What are metamorphic rocks?

Metamorphic rocks are created by the physical or chemical alteration by heat and pressure of an existing igneous or sedimentary material into a denser form. Due to the action of plate tectonics, compression, stress and shearing forces over long periods of time, rocks can be essentially warped and deformed, causing them to be compacted into a smaller volume of space. As a consequence, metamorphic rocks are always more dense than their original material, and also much less susceptible to erosional breakdown. As the Earth's plates move over geologic time, a plate containing igneous or sedimentary rock may become subducted under another plate. The sheer weight of the material above it can cause the rock to undergo metamorphism. In some cases, heat from the Earth's interior can melt the rock slightly, in a process termed "contact metamorphism." Examples of metamorphic rocks are schist (converted basalt), quartzite (compressed sandstone), and marble (compressed limestone or dolomite). 

A metamorphic rock is a result of a transformation of a pre-existing rock. The original rock is subjected to very high heat and pressure, which cause obvious physical and/or chemical changes. Examples of theserock types include marble, slate, gneiss, schist.