Sometimes sedimentary and igneous rocks are subjected to pressures so intense or heat so high that they are completely changed. They become metamorphic rocks, which form while deeply buried within the Earth's crust. The process of metamorphism does not melt the rocks, but instead transforms them into denser, more compact rocks. New minerals are created either by rearrangement of mineral components or by reactions with fluids that enter the rocks. Some kinds of metamorphic rocks--granite gneiss and biotite schist are two examples--are strongly banded or foliated. (Foliated means the parallel arrangement of certain mineral grains that gives the rock a striped appearance.) Pressure or temperature can even change previously metamorphosed rocks into new types.
Common
metamorphic rocks include schist, marble, and gneiss. Sedimentary
rock shale (formed mostly of clay sediments) when buried and heated to
high temperatures (300-500°C) becomes transformed or metamorphosed into
schist.
Information provided by: http://interactive2.usgs.gov