The Indian plate: boundaries and the journey north

Detail

What the Indian plate is

• A lithospheric plate is a large, stiff slab of the Earth's outer shell (crust plus the top part of the mantle). It moves slowly over the softer layer below it.
• The Indian plate is one of these slabs. It carries:
• Peninsular India, the old and stable land block south of the Himalayas.
• The continental parts of Australia. So in NCERT's scheme, India and Australia ride on one plate, called the Indo-Australian plate.
• The ocean floor between them, which covers most of the Indian Ocean.

Boundaries of the Indian plate

Side	Where the boundary runs	Type of boundary
North	Along the Himalayas	Convergent. It is a continent-continent convergence: two land masses push into each other, and neither sinks easily.
East	Through the Rakinyoma (Arakan Yoma) mountains of Myanmar, then on to the island arc along the Java Trench (Sunda Trench)	Convergent. The oceanic plate sinks under the plate next to it (subduction).
Far east	An oceanic ridge east of Australia, in the south-west Pacific	Divergent (a spreading site)
West	Along the Kirthar mountains of Pakistan, then the Makran coast, then joining the spreading site from the Red Sea rift. From there it runs south-east along the Chagos Archipelago.	Mixed. There is collision and sideways sliding on land, subduction at Makran, and spreading in the ocean.
South	An oceanic ridge with the Antarctic plate that runs roughly west to east and joins the spreading site south of New Zealand	Divergent. New ocean floor is made 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. It forms above a subduction zone. Example: Sumatra and Java along the Java Trench.
• Oceanic ridge / spreading site: an underwater mountain chain. There, plates move apart and magma rises to make new sea floor. This is sea-floor spreading, the idea Hess put forward in the 1960s.
• Rift: a long crack where land is being pulled apart. Example: the Red Sea rift between Africa and Arabia.

The Burma microplate and the 2004 earthquake

• A microplate is a small plate caught between big plates.
• The Burma microplate lies between the Indian plate and the Sunda plate. It carries the Andaman and Nicobar Islands and western Myanmar.
• How the boundary works:
• The Indian plate is oceanic here. It is pushed down beneath the Burma microplate, which is part of the larger Sunda plate [3].
• It goes down at an average of about 6 cm a year [3].

• Strain builds up over centuries along this locked fault. Then it is released all at once as a great earthquake.

• The 26 December 2004 Sumatra-Andaman earthquake:

• Magnitude Mw 9.1. It struck at 00:58:53 UTC [3].
• It broke about 1,300 km of the subduction boundary, from northern Sumatra up to the Andaman Islands.
• The rupture went on for about 500 seconds, which is unusually long [4].
• Result: the sea floor was pushed up suddenly. This lifted the water above it and set off the Indian Ocean tsunami, which hit India's east coast and the Andaman and Nicobar Islands.

The journey north (NCERT Figure 4.6)

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

• Gondwana: its southern part, made up of South America, Africa, India, Australia and Antarctica.

• Tethys Sea: an ancient ocean between Gondwana and Asia. It separated India from Asia until about 225 million years ago. Fossils show that this sea flooded the Tibetan region from time to time while India moved north [2].

• Timeline:

Time	Event
~225 million years ago	Tethys Sea separates India from Asia
~200 million years ago	Pangaea breaks up. India starts moving north.
~140 million years ago	India lies as far south as 50°S. Tibet is closer to Asia.
~80 million years ago	India is about 6,400 km south of Asia. It moves north at about 9 m a century (~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). The Himalayas rise quickly.
Today	The Himalayas still rise by more than 1 cm a year, which adds up to about 10 km in a million years [2]

• Speed:
• After leaving Gondwana, India moved unusually fast, at up to about 15–20 cm a year. USGS gives about 9 cm a year for the time around 80 million years ago [2].

• When India hit Asia, its northward movement slowed by about half [2].

• How the Himalayas formed:

• The ocean floor of the Tethys sank under Asia (subduction).
• The Tethys closed, and the continental crust of India met Asian crust.
• Continental crust is light, so it does not sink. Instead it crumples, folds and thickens.
• Sediments from the old Tethys floor were pushed up. The result was the Himalayas and the Tibetan Plateau.

