Maarg Manthan · Topic 3.5

Plate Tectonics and Types of Plate Boundaries

Geography › Geomorphic Movements and Plate Tectonics · Topic 3.5

Plate tectonics is the theory that the rigid lithosphere is broken into a few large and many small tectonic plates that move slowly over the weak asthenosphere at a few centimetres a year. Most earthquakes, volcanoes and mountain belts occur at plate boundaries, which are divergent where plates move apart, convergent where they collide, and transform where they slide past one another.

5 min read · UPSC Prelims · Geography · Geomorphic Movements and Plate Tectonics

At a Glance

Proposed Late 1960s, by McKenzie, Parker and Morgan
Major plates Seven, with many minor plates
Boundary types Divergent, convergent, transform
Speed About 1 to 15 cm a year
Driving forces Mantle convection, slab pull, ridge push
Exam link UPSC Prelims, Geography: Geomorphic Movements and Plate Tectonics

Where Does It Fit?

This is the fifth topic of the category on geomorphic movements and plate tectonics. The previous topic gave the evidence of seafloor spreading, and this one presents the complete theory. The next topic follows the Indian plate. Previous topic: Seafloor Spreading and Paleomagnetism. Next topic: Movement of the Indian Plate.

What Is Plate Tectonics?

In short: the outer shell of the Earth is a mosaic of moving plates, and their edges are where most geological action happens.

The theory of plate tectonics was proposed in 1967 by McKenzie and Parker, and independently by Morgan, building on continental drift and seafloor spreading. A tectonic plate is a rigid slab of lithosphere, made of the crust and the uppermost mantle, which floats on the asthenosphere and moves as a unit. A plate may contain a continent, an ocean floor or both, and the tectonic plates fit together like the pieces of a puzzle. The plates are 100 km thick on average and move at speeds of about 1 to 15 cm a year, similar to the speed at which fingernails grow. The structure of the lithosphere is described in Crust, Mantle and Core – Internal Structure of the Earth.

Which Are the Major Plates?

Seven large plates are generally recognised as major plates, along with a number of minor ones.

Category	Plates
Major plates	Pacific, North American, South American, Eurasian, African, Indo-Australian and Antarctic
Notable minor plates	Cocos, Nazca, Arabian, Philippine, Caribbean, Caroline, Fuji, Juan de Fuca, Scotia

The Pacific plate is the largest and is almost entirely oceanic. Many geologists treat the Indian and Australian parts of the Indo-Australian plate as separate plates, since the plate is believed to be breaking up along a deformation zone in the Indian Ocean, but the standard classification in textbooks keeps the seven-plate scheme.

What Are the Types of Plate Boundaries?

Boundary	Plate motion	Features	Examples
Divergent	Plates move apart	New crust forms; mid-ocean ridges, rift valleys, shallow earthquakes	Mid-Atlantic Ridge, East African Rift
Convergent	Plates move toward each other	Subduction zones, trenches, volcanic arcs, fold mountains, deep earthquakes	Andes, Himalayas, Japan
Transform	Plates slide past each other	Crust is neither created nor destroyed; strike-slip faults and shallow earthquakes	San Andreas Fault, Alpine Fault

At divergent boundaries the plates move apart and new crust is created, which is covered in Divergent Boundaries and Rift Valleys. At a convergent boundary, a denser oceanic plate subducts below a lighter plate, and where two continental plates collide neither subducts and mountains rise, as shown in the topics on Ocean-Ocean Convergence and Island Arcs and on Continent-Continent Convergence – Himalayas and Tibetan Plateau. A transform boundary has no subduction and no new crust. The San Andreas Fault in California, which marks the boundary between the Pacific and North American plates, is the best-known transform fault.

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.

What Does Plate Tectonics Explain?

The theory explains the distribution of earthquakes and volcanoes, which are concentrated along plate boundaries and not randomly spread, as in Distribution of Volcanoes and Earthquakes – Ring of Fire. It explains the origin of mountain belts, ocean trenches, island arcs, mid-ocean ridges and rift valleys, and the movement of continents over geological time. It also explains why the oceans are young, since ocean floor is continually recycled at subduction zones. Intraplate features such as hotspots, which are not at boundaries, are explained by mantle plumes in Hotspots, Mantle Plumes, Geysers and Hot Springs.

Why Is This Topic Important?

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

What Came Next?

The next topic describes the movement of the Indian plate. Next topic: Movement of the Indian Plate.

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: major plates, plate boundaries, driving forces. GS Paper 1: geomorphology and distribution of earthquakes and volcanoes.

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.