

Human: What exactly causes volcanoes to form?

Assistant: 
From Wikipedia, “The development of volcanoes can be explained by the displacement of crustal plates. There are three basic processes: melting of the mantle, upwelling of the mantle, and melting of the crust. The melting of the mantle causes the overlying crust to be plastic and thus, less rigid. Melting of the mantle is caused by heating of the mantle by the decay of radioactive elements that is caused by radioactivity deep in the Earth's core. Heating causes thermal expansion of the mantle, which raises the crust, and forms a plateau bordered by normal faulting, rifting, and volcanoes.
Volcanic activity in the crust of the Earth takes place as part of the "Plate Tectonics" process.  During this process, pieces of the Earth's crust slide past each other, like pieces of a jigsaw puzzle, and sometimes friction in the plates can cause them to melt and form magma, which eventually causes volcanoes to erupt.  The formation of new volcanoes on the Earth is an ongoing process.