Volcanoes, earthquakes and tectonic plates

• The crust is the outermost layer, up to about 60 km thick.
• The mantle lies directly below the crust: hard and semi-molten rock.
• The core is made mostly of iron and nickel and is the hottest part. The outer core is liquid (molten); the inner core is solid because of the huge pressure.

The crust is broken into large pieces: tectonic plates. They drift a few centimetres a year on the hot, slowly flowing rock of the mantle. Alfred Wegener proposed that the continents were once joined. The plates move because of convection currents: heat from the core makes the soft mantle rock rise, spread out, cool and sink again, in slow circles that drag the plates along.

• Where plates collide, mountains form or one plate sinks under the other.
• Where plates move apart, new crust forms.
• Where plates slide past each other, they get stuck and then suddenly jerk loose: this causes earthquakes.
• A fault line is a crack in the crust along which plates move.

Deep under a volcano is the magma chamber. Magma rises and bursts out at the crater; above ground it is called lava. An ash cloud rises. The greatest danger is hot ash and gas rushing downhill. People still live near volcanoes, because the soil is very fertile. Magma rises where plates move apart (e.g. Iceland), where one plate sinks under another and melts (e.g. Japan and the Andes), and over hot spots in the middle of a plate (e.g. Hawaii).

Meerkat tip

An active volcano erupts often or has erupted recently. A dormant volcano has not erupted lately but could. An extinct volcano will not erupt again. Most volcanoes lie around the Pacific Ocean.

An earthquake happens when plates suddenly shift. It starts deep down at the focus; the point on the surface right above it is the epicentre, where damage is usually worst. A seismograph records the waves, and we measure the strength on the Richter scale. A big earthquake under the sea can cause a tsunami.