Indian plate

Also called: Indo-Australian plate · Topic: Continental Drift, Sea-floor Spreading and Plate Tectonics · NCERT: Class 11, Ch 4 "Distribution of Oceans and Continents"

Meaning

The Indian plate (also called the Indo-Australian plate) is a large, stiff slab of the Earth's outer shell. It carries Peninsular India, the continental parts of Australia and the ocean floor between them, which covers most of the Indian Ocean. Its edges run along the Himalayas in the north, the Arakan Yoma and the Java Trench in the east, the Kirthar range and Makran coast in the west, and ocean ridges in the south and far east.

It matters because the plate's long trip north explains India's main landforms: the Himalayas, the Deccan Traps and the old Peninsular plateau. Its edges also decide where India faces the biggest earthquake and tsunami risk.

Explanation

What the plate is and what it carries

• A lithospheric plate is a large, stiff slab of crust plus the top part of the mantle. It moves slowly over the softer layer below it.
• The Indian plate carries three things:
• Peninsular India, the old and stable block of land south of the Himalayas.
• The continental parts of Australia. In NCERT's scheme, India and Australia ride on one plate. That is why it is called the Indo-Australian plate.

• The ocean floor between them, which covers most of the Indian Ocean.

• The Andaman and Nicobar Islands are not on the Indian plate. They sit on the Burma microplate (a small plate caught between big plates), which lies between the Indian plate and the Sunda plate.

Boundaries: each side works in a different way

Side	Where it runs	Type
North	Along the Himalayas	Convergent (plates push together). It is a continent-continent collision, so neither side sinks easily.
East	Through the Rakinyoma (Arakan Yoma) mountains of Myanmar, then along the island arc of the Java (Sunda) Trench	Convergent. Here the plate is ocean floor, and it sinks under the next plate (subduction).
Far east	An oceanic ridge east of Australia, in the south-west Pacific	Divergent (plates move apart)
West	Kirthar mountains (Pakistan) → Makran coast → joins the spreading site from the Red Sea rift → runs south-east along the Chagos Archipelago	Mixed: collision and sideways sliding on land, subduction at Makran, spreading in the ocean
South	An oceanic ridge with the Antarctic plate. It runs roughly west to east and joins the spreading site south of New Zealand.	Divergent. New ocean floor forms here.

• Key terms:
• Subduction zone: a place where one plate dives under another and sinks into the mantle. Deep trenches, volcanoes and big earthquakes form there.
• Island arc: a curved chain of volcanic islands above a subduction zone. Examples are Sumatra and Java.
• Oceanic ridge (spreading site): an underwater mountain chain. Plates move apart there, and magma rises to make new sea floor. Hess called this sea-floor spreading in the 1960s.
• Rift: a long crack where land is being pulled apart, such as the Red Sea rift between Africa and Arabia.

The journey north: cause → process → result

• Start:
• India was a large island off the coast of Australia.
• It was part of Gondwana, the southern half of Pangaea (the single supercontinent Wegener described in 1912).

• The Tethys Sea lay between India and Asia until about 225 million years ago. Fossils show that this sea flooded the Tibetan region from time to time as India moved north [2].

• Timeline:

Time	Event
~200 million years ago	Pangaea breaks up. India starts moving north.
~140 million years ago	India lies as far south as 50°S
~80 million years ago	India is about 6,400 km south of Asia and moves at about 9 cm a year [2]
~60–66 million years ago	Deccan Traps lava pours out. India is near the equator.
~40–50 million years ago	India hits Asia (Class 7 NCERT: about 50 million years ago)
Today	The Himalayas still rise by more than 1 cm a year [2]

• Speed:
• India moved unusually fast after leaving Gondwana, at up to about 15–20 cm a year.

• After it hit Asia, its northward movement slowed by about half [2].

• How the Himalayas formed:

• The Tethys ocean floor sank under Asia (subduction), and the Tethys closed.
• India's continental crust then met Asian crust. Continental crust is light, so it does not sink. It crumples, folds and thickens instead.
• Sediments from the old Tethys floor were pushed up. This formed the Himalayas and the Tibetan Plateau.

Hotspot tracks: a record of the plate's movement

• Hotspot: a fixed, very hot spot deep in the mantle that sends up plumes of magma. The plate moves over it, but the hotspot stays put. So it leaves a line of volcanoes behind, called a hotspot track, like a candle burning a line on paper moved over it.
• Reunion hotspot:
• About 66 million years ago it lay under western India and made the Deccan Traps. These are thick, step-like sheets of basalt (dark volcanic rock). Trap comes from the Swedish word for "stairs".
• As India moved north, the hotspot left the Chagos-Laccadive Ridge: Lakshadweep → Maldives → Chagos, from north to south.

