High pressures and temperatures within the Earth's crust can change rocks from one type to another. Scientists call this process metamorphism, which comes from the Greek words for "after" and "form." At the junctions where the Earth's crustal plates collide, pushing up mountain ranges, or where one plate slides underneath another, the original rocks, or protoliths (from the Greek words meaning "first" and "rock") undergo reactions that change the chemical or crystal structure of the rocks with little or no actual melting. The grains of minerals within many metamorphic rocks are aligned in parallel due to the forces pushing on them. The pressure-temperature combination causes some sedimentary rocks to lose water molecules or become anhydrous.