Metamorphic rocks are rocks that have been changed. Previously, they were igneous, sedimentary, or other metamorphic rocks. Almost all metamorphism happens deep in young mountain ranges as rocks are folded and compressed underneath other rocks. Although rock never actually melts, the texture and nature of the rock may become like new, made of different, metamorphic crystals (p. 23), with no sign of the original minerals or textures. Part of this occurs in response to pressure, and part is the result of the heat. Metamorphism takes a long time, first for the rock to be buried to a depth where metamorphism can begin, and then for the solid recrystallization to take place. Eventually, metamorphic rocks are exposed at the Earth's surface, but only after the mountain chain is uplifted and eroded deeply (pp. 46-47). This process in which sediment is buried and made into metamorphic rocks may take 100 million years.