Plate Tectonics

What are tectonic plates?

The Earth's outer shell, the lithosphere, is broken into large pieces called tectonic plates. These plates sit on top of the semi-molten asthenosphere and move very slowly, typically a few centimetres per year, driven by convection currents in the mantle. Heat from the core causes magma to rise, spread sideways, cool and sink again, dragging the plates along with it.

There are around fifteen major plates, including the Eurasian, Pacific, African, North American and South American plates. The boundaries where plates meet are the most geologically active areas on Earth, which is why earthquakes and volcanoes cluster along these edges.

Types of plate boundary

Constructive (divergent) boundaries

At a constructive boundary two plates move apart from each other. Magma rises from the mantle to fill the gap, creating new crust. This process forms mid-ocean ridges such as the Mid-Atlantic Ridge, where Iceland sits. Volcanic eruptions at constructive boundaries tend to be gentle, with runny basaltic lava flowing rather than exploding. Iceland provides a clear example: the island straddles the boundary between the Eurasian and North American plates, and volcanic activity is frequent but relatively predictable.

Destructive (convergent) boundaries

At a destructive boundary two plates move towards each other. When an oceanic plate meets a continental plate, the denser oceanic plate is forced beneath the lighter continental plate in a process called subduction. The subducted plate melts in the mantle, and the resulting magma can rise through weaknesses in the crust to form explosive composite volcanoes. Deep ocean trenches also form at these boundaries. Japan sits on a destructive boundary where the Pacific Plate subducts beneath the Eurasian Plate, making it extremely prone to both earthquakes and volcanic eruptions. The devastating 2011 Tohoku earthquake and tsunami, measuring magnitude 9.1, demonstrated the immense power released at destructive margins.

Conservative (transform) boundaries

At a conservative boundary two plates slide past each other horizontally. No crust is created or destroyed, but friction between the plates causes stress to build up. When the stress is finally released, it produces powerful earthquakes. The San Andreas Fault in California is the world's most famous conservative boundary, where the Pacific Plate slides north-west past the North American Plate. There are no volcanoes at conservative boundaries because no magma reaches the surface, but the earthquakes can be severe; the 1906 San Francisco earthquake caused widespread destruction.

Why do earthquakes happen?

Earthquakes occur when stress that has built up along a fault line is suddenly released as seismic waves. The point underground where the earthquake originates is called the focus, and the point on the surface directly above is the epicentre. Shallow-focus earthquakes tend to cause the most damage because the energy does not have far to travel. Earthquake magnitude is measured using the moment magnitude scale, and the effects depend on depth, distance from the epicentre, population density and the quality of buildings.

Why do volcanoes form?

Volcanoes form where magma from the mantle reaches the Earth's surface. At constructive boundaries, magma fills the gap between diverging plates. At destructive boundaries, the melting of the subducted plate generates magma that rises through the overlying crust. Some volcanoes, like those in Hawaii, form over hotspots where plumes of exceptionally hot magma rise from deep within the mantle, independent of plate boundaries.