Asthenosphere A soft layer in the Earth’s upper mantle. It lies just beneath the lithosphere. It is partly molten and allows the lithosphere to slide about over it.

Continental drift The movement of the continents around the globe. The continents are attached to the tectonic plates, and are carried by their movements.

Convection current The movement of heat through liquids and gases. Heated from below, a liquid or gas will expand, become less dense, and rise. Away from the source of heat, it will cool down and sink. Convection currents in the Earth’s mantle, together with “pull” from subducting lithosphere, are responsible for continental drift.

Convergent boundary A boundary between two tectonic plates that are pushing against each other. If one of the plates is oceanic, its edge may be forced down under a heavier continental plate. If both plates are continental, they will collide and crumple up into mountains.

Divergent boundary A boundary between tectonic plates that are moving away from each other. Molten rock wells up to fill the gap between the plates. Most divergent boundaries are between oceanic plates and are called mid-oceanic ridges.

Earthquake A shaking or trembling of the ground, caused by the sudden movement of rocks in the Earth’s crust. Most earthquakes happen along the edges of tectonic plates, when plates collide or slide past each other. The two plates lock together before the pressure becomes too much and the rocks suddenly snap apart.

Fault A crack in the Earth’s crust, along which there is movement of one side relative to the other. Faults form when rocks are stretched or bent and crack along weak points.

Fold A bend in the rocks of the Earth’s crust. Folds occur when intense pressure is exerted on rocks by movements in the crust. Folds vary in size from small wrinkles to large mountains.

Lithosphere A layer of the Earth made up of its crust, plus a thin layer of the upper mantle. It is split into large pieces called lithospheric, or tectonic, plates. Tectonic plates slide about on top of the asthenosphere.

Mid-Oceanic Ridge A long mountain range running along the ocean floor. The Mid-Oceanic Ridge is formed where lava erupts between tectonic plates. A series of cracks, called transform faults, runs at right-angles to the ridge.

Plate tectonics The Earth’s outer layer, the lithosphere, is divided into enormous irregular slabs, called tectonic plates. These fit together like a ball-shaped jigsaw. The plates “float” on top of the liquid asthenosphere. Convection currents in the mantle push the plates and makes them move. In some places, the plates push against each other and crumple, forming mountains. In other places, they pull away from one another and molten rock wells up between them. Sometimes plates lock together for a time, then suddenly jolt apart, causing earthquakes.

Rift A place where the Earth’s crust is being pulled apart by the movement of tectonic plates. Rifting causes rocks at the Earth’s surface to crack, creating faults. A block of land may slip down between faults to form a rift valley.

Subduction The process by which one tectonic plate slides beneath another. The place where this occurs is called a subduction zone. A thinner, denser oceanic plate will slide beneath a continental plate when the two collide.

Tectonic plates The large slabs into which the Earth’s surface is divided. Tectonic plates are sometimes called lithospheric plates. Each plate consists of a piece of the Earth’s crust, plus a portion of upper mantle, which together form the lithosphere.

Transform fault A boundary between tectonic plates that are sliding past each other. The San Andreas Fault, which stretches 1200 kilometres (750 miles) along the west coast of North America, is an example of a transform fault.

Consultant: Ian Fairchild