The Earth's outer layer, the crust, is broken into huge pieces called tectonic plates. They float on hot, partly melted rock and move very slowly — only a few centimetres each year, about as fast as your fingernails grow.

Where plates meet, their edges can lock together. Pressure builds up until the rock suddenly slips. The energy released travels through the ground as waves, causing an earthquake.

Deep underground it is hot enough to melt rock into magma. Magma can rise through cracks in the crust. When it erupts from a volcano it is called lava, and it can come with ash and gases.

When an earthquake happens under the sea, the sea floor can suddenly lift or drop, pushing a huge amount of water. This makes a tsunami. In deep ocean the waves are low but move as fast as a jet plane. As they reach shallow water near the coast they slow down and grow very tall.

Warning centres use seismometers and ocean sensors to detect earthquakes and tsunamis and send out alerts. Scientists also watch volcanoes for small earthquakes, swelling ground and gas, which can be signs an eruption is coming.