Volcanoes are weak points in the earth’s crust where the liquid magma can build up close to the surface. Volcanoes can take up many different shapes. In the middle of the atlantic, two continental plates are separating and the magma is oozing up between them in a very flat crevice like volcano. Yellowstone national park in the US is a supervolcano. In the pacific, Hawaii and other islands were formed due to convection currents under the mantle forming hot spots. These hot spots remain in the same spot as the plates move over them, and when the magma bursts through under pressure it can rapidly form island. This is why volcanic islands tend to form as chains. Most volcanoes erupt because magma inside them begins to increase in volume. This volume increase causes the pressure to increase, increasing the forces on the solid surface of the volcano. Just prior to an eruption, there will often be a period of minor earthquake tremors and a bulge may form on the side. Once it erupts the pressure is released all in one go and this can be devastating. Mount St Helens, Vesuvius and Krakatoa are all famous examples that you can google.