Plate tectonics

Topic: Continental Drift, Sea-floor Spreading and Plate Tectonics · NCERT: Class 9, Ch 2 "Shaping of the Earth's Surface"; Class 11, Ch 4 "Distribution of Oceans and Continents"

Meaning

Plate tectonics is the theory that the earth's hard outer shell, the lithosphere, is broken into seven major and several minor rigid pieces called plates. Each plate moves as one unit over the soft asthenosphere below it, and it carries both continents and ocean floor with it. McKenzie and Parker put the theory forward in 1967, and Morgan reached the same idea on his own (his paper came out in 1968).

It matters because it explains why earthquakes, volcanoes and young fold mountains are not scattered at random. They are found in belts along plate edges, such as the Ring of Fire, the Andes and the Himalayas.

Explanation

How the theory came about

• It joined two earlier ideas into one theory:
• Continental drift (Wegener, 1912): the continents move across the globe.

• Sea-floor spreading (Hess, 1960s): new ocean floor forms at mid-ocean ridges and spreads outwards.

• Who proposed it: McKenzie and Parker (1967). Morgan reached it independently. Class 9 NCERT credits W.J. Morgan, whose paper was published in 1968.

• 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 plate is and what it moves on

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

• The whole slab moves sideways as one rigid unit.

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

• Under the oceans it is 5-100 km thick. Under the continents it is about 200 km thick (NCERT).

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

• What drives the plates: convection currents in the mantle (hot rock rises, cools and sinks again in a slow loop), slab pull (the sinking edge of a plate pulls the rest of the plate after it) and ridge push (new, raised crust at a ridge pushes the plate away).
• Ocean floor is renewed all the time. It is made new at ridges and destroyed at trenches. This is why the ocean floor is young.
• Plates have moved all through earth's history, and they will keep moving. Pangaea is just one stage in this long process.

Types of plates and plate boundaries

• A plate is named after whichever part of it 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 areas of both land and ocean floor.

• 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 (Himalayas, Alps, Andes).
• Trench: a long, narrow, very deep valley in the sea floor, where one plate sinks under another.

• Fault: a crack in the crust along which blocks of rock have moved.

• What happens at plate edges:

• Subduction: one plate, usually the heavier oceanic one, dives under another and sinks into the mantle. This gives trenches, volcanoes and fold mountains.
• Transform boundary: two plates slide sideways past each other. Crust is neither made nor destroyed.

The seven major and the minor plates (NCERT)

Major plate	What it includes
Antarctic	Antarctica and the ocean around it
North American	North America and the floor of the western Atlantic. Separated from the South American plate along the Caribbean islands
South American	South America and the floor of the western Atlantic
Pacific	Most of the Pacific Ocean floor. The largest plate
India-Australia-New Zealand	India, Australia, New Zealand and parts of the Indian Ocean
African	Africa and the floor of the eastern Atlantic
Eurasian	Europe, Asia and the ocean next to them

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

• Beyond NCERT:
• Many geologists now treat the Indo-Australian plate as two plates, Indian and Australian. This changes the count of major plates, so check which classification a question uses.
• Other minor plates include Juan de Fuca (off north-west USA and Canada), Caribbean, Scotia (between South America and Antarctica) and Sunda (South-east Asia). Lists differ between sources.

How fast plates move

• Plates usually move a few centimetres a year (NCERT). That 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].

Where it occurs

World

• Pacific plate and the Ring of Fire: the Pacific is the largest plate and is almost entirely oceanic. Subduction zones surround it on almost every side. Together they form the Ring of Fire, a horseshoe-shaped belt of volcanoes and earthquakes around the Pacific Ocean.
• San Andreas fault (California, USA): a transform boundary where the Pacific plate meets the North American plate. The Pacific plate has been grinding past the North American plate for 10 million years, at about 5 cm a year on average [3].
• Nazca plate and the Andes (Peru-Chile):
• The heavier oceanic Nazca plate sinks under the continental South American plate 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 plates meet. When it is suddenly released, earthquakes hit Peru and Chile.

