Some of the changes occur because the rocks are at great depths. As more
and more younger rocks are deposited on top of them, the older rocks
become deeply buried. The great thicknesses of younger rocks are heavy,
and they squeeze and press down on the rocks beneath them. The deeply
buried rocks are also hotter than surface rocks. In general, the
temperature increases about 1° Fahrenheit for each 50 feet of depth
below the surface. The change of deeply buried rocks into new rocks by
pressure and heat is known as _static metamorphism_.