• Today the hotspot sits under Réunion Island, east of Madagascar. The volcanoes get younger from north to south.

• Kerguelen hotspot:

• It made the Ninety East Ridge, a straight underwater ridge about 5,000 km long along about 90°E.
• The ridge is straight because India moved almost due north over this hotspot.

Where it occurs

• India:
• Peninsular plateau: the stable heart of the plate.
• Himalayas: the northern collision boundary.
• Maharashtra, Gujarat and Madhya Pradesh: large areas are covered by Deccan Traps basalt, which weathered into black (regur) soil.

• Lakshadweep: the northern part of the Reunion hotspot track.

• Neighbouring boundary zones:

• Arakan Yoma (Myanmar): eastern boundary on land.
• Kirthar range and Makran coast (Pakistan): western boundary.

• Andaman and Nicobar Islands and western Myanmar: on the Burma microplate, next to the Indian plate.

• Indian Ocean:

• Java (Sunda) Trench, with Sumatra and Java as the island arc.
• Maldives and Chagos on the Chagos-Laccadive Ridge.
• Ninety East Ridge, which runs north towards the Bay of Bengal.

• Réunion Island and the Kerguelen Islands, where the two hotspots sit today.

• Beyond:

• Australia's continental parts.
• The ridge with the Antarctic plate that joins the spreading site south of New Zealand.
• The divergent ridge in the south-west Pacific.

Don't confuse with

• Burma microplate: this small plate carries the Andaman and Nicobar Islands and western Myanmar. The Indian plate sinks under it. The islands are not on the Indian plate.
• Ocean-continent convergence (Java Trench): the Himalayan boundary is a continent-continent collision, where neither side sinks easily. Subduction and island arcs belong to the eastern edge.
• Island arc vs hotspot track: Sumatra and Java are an island arc formed above subduction. Lakshadweep, Maldives and Chagos are a hotspot track formed by the Reunion hotspot.
• Reunion vs Kerguelen hotspot: Reunion made the Deccan Traps and the Chagos-Laccadive Ridge. Kerguelen made the Ninety East Ridge.

Prelims Hooks

• Northern boundary = Himalayas = continent-continent convergent boundary, not ocean-continent.
• Eastern boundary order: Arakan Yoma (Myanmar) → island arc along the Java Trench. Western boundary order: Kirthar → Makran coast → Red Sea rift spreading site → along the Chagos Archipelago.
• Southern boundary: a divergent oceanic ridge with the Antarctic plate.
• Andaman and Nicobar Islands sit on the Burma microplate. The Indian plate sinks beneath it at about 6 cm a year. The 26 December 2004 earthquake there was Mw 9.1 [3], and the rupture lasted about 500 seconds [4].
• Position of India over time: at about 50°S about 140 million years ago, and near the equator during the Deccan Traps eruptions. It hit Asia 40–50 million years ago.
• Hotspot pairs: Deccan Traps and Chagos-Laccadive Ridge → Reunion. Ninety East Ridge → Kerguelen.

Mains Points

• The plate setting explains India's physical divisions (GS-I):
• Collision in the north → young fold mountains (the Himalayas) and the Indo-Gangetic foredeep, which filled with river sediment.
• Stable shield in the south → the old Peninsular plateau.

• A past hotspot → the Deccan Traps and their black soils.

• Earthquake and tsunami risk follows the plate's edges (GS-III, disasters):

• India keeps pushing into Asia. So the Himalayas and the North-East are in Seismic Zones IV–V.
• The Andaman-Sumatra subduction zone produces great earthquakes. In 2004, about 1,300 km of the boundary broke, the sea floor lifted, and the Indian Ocean tsunami followed (Mw 9.1 [3]).

• This is why India needs earthquake-resistant building and the INCOIS tsunami early-warning system.

• The Indian Ocean floor is evidence for plate tectonics:

• Hotspot tracks show which way India moved and how fast. India moved at about 9 cm a year and slowed by about half after the collision [2].
• Sea floor gets older the farther it is from the ridges. This supports Hess's sea-floor spreading and the 1967 plate tectonics theory over Wegener's drift alone.
• India's push still continues. It keeps raising the Himalayas, building up strain and feeding sediment to the Ganga-Brahmaputra system.

Related concepts

• Northward movement of the Indian plate
• Tethys Sea
• Burma microplate
• Reunion hotspot
• Ninety East Ridge

Read more

• The Indian plate: boundaries and the journey north