Plate tectonics

Plate tectonics Juan plate Phillipine plate North American plate North American plate Caribbean plate Arabian plate Eurasian plate Australian plate African plate South American plate Cocos plate Antarctic plate Indian plate Pacific plate Pacific plate Scotia plate Pacific plate Nazca plate

Observations show that most earthquakes are associated with tectonic plate
boundaries. The theory of plate tectonics can be used to provide a simplified explanation of the global distribution of earthquakes, while some of the characteristics of earthquakes can be explained by using a simple elastic rebound theory.

Movement of tectonic plates

The Earth’s outermost layer is fragmented into about 15 major slabs called tectonic plates; these slabs form the lithosphere. The lithosphere comprises the crust (continental and oceanic) and the upper part of the mantle.

The tectonic plates are large solid pieces of lithosphere, ranging in thickness from 5–10 km in deep oceans to over 70 km beneath large mountain ranges, which together form the Earth’s crust.

The Earth's tectonic plates move very slowly relative to each other, typically a few centimetres per year. Hover over the adjacent tectonic map of the world to see how quickly each plate is moving and its direction of movement.

Seismologists associate different kinds of seismic activity with what is happening at different types of plate boundaries. Boundaries between tectonic plates are made up from a system of faults. Each type of boundary is associated with one of three basic types of fault, called normal, reverse and strike-slip faults.

Divergent boundary

Divergent boundary

Plates can move apart at a boundary. This type of boundary is called a divergent boundary. It is also referred to as a constructive plate boundary, as new material is being produced at the boundary surface. This type of boundary is dominated by normal faulting although other types of faulting may be observed.

Hot magma rises from the mantle at mid-ocean ridges pushing the plates apart. Earthquakes occur along the fractures that appear as the plates move apart.

Examples include the East African rift and mid-ocean ridges where two ocean plates are moving apart, such as the regions near the Azores and Iceland.

They are associated with volcanic activity, and the earthquakes in these zones tend to be frequent and small.

Convergent boundary

Continental collisions result in the creation of mountains and fold belts as the rocks are forced upwards.

Plates can move towards each other at a boundary. This type of is called a convergent boundary. This type of boundary is dominated by reverse faulting although other types of faulting may be observed.

Destructive boundary

Destructive boundary

When the boundary is between an oceanic plate and a continental plate, it is also referred to as a destructive plate boundary.

At subduction zones, the oceanic plate is pushed down, or subducted, below the continental lithosphere. As the oceanic slab decends, earthquakes are generated within the slab and at the interface between the plates.

Examples include deep ocean trenches like the Peru–Chile trench, where the Nazca plate (an oceanic plate) is being subducted under the South American (continental) plate, i.e. the oceanic plate is forced underneath the continental plate.

These boundaries tend to produce most of the earthquakes that have magnitudes greater than 6.0, and subduction zones produce the deepest earthquakes.

Continental collisions

Convergent boundary

Where the boundary is between two continental plates, one plate crumples upwards over the other instead of one plate being subducted.

Examples include the boundary between the Eurasian plate and the African plate, and the boundary between the Indian plate and the Eurasian plate, where the Himalayas are formed where the Eurasian plate is forced up and over the Indian plate.

This type of boundary tends to produce a diffuse zone of activity.

Continental collisions result in the creation of mountains and fold belts as the rocks are forced upwards.

Transform boundary

Transcurrent boundary

Plates can move past each other in the same plane at a boundary. This type of boundary is called a transform boundary. This type of boundary is dominated by strike-slip faulting although other types of faulting may be observed.

Where two plates slide past each other, earthquakes originate at shallow depths. California is a good example of this type of boundary.

This type of plate boundary is also referred to as a conservative plate boundary, as it involves movement but no loss or creation of material at the surface.

Examples include the San Andreas fault and the Anatolian fault.

Transform boundaries typically produce large, shallow-focus earthquakes. Although earthquakes do occur in the central regions of plates, these regions do not usually have large earthquakes.

Classroom activities

What is an earthquake? Movement of faults and boundaries using foam blocks

Using pieces of foam or card you can model the movement of tectonic plates in different kinds of faults and boundaries.

Modelling an earthquakeModelling an earthquake

In these three tasks, you are going to use a brick being pulled along a surface covered in sandpaper to model the behaviour of an earthquake.

External links

Earth Learning Idea Continents in collision | Earth Learning Idea

Modelling processes at a destructive (convergent) plate margin.

Related topics

Elastic rebound Elastic rebound theory

The process of ground being subjected to a growing force until it snaps or breaks is explained in a theory called the elastic rebound theory.

Driving tectonic plates What drives the movement of tectonic plates?

There are a number of competing theories that attempt to explain what drives the movement of tectonic plates.

Core structure The structure of the Earth

Studying the signals from distant earthquakes has allowed scientists to determine the internal structure of the earth.

Map of quakes Why and where?

Earthquakes do not occur randomly on the Earth. The pattern of earthquake locations can be explained by assuming the Earth's surface is made up from rigid plates that are in motion relative to each other.