Plate tectonics: the theory and the plates

Detail

How the theory came about

• Plate tectonics (the theory that the earth's outer shell is broken into large rigid pieces, called plates, that move) was put forward in 1967.
• McKenzie and Parker proposed it in 1967. Morgan reached the same idea on his own.
• They brought together two earlier ideas: continental drift (Wegener, 1912: continents move across the globe) and sea-floor spreading (Hess, 1960s: new ocean floor forms at mid-ocean ridges and spreads outwards).

• Class 9 NCERT credits W.J. Morgan. His paper was published in 1968.

• The order to remember: Pangaea broke up about 200 million years ago → Wegener's drift (1912) → Hess's sea-floor spreading (1960s) → plate tectonics (1967).

What a tectonic plate is

• A tectonic plate (also called a lithospheric plate) is a huge slab of solid rock with an irregular shape.
• It usually has both a continental part and an oceanic part.

• The whole slab moves sideways as one rigid unit over the asthenosphere.

• Asthenosphere: the soft, partly molten layer of the upper mantle that lies below the lithosphere. Plates slide over it.

• Lithosphere: the crust plus the top part of the mantle. It is hard and brittle.

Part of the lithosphere	Thickness (NCERT)
Under the oceans	5-100 km
Under the continents	about 200 km

Types of plates

• A plate gets its name from whichever part is larger.
• Oceanic plate: mostly ocean floor. Example: the Pacific plate.
• Continental plate: mostly land. Example: the Eurasian plate.
• Mixed plate: Class 9 adds this third type, for plates with large parts of both land and ocean floor.

How fast plates move

• Plates usually move a few centimetres a year (NCERT). This is about as fast as your fingernails grow [2].
• Each plate moves at its own speed and in its own direction [2].
• Slowest spreading: the Arctic Ridge, at less than 2.5 cm a year [3].
• Fastest spreading: the East Pacific Rise near Easter Island, in the South Pacific about 3,400 km west of Chile, at more than 15 cm a year [3].
• India today: GPS (satellite positioning) readings in the Garhwal-Kumaun Himalaya (Uttarakhand) show strain building up on the Main Himalayan Thrust. The "slip deficit" (movement that is stored and not yet released) is 18 mm a year across a belt about 100 km wide (study published 2017) [4].

The seven major plates

• Major plates are bounded by young fold mountain ridges, trenches and/or faults (NCERT Figure 4.5).
• Fold mountains: young mountains made when rock layers are squeezed and bent, such as the Himalayas, Alps and Andes.
• Trench: a long, narrow, very deep valley in the sea floor. It forms where one plate sinks under another.
• Fault: a crack in the earth's crust along which blocks of rock have moved.

#	Major plate	What it includes
1	Antarctic	Antarctica and the ocean around it
2	North American	North America and the floor of the western Atlantic. It is separated from the South American plate along the Caribbean islands
3	South American	South America and the floor of the western Atlantic
4	Pacific	Most of the Pacific Ocean floor. The largest plate
5	India-Australia-New Zealand	India, Australia, New Zealand and parts of the Indian Ocean
6	African	Africa and the floor of the eastern Atlantic
7	Eurasian	Europe, Asia and the ocean next to them

• Beyond NCERT: many geologists now treat the Indo-Australian plate as two separate plates, Indian and Australian. If so, the count of major plates changes. Check which classification a question uses.

Minor plates (NCERT list)

Minor plate	Where it lies
Cocos	Between Central America and the Pacific plate
Nazca	Between South America and the Pacific plate
Arabian	Mostly the Saudi Arabian landmass
Philippine	Between the Asiatic (Eurasian) and Pacific plates
Caroline	Between the Philippine and Indian plates, north of New Guinea

• Other minor plates (beyond NCERT): Juan de Fuca (off the coast of north-west USA and Canada), Caribbean, Scotia (between South America and Antarctica) and Sunda (South-east Asia). Lists differ between sources.

Drift vs plate tectonics: the key difference

• Wegener: the continents themselves move. They plough through the ocean floor.
• Plate tectonics: the continent does not move on its own. The whole plate moves, and it carries the continent with it, like a box on a moving conveyor belt.
• All plates have moved throughout earth's history, and they will keep moving. Pangaea is just one stage in this long process.

Point	Continental drift (Wegener, 1912)	Plate tectonics (1967)
What moves	Continents only	Lithospheric plates (continent + ocean floor)
Moves over	The ocean floor	The asthenosphere
Force behind it	Pole-fleeing force and tidal force (too weak)	Convection currents in the mantle, slab pull and ridge push
Ocean floor	Treated as fixed	Made new at ridges and destroyed at trenches
Time frame	Mainly the break-up of Pangaea	All of earth's history, still going on

The Pacific plate

• It is the largest plate, and it is almost entirely oceanic.
• Subduction zones ring it on almost every side.
• Subduction: one plate (usually the heavier oceanic one) dives under another and sinks into the mantle.

• This ring of sinking plates is the Ring of Fire: a horseshoe-shaped belt of volcanoes and earthquakes around the Pacific Ocean.

