
Assam (oldest oil region) Digboi, Naharkatiya, Moran
Gujarat Ankleshwar (largest in Gujarat), Kalol
Offshore Arabian Sea Mumbai High (largest producer, about 160 km off Mumbai)

Lignite

  • Found in Tertiary sediments in Tamil Nadu, Puducherry, Gujarat and Rajasthan, and also in J&K and Odisha [1].
  • Resources: about 46.024 billion tonnes as on 1 April 2022. Tamil Nadu holds 79.3%, then Rajasthan (13.98%) and Gujarat (5.89%) [1].
  • Production in 2021-22: 47 million tonnes. By share, Tamil Nadu (about 50%) > Gujarat (about 28%) > Rajasthan (about 22%) [1].
  • Mines: 20 lignite mines were working in 2021-22. 10 were in Gujarat, 7 in Rajasthan and 3 in Tamil Nadu. All of them are opencast (open-pit) mines [1].
  • Neyveli (Tamil Nadu) is the largest opencast mine in India. NLC runs three opencast mines at Neyveli and one at Barsingsar (Rajasthan) [1].
  • Other centres: Barmer and Palana (Bikaner) in Rajasthan, and Kutch in Gujarat.

North-east coal

  • States: Assam, Arunachal Pradesh, Nagaland and Meghalaya [1].
  • Examples: Makum (Assam), Garo–Khasi–Jaintia hills (Meghalaya), Namchik–Namphuk (Arunachal Pradesh).

Ophiolite belts

  • Indus–Tsangpo suture (Ladakh): a suture is the line where two plates have joined together. This one marks where the Indian plate met the Eurasian plate.
  • Naga–Manipur hills: an ophiolite belt with chromite and nickel.
  • Andaman Islands: ophiolites formed where the Indian plate sinks below the Burma microplate.

Don't confuse with

  • Gondwana system: much older, about 300–150 million years. It holds about 98% of India's coal, found in the Peninsular river valleys (Damodar, Son, Mahanadi, Godavari, Wardha). Tertiary coal is only a very small share and is high in sulphur [1].
  • Deccan Traps: also about 66 million years old, but they are igneous flood basalt (lava that hardened), not sedimentary rock. They give black soil and building stone, but no oil or lignite.
  • Quaternary system: younger, covering the last 2.6 million years. It is loose alluvium and sand (Indo-Gangetic plain, deltas, Thar), with almost no economic minerals. Tertiary rocks hold oil and lignite.
  • Lignite vs bituminous coal: lignite is a soft, moist brown coal with little carbon, and it is younger. Most Gondwana coal is bituminous, which is harder and has more carbon.

Prelims Hooks

  • Order from oldest to youngest: Gondwana → Deccan Traps → Tertiary (Siwalik, lignite, oil) → Quaternary (alluvium).
  • Oilfield and place pairs: Digboi, Naharkatiya and Moran are in Assam. Ankleshwar and Kalol are in Gujarat. Mumbai High is offshore in the Arabian Sea.
  • Oil traps: anticlines (upward folds) and fault traps.
  • Lignite trap: Tamil Nadu has the most lignite resources (79.3%) and the most production (about 50%), but Gujarat has the most working mines (10 of 20) [1]. Neyveli is India's largest opencast mine [1].
  • Tertiary coal is found in Assam, Arunachal, Nagaland and Meghalaya and is high-sulphur [1]. It is about 55 million years old, not Gondwana coal.
  • Ophiolite belts: Indus–Tsangpo suture (Ladakh), Naga–Manipur hills (chromite, nickel) and the Andaman Islands.

Mains Points

  • Rock age decides where India's energy is found (GS-III):
  • Young Tertiary basins hold oil, so petroleum comes from Assam, Gujarat and offshore Mumbai High.
  • Tertiary lignite supports power and industry in Tamil Nadu (Neyveli), Gujarat (Kutch) and Rajasthan (Barmer, Barsingsar). All lignite mines are opencast [1].

  • So these states play a key part in India's energy security, apart from the Gondwana coal belt.

  • North-east coal and the environment (GS-III):

  • North-east Tertiary coal is high in sulphur [1].

  • This limits its use in industry and causes pollution, such as acid mine drainage in Meghalaya (acidic water flowing out of mines into streams).

  • A record of plate tectonics (GS-I):

  • The Siwalik sediments and the Indus–Tsangpo, Naga–Manipur and Andaman ophiolites record India's collision with Eurasia and the rise of the Himalaya.
  • This is useful in answers on plate movement and how India's landforms evolved.

Related concepts

Read more

Sources

  1. 1Indian Minerals Yearbook 2022, "Coal and Lignite", Indian Bureau of Minesibm.gov.in · tier 1