Tethys Sea

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

Meaning

The Tethys Sea (also called the Tethys Ocean) was an ancient ocean that lay between Gondwana (the southern part of Pangaea, which included India) and Asia (part of Laurasia, the northern part of Pangaea).

It matters because the Himalayas and the Tibetan Plateau were built from it. As India drifted north, the floor of the Tethys sank under Asia and the sea closed. The sediments (layers of mud, sand and shells) on its floor were then folded and pushed up.

Explanation

Setting: where the Tethys lay

• Pangaea: the single supercontinent described by Wegener in 1912.
• Gondwana: its southern half, made up of South America, Africa, India, Australia and Antarctica.
• India's position: India was a large island off the coast of Australia and was part of Gondwana.
• Tethys Sea: the ancient ocean between Gondwana and Asia. NCERT says it separated India from Asia until about 225 million years ago.
• Evidence: fossils show that this sea flooded the Tibetan region from time to time while India moved north [2].

How the Tethys closed: India's journey north

Time	What happened to India and the Tethys
~225 million years ago	The Tethys Sea separates India from Asia
~200 million years ago	Pangaea breaks up. India starts moving north and the Tethys begins to shrink.
~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. It moves north at about 9 cm a year [2].
~60–66 million years ago	The 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 Tethys is closed.

• Speed: after leaving Gondwana, India moved unusually fast, at up to about 15–20 cm a year.
• Slowing down: once India hit Asia, its northward movement slowed by about half [2].

From sea floor to mountains: cause → process → result

• Cause: the Indian plate pushed north.
• The ocean floor of the Tethys was in front of India.

• This oceanic floor sank under Asia. This is subduction (one plate diving under another and sinking into the mantle).

• Process: two continents met.

• When all the Tethys floor had sunk, the sea was closed.
• India's continental crust then met Asia's continental crust. This is a continent-continent convergence (two land masses pushing into each other).

• Continental crust is light, so it does not sink. Instead it crumples, folds and thickens.

• Result: the Himalayas rose.

• Sediments from the old Tethys floor were pushed up. They formed the Himalayas and the Tibetan Plateau.
• The Himalayas are still rising by more than 1 cm a year, which adds up to about 10 km in a million years [2].

Conditions it needed

• A gap between two continents: this allowed a sea to exist, and sediments built up on its floor over millions of years.
• A moving plate: the Indian plate drifted north and closed the gap.
• Light continental crust on both sides: neither land mass could sink, so the squeezed sediments had to go up, not down.

Where it occurs

• Himalayas: they run along the northern boundary of the Indian plate. They are made of uplifted Tethys sediments.
• Tibetan Plateau: it was flooded by the Tethys from time to time, as fossils show [2]. It was later raised by the collision.
• Indo-Gangetic foredeep: a low trough south of the Himalayas. It formed during the collision and later filled with river sediment.
• Ganga-Brahmaputra system: these rivers carry eroded material from the rising Himalayas.
• Himalayas and North-East India: they lie in Seismic Zones IV–V because India is still pushing into Asia.
• Original setting: between Gondwana (with India off the coast of Australia) and Asia.

Don't confuse with

• Pangaea: the single supercontinent. The Tethys was a sea, not land, and it lay between Gondwana and Asia.
• Gondwana: the southern landmass that included India. The Tethys was the water body that separated it from Asia.
• Indian Ocean: a present-day ocean that is still opening. New sea floor forms at its divergent ridges, such as the one with the Antarctic plate. The Tethys was closed by subduction and collision.
• Deccan Traps: volcanic basalt sheets made by the Reunion hotspot (a fixed hot spot deep in the mantle) when India was near the equator. They have nothing to do with Tethys sediments or with folding.

Prelims Hooks

• Tethys Sea → Himalayas: the Himalayas formed from folded and uplifted Tethys sediments. They are young fold mountains.
• Position: the Tethys lay between Gondwana and Asia. It separated India from Asia until about 225 million years ago (NCERT).
• Type of boundary formed: the closing of the Tethys created a continent-continent convergent boundary along the Himalayas, not an ocean-continent one.
• Fossil evidence: marine fossils show the Tethys flooded the Tibetan region from time to time [2].
• Distance and speed: about 80 million years ago, India was about 6,400 km south of Asia and moved at about 9 cm a year [2].
• Trap: the Tethys floor sank (was subducted) under Asia. It did not spread apart. Divergence is how oceans open, not how the Tethys closed.

Mains Points

• The Tethys explains India's northern landforms (GS-I):
• When the Tethys closed, its sediments were folded into the Himalayas and the Tibetan Plateau.
• The Indo-Gangetic foredeep formed in front of them and filled with river sediment, creating the fertile northern plains.

• In contrast, the Peninsular plateau in the south is an old, stable shield.

• The collision still goes on (GS-III, disasters):

• India keeps pushing into Asia, though more slowly than before. Its speed dropped by about half after the collision [2].
• Strain builds up, so the Himalayas and North-East lie in Seismic Zones IV–V. Earthquake-resistant building is needed there.

• The Himalayas are still rising by more than 1 cm a year [2]. This drives erosion and sends sediment to the Ganga-Brahmaputra system.

• The Tethys is evidence for plate tectonics:

• Marine fossils high in the Tibetan region show it was once under the sea [2].
• This supports Wegener's continental drift (1912), which was later explained by Hess's sea-floor spreading (1960s) and the plate tectonics theory (1967).

Related concepts

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

Read more

• The Indian plate: boundaries and the journey north