Deccan Traps and hotspot tracks

• Hotspot: a fixed, very hot spot deep in the mantle. Plumes of magma rise from it. The plate moves over it, but the hotspot stays put. So it leaves a line of volcanoes behind, called a hotspot track, much like a candle under moving paper.
• Deccan Traps:
• These are thick, step-like sheets of basalt (dark volcanic rock). The word trap comes from the Swedish for "stairs".
• The lava began pouring out about 60 million years ago (NCERT). The main pulse is dated to about 66 million years ago, close to the end-Cretaceous extinction that wiped out the dinosaurs.
• The eruptions went on for a long time.
• India was still close to the equator then.

• Today the Traps cover large parts of Maharashtra, Gujarat and Madhya Pradesh. They weathered into black (regur) soil.

• Reunion hotspot track:

• About 66 million years ago, the Reunion hotspot lay under western India and made the Deccan Traps.
• India kept moving north, so the hotspot "drew" a line southward across the sea floor. That line is the Chagos-Laccadive Ridge, with Lakshadweep (India) → Maldives → Chagos from north to south.
• Today the hotspot sits under Réunion Island, east of Madagascar.

• Its volcanoes get younger from north to south: India is the oldest, then Lakshadweep and Maldives, then Chagos, then Mauritius and Réunion.

• Ninety East Ridge:

• A straight underwater ridge along about 90°E in the eastern Indian Ocean. It runs north towards the Bay of Bengal.
• It is about 5,000 km long.
• It is the track of the Kerguelen hotspot, which now lies under the Kerguelen Islands in the southern Indian Ocean.
• It is straight because India moved almost due north over the hotspot.

Feature	Hotspot	Location	Direction of ageing
Deccan Traps + Chagos-Laccadive Ridge	Reunion	Western India, then Arabian Sea and western Indian Ocean	Oldest in India, youngest at Réunion
Ninety East Ridge	Kerguelen	Eastern Indian Ocean, along ~90°E	Oldest in the north, youngest in the south

Prelims Hooks

• Northern boundary of the Indian plate: the Himalayas, a continent-continent convergent boundary. It is not ocean-continent.
• Eastern boundary order: Arakan Yoma (Myanmar) → island arc along the Java Trench.
• Western boundary order: Kirthar mountains (Pakistan) → 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, not on the Indian plate. The 2004 earthquake was Mw 9.1 [3].
• About 140 million years ago, India lay at about 50°S. During the Deccan Traps it lay near the equator.
• Collision with Asia: 40–50 million years ago (Class 7 NCERT: ~50 million years). The Himalayas still rise by more than 1 cm a year [2].
• Hotspot pairs: Deccan Traps and Chagos-Laccadive Ridge → Reunion. Ninety East Ridge → Kerguelen.
• Trap: Lakshadweep, Maldives and Chagos are hotspot tracks. They are not island arcs or subduction features.

Mains Points

• Plate setting explains India's landforms:
• Collision in the north → young fold mountains (the Himalayas) and the Indo-Gangetic foredeep, which filled with river sediment.
• A stable shield in the south → the old Peninsular plateau.

• A hotspot in the past → the Deccan Traps and their black soils (GS-I).

• Earthquake risk follows plate boundaries (GS-III, disasters):

• The Himalayas and North-East are in Seismic Zones IV–V because India keeps pushing into Asia.
• The Andaman-Sumatra subduction zone is where great earthquakes and tsunamis start (2004, Mw 9.1 [3]).

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

• The Indian Ocean floor holds evidence for plate tectonics:

• Hotspot tracks (the Chagos-Laccadive and Ninety East ridges) show which way and how fast India moved.
• Ocean floor that gets older away from the ridges supports Hess's sea-floor spreading.

• Together they support the 1967 plate tectonics theory over Wegener's drift alone.

• Fast drift, then slowing:

• India moved at about 9 cm a year about 80 million years ago, and slowed by about half after the collision [2].
• Its push still continues. This raises the Himalayas, builds up strain and drives erosion and sediment supply to the Ganga-Brahmaputra system.