• San Andreas fault (California, USA): here the Pacific plate meets the North American plate.

• This is a transform boundary: the two plates slide sideways past each other. Crust is neither made nor destroyed.
• The Pacific plate has been grinding past the North American plate for 10 million years, at about 5 cm a year on average [3].

The Nazca plate and the Andes

• What happens:
• The oceanic Nazca plate is heavier than the continental South American plate, so it sinks under it along the Peru-Chile Trench.
• The edge of South America is squeezed and pushed up into the Andes, a young fold mountain range with volcanoes.

• Stress builds up where the two plates meet. When it is suddenly released, earthquakes hit Peru and Chile.

• Speed: compared with a fixed South American plate, the Nazca plate moves slightly north of east. Its speed ranges from about 80 mm a year in the south to about 70 mm a year in the north [5].

• Chile, 22 May 1960 (Valdivia earthquake): Mw 9.5. This is the largest earthquake ever recorded [6].
• Mw (moment magnitude): the scale that measures the total energy released by an earthquake.
• Epicentre (the point on the surface right above the earthquake's starting point): 38.143°S, 73.407°W. Focal depth: 25 km [6].
• It happened on the Nazca-South American boundary. There, the plates converge (move towards each other) at about 70 mm a year [6].
• The edge of the South American plate jerked by up to about 18 m (60 feet) over an area about 965 km (600 miles) long and more than 160 km (100 miles) wide [7].
• Its tsunami (a series of huge sea waves set off by the earthquake) crossed the Pacific. It killed 61 people in Hawaii and 122 in Japan [6].

India and plate tectonics

• How the Himalayas formed (Class 7): the Indian plate moved north and collided with the Eurasian plate. The sediments of the sea between them, the Tethys Sea, were squeezed and folded up into the Himalayas.
• The Indian plate is still moving north, and it is still pushing under the Eurasian plate. This is why the Himalaya has so many earthquakes [8].
• Ladakh Magmatic Arc (north-west Himalaya): a belt of rocks formed from molten material. It records the subduction, growth and collision of the Indian and Eurasian plates over about 130 million years [9].
• Major thrusts of the Himalaya: the Himalayan Frontal Thrust (HFT), the Main Boundary Thrust (MBT) and the Main Central Thrust (MCT). A thrust is a low-angle fault where one block of rock is pushed up over another [10].
• North-east India: the north-eastern edge of the Indian plate meets the Eastern Himalaya and the Indo-Burma Ranges. This zone is seismically active and has been linked to the great Assam earthquake [11].

Prelims Hooks

• 1967, McKenzie and Parker: proposed plate tectonics. Morgan reached it independently (his paper came out in 1968). Wegener (1912) = continental drift. Hess (1960s) = sea-floor spreading.
• Lithosphere thickness: 5-100 km under oceans and about 200 km under continents. Plates move over the asthenosphere, not over the ocean floor.
• Trap: the Arabian, Nazca, Cocos, Philippine and Caroline plates are minor plates (NCERT). The Pacific is the largest major plate.
• Plate and its location: Cocos – Central America; Nazca – western South America; Caroline – north of New Guinea; Philippine – between the Eurasian and Pacific plates; Arabian – Saudi Arabia.
• North American vs South American plates: they are separated along the Caribbean islands.
• San Andreas fault: a transform boundary between the Pacific and North American plates, moving about 5 cm a year [3].
• Fastest spreading: East Pacific Rise, more than 15 cm a year. Slowest: Arctic Ridge, less than 2.5 cm a year [3].
• Peru-Chile Trench: Nazca plate sinks under the South American plate → Andes. Chile 1960, Mw 9.5: largest earthquake ever recorded [6].
• Pacific = oceanic plate; Eurasian = continental plate. A plate's type depends on which part is larger.
• Himalayan thrusts from south to north: HFT → MBT → MCT.

Mains Points

• Drift vs plate tectonics (GS-I): Wegener said the continents move on their own, pushed by forces that were too weak. Plate tectonics says the whole lithospheric plate moves over the asthenosphere and carries the continent with it. It also explains why the ocean floor is young, and why earthquakes, volcanoes and fold mountains occur in belts along plate edges.
• Distribution pattern (GS-I): earthquakes, volcanoes and fold mountains are not scattered at random. They follow plate edges.
• The Ring of Fire follows the subduction zones around the Pacific plate.
• The Andes rise where the Nazca plate sinks under South America.

• The Himalayas rise where India collides with Eurasia.

• Disaster angle (GS-III): the Indian plate is still moving north, and strain is building up on the Main Himalayan Thrust (a slip deficit of 18 mm a year [4]). This keeps the Himalaya and north-east India in high seismic zones. India needs GPS monitoring, earthquake-safe building codes and microzonation (detailed local maps of earthquake hazard).

• Lesson from Chile 1960 (GS-III): a single subduction earthquake sent a deadly tsunami across the whole Pacific [6]. This is the case for warning systems that cover a whole ocean, like India's tsunami warning centre at INCOIS for the Andaman-Sumatra subduction zone.