The Earth's crust is made up of HUGE plates, the plates fit together like a jigsaw puzzle. These plates sometimes move. When they move, it causes movement which then creates earthquakes and magma eruptions at the side of the plates. After these small eruptions the magma makes its way to the surface. When the magma is at the surface it erupts to form lava flows and ash clouds. Volcanic ash can go over 30km into the air. Some volcanoes can take thousands of years to form, but some volcanoes can form quite quickly.