Volcanoes are usually formed at the boundaries of tectonic plates. When two plates collide, one plate can be forced beneath the other in a process called subduction. The subducted plate sinks into the Earth's mantle and rises in the process, creating magma. Magma is hot, molten rock. it can come to the Earth's surface through weak points in the crust. When magma reaches the surface, it erupts. This leads to the formation of volcanoes.
