Maarg Manthan · Topic 4.7

Rates and Forces of Plate Movement

Geography › Plate Tectonics and Distribution of Continents and Oceans · Topic 4.7

Plates move at speeds of about 1 to 15 centimetres a year, roughly as fast as fingernails grow. The slowest spreading is at the Arctic ridge and the fastest is on the East Pacific Rise. The Indian plate now moves north at about 5 cm a year. The main forces are slab pull, the weight of a cold sinking plate edge pulling the plate; ridge push from the raised mid-ocean ridge; and drag by slow convection in the mantle. Motions are measured with GPS, satellites and radio telescopes.

5 min read · UPSC Prelims · Geography · Plate Tectonics and Distribution of Continents and Oceans

At a Glance

Typical speed 1 to 15 cm a year
Fastest spreading East Pacific Rise
Indian plate About 5 cm a year
Strongest force Slab pull
Measured by GPS, satellites, VLBI
Exam link UPSC Prelims, Geography: Plate Tectonics and Distribution of Continents and Oceans

Where Does It Fit?

This topic completes the macro by answering the question that stopped Wegener, which is why and how fast the plates move. It ties together the convection, sea-floor spreading and plate boundary topics. Previous topic: Plate Boundaries: Divergent, Convergent, Transform. Next topic: Composition of the Atmosphere: Gases, Water Vapour, Dust Particles.

What Drives the Plates?

Plates move because of forces that act together. Heat in the interior drives slow convection currents in the mantle, which were the first mechanism proposed. Two forces acting on the plates themselves are now considered the strongest. Slab pull is the pull of a cold, dense subducting plate as it sinks into the mantle and drags the rest of the plate behind it. Ridge push arises from the elevated position of the mid-ocean ridge, from which the newly formed plate slides downslope. Plates with long subduction edges, such as the Pacific plate, move fastest, which suggests that slab pull is the dominant force. As an analytical point, mantle convection and plate motion are part of a single system, so scientists now treat plates as the surface layer of mantle circulation and not as a separate cause.

How Fast Do the Plates Move?

Plate or boundary	Approximate speed
Arctic (Gakkel) ridge	Less than 1 cm a year
Mid-Atlantic Ridge	About 2 to 2.5 cm a year (each side about half)
Indian plate (now)	About 5 cm a year northward
Nazca plate	About 7 to 8 cm a year towards South America
Pacific plate	About 7 to 10 cm a year
East Pacific Rise	Up to about 15 cm a year (the fastest spreading)

The Indian plate moved north at 15 to 20 cm a year before the collision with Asia, and slowed down to about 5 cm a year afterward as the Tibetan crust thickened. The Himalaya are still rising by roughly 4 to 5 millimetres a year. The figures are approximate and change with new measurements, so the latest data should be checked.

What Are the Forces That Move the Plates?

Slab pull: the cold, dense edge of an oceanic plate sinks into the mantle at a subduction zone and pulls the rest of the plate with it. This is thought to be the strongest force, since plates attached to sinking slabs, like the Pacific, move fastest.
Ridge push: new crust at a ridge stands higher than the old sea floor, and gravity makes it slide off the raised ridge, pushing the plate away.
Mantle convection and drag: slow currents in the mantle, as in Convectional Current Theory and Mapping of the Ocean Floor, drag the base of the plates, though how much they drive the plates is still debated.
Trench suction: the overriding plate is drawn towards the trench as the slab rolls back.
Mantle plumes: columns of hot rock rising from deep in the mantle may help to split plates, as in East Africa and with the Deccan Traps.

How Are the Movements Measured and Where Do They End?

Scientists measure plate motion using GPS stations, laser ranging to satellites and radio telescopes (VLBI), which detect shifts of millimetres a year. Older records come from the magnetic stripes of the sea floor and from chains of volcanic islands over hotspots, such as Hawaii, that record the direction and speed of the plate. Plates do not move for ever in the same way. The Wilson cycle describes how an ocean opens by rifting, grows by spreading, and later closes by subduction and collision, over a span of several hundred million years, as the Tethys did. The boundaries where this happens are in Plate Boundaries: Divergent, Convergent, Transform. As an analytical point, plate motion is slow in human terms but its effects, in earthquakes and mountains, are the main source of geological hazard.

Why Is This Topic Important?

Unifying theory Explains most landforms
Boundaries Core Prelims content
Hazards Earthquakes and volcanoes

What Came Next?

The next macro moves from the solid Earth to the atmosphere, beginning with its composition. Next topic: Composition of the Atmosphere: Gases, Water Vapour, Dust Particles.

Key People and Terms

Tectonic plate Rigid slab of lithosphere
Subduction Sinking of one plate under another
Slab pull Pull of a sinking plate
Ridge push Push from the elevated ridge
Transform fault Plates slide past each other
Divergent boundary Plates move apart

Exam Corner

Points to Remember

Seven major plates, with many minor ones.
Pacific is the largest plate.
Three boundaries: divergent, convergent, transform.
Slab pull is the strongest driving force.
San Andreas is a transform fault.

Do Not Confuse With

Divergent and convergent Apart versus together
Transform and convergent Sliding versus colliding
Plate and continent A plate can hold continent and ocean

Memory Hook

Apart makes ridges, together makes mountains, sideways makes shakes.

Mains Angle

Questions ask for boundaries and features. These points add depth.

Boundaries Three types with distinct landforms
Forces Convection, slab pull, ridge push
Hazards Boundaries match quake belts

A Question You May Face

An original practice question, not a past paper question.

“Explain the theory of plate tectonics and the landforms associated with the different types of plate boundaries.”

How to Answer

Theory Plates and asthenosphere.
Boundaries Three types.
Landforms Ridges, trenches, mountains.
Forces Convection and slab pull.

GS Relevance

Prelims: rates of movement, slab pull, ridge push, mantle convection. GS Paper 1: plate tectonics and mantle dynamics.

Frequently Asked Questions

What is plate tectonics?

Plate tectonics is the theory that the rigid outer shell of the Earth, the lithosphere, is divided into plates that move slowly over the weak asthenosphere. Most earthquakes, volcanoes and mountain building occur at the boundaries where the plates interact.

What are the major tectonic plates?

The seven major tectonic plates are the Pacific, North American, South American, Eurasian, African, Indo-Australian and Antarctic plates. There are also many minor plates such as the Cocos, Nazca, Arabian, Philippine and Caribbean plates, which are smaller in area.

What are the types of plate boundaries?

There are three types: divergent boundaries, where plates move apart and new crust forms; convergent boundaries, where plates collide and one may subduct; and transform boundaries, where plates slide horizontally past each other without creating or destroying crust.

What drives the movement of tectonic plates?

Plates are driven by heat from the interior through mantle convection, together with slab pull, the pull of a cold sinking plate, and ridge push, the sliding of new plate away from the elevated ridge. Slab pull is considered the strongest force.

What is a transform boundary?

A transform boundary is a plate margin where two plates slide past each other horizontally. Crust is neither created nor destroyed, and shallow earthquakes occur along the strike-slip faults. The San Andreas Fault in California is a well-known example.

PYQ Practice — Statement Analysis

1 The Pacific plate is the largest tectonic plate.

True

It is almost entirely oceanic.

2 New crust is created at transform boundaries.

False

It is created at divergent boundaries.

3 The San Andreas Fault is a transform fault.