Inside their cocoons, caterpillars change into butterflies. This is called metamorphism, from a Latin word that means “to transform.” Inside Earth, heat and pressure change a rock’s minerals from one set to another. This, too, is called metamorphism. In each case, the basic ingredients stay the same. They’re just reassembled into a new form.

The basalt pictured above is made up of brown glass and the mineral olivine. As it erupted and cooled, escaping gas bubbles created its spongelike texture.

If basalt is buried to 2,000-4,000 atmospheres pressure, equivalent to depths of 7-15 km, and heated to 350°C (660°F), its minerals and glass react to form new minerals-epidote and albite. The result is a metamorphic rock: greenstone.

If greenstone is buried to 18,000 atmospheres, equivalent to a depth of 60 km, and heated to 700°C (1,290°F), its minerals convert to other ones, including garnet and clinopyroxene. The result is a new metamorphic rock: eclogite.