• Measured against 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, the largest earthquake ever recorded [6].

• Mw (moment magnitude): the scale that measures the total energy an earthquake releases.
• Epicentre (the point on the surface right above where the earthquake starts): 38.143°S, 73.407°W. Focal depth: 25 km [6].
• It struck on the Nazca-South American boundary, where 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

• Himalayas:
• The Indian plate moved north and collided with the Eurasian plate.
• Sediments of the sea between them, the Tethys Sea, were squeezed and folded up into the Himalayas (Class 7).

• The Indian plate is still moving north and still pushing under the Eurasian plate. This is why the Himalaya has so many earthquakes [8].

• Major Himalayan thrusts: 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].

• Garhwal-Kumaun Himalaya (Uttarakhand): GPS (satellite positioning) readings 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].

• 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].

• 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].

Don't confuse with

• Continental drift (Wegener, 1912): in drift, the continents alone move and plough through a fixed ocean floor, pushed by pole-fleeing and tidal forces that are too weak. In plate tectonics, the whole plate (continent plus ocean floor) moves over the asthenosphere and carries the continent like a box on a conveyor belt.
• Sea-floor spreading (Hess, 1960s): this explains only how new ocean floor forms at mid-ocean ridges and spreads outwards. Plate tectonics is the larger theory that combines it with drift and covers the whole lithosphere.
• Lithosphere vs asthenosphere: the lithosphere (crust plus top of the mantle) is hard and brittle and makes up the plates. The asthenosphere is the soft, partly molten layer of the upper mantle that the plates slide over.
• Major vs minor plates: the Arabian, Nazca, Cocos, Philippine and Caroline plates are minor plates (NCERT), even though they often appear in news about earthquakes. The Pacific is the largest major plate.

Prelims Hooks

• 1967, McKenzie and Parker proposed plate tectonics. Morgan reached it independently (paper published 1968). Wegener (1912) = continental drift. Hess (1960s) = sea-floor spreading.
• Plate–location pairs: Cocos – Central America; Nazca – western South America; Caroline – north of New Guinea; Philippine – between the Eurasian and Pacific plates; Arabian – Saudi Arabia.
• Trap: a plate's type depends on which part is larger. Pacific = oceanic plate; Eurasian = continental plate. The North American and South American plates are separated along the Caribbean islands.
• Spreading rates: fastest is the East Pacific Rise (more than 15 cm a year); slowest is the Arctic Ridge (less than 2.5 cm a year) [3]. The San Andreas fault is a transform boundary (Pacific–North American), moving about 5 cm a year [3].
• Process–landform pair: the Nazca plate sinks under the South American plate at the Peru-Chile Trench → Andes. Chile 1960, Mw 9.5 is the largest earthquake ever recorded [6].
• Himalayan thrusts from south to north: HFT → MBT → MCT.

Mains Points

• Distribution of landforms and hazards (GS-I): earthquakes, volcanoes and fold mountains follow plate edges. They are not scattered at random.
• 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.

• Plate tectonics also explains what drift could not: a believable driving force (mantle convection, slab pull, ridge push) and why the ocean floor is young.

• Himalayan seismic risk (GS-III, disaster management): the Indian plate is still moving north, and strain is stored on the Main Himalayan Thrust (slip deficit of 18 mm a year [4]).

• This keeps the Himalaya and north-east India in high seismic zones [8] [11].

• India needs continuous GPS monitoring, earthquake-safe building codes and microzonation (detailed local maps of earthquake hazard).

• Ocean-wide tsunami preparedness (GS-III): one subduction earthquake in Chile in 1960 sent a deadly tsunami across the whole Pacific to Hawaii and Japan [6].

• This supports warning systems that cover a whole ocean.
• An Indian example is the tsunami warning centre at INCOIS, which watches the Andaman-Sumatra subduction zone.

Related concepts

• Tectonic plate
• Major plates
• Minor plates
• Pacific plate
• Nazca plate

Read more

• Plate tectonics: the theory and the plates