Heat and pressure
Pressure is when a force presses on an area. When heat and pressure combine they can do wonders. They can change a igneous rock to a metamorphic rock. Heat and pressure could also change a sedimentary rock to a metamorphic rock. The deeper the rock goes beneath the earth's surface the greater the pressure. Think about the ocean for an example. The deeper a submarine goes the more water and pressure is built on the submarine. That's why people can't go to the very depths of the ocean without gear. Back to heat and pressure though. Under great pressure and very high temperatures does the rock's minerals change. When the minerals in the rock change the type of rock (igneous, sedimentary, metamorphic) changes as well.