The surface of the Earth is made up of rigid plates that move at a rate of a few centimetres per year.
When they collide, one plate can be pushed beneath another. As the plate sinks it heats up and dehydrates: water is released from minerals and cracks in the sinking plate. This water is hotter than the surrounding rocks and rises up into the mantle. The addition of these hot fluids lowers the pressure and causes the mantle rocks to melt. This molten rock then rises and erupts on the surface building up a volcano.
Volcanoes also form when plates move apart. Magma rises up and erupts on the surface as lava where the plates separate, for example, along the Mid Atlantic Ridge.
This is why we see volcanoes along plate edges, for example all around the Pacific plate; North and South America, Japan and the Philippines. Sometimes volcanoes can form in the middle of a plate like Hawaii. Hawaii is there because of a hot mantle plume which rises up from very deep in the mantle, bringing hot magma to the surface. Different types of volcanoes form at different tectonic settings.
There are over 1500 named volcanoes in the world. To learn more about them, visit volcanoes of the world.
The BGS studies volcanoes all over the world, including volcanoes on Iceland and on the Caribbean island of Montserrat. We are also studying the volcanic activity in the Afar Region of Ethiopia where the tectonic plates are moving rapidly apart.
The BGS managed the Montserrat Volcanic Observatory from 1997 to 2008. The scientist's work included: seismic, deformation, environmental and volcanological monitoring. Volcano observatories use real-time and close to real-time data from observations, sampling and monitoring equipment to draw conclusions about the state of a volcano and forecast likely short-term behaviour, often quite successfully if monitoring networks are good. Long-term forecasts tend to rely on the past behaviour of the volcano and are based on historical and geological evidence.