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 begins to melt. This molten rock then rises and erupts on the surface as lava, building up a volcano.
This is why we see volcanoes along plate edges. Sometimes volcanoes can form in the middle of a plate because there happens to be a hot mantle plume there e.g. Hawaii.
The types of volcano, and their shape, are determined by the kind of lava that erupts.
We also are studying the volcanic activity in the Afar Region of Ethiopia where the tectonic plates are moving rapidly apart.
There are over 1500 named volcanoes in the world. To learn where they are go to volcanoes of the world.
The BGS managed the Montserrat volcanic observatory from 1997 to 2008. The scientist's work included: seismic, deformation, environmental and volcanological monitoring. Predictions by volcanic observatories are becoming more reliable, but remain imperfect.