There are 7 major tectonic plates. When they move together or slide by each other it creates earthquakes. When they press together they form mountains. Thats how the alps were formed! Just like that if the two tectonic plates that had formed a mountain separate, lava will come out and therefor we call them volcanos , as there is magma under the plates when it comes to the surface it is then called lava.
Hope this helped!
(don't know if this is what your looking for)