metamorphic rock

Aggregate quarry. Aggregate is a material used in construction and building as a component of concrete or mortar. Some commonly used aggregrates are sand, crushed stone, burned clay, gravel, and crushed or broken slag from blast furnaces.

Rock altered in structure, composition, and texture by pressure or heat after its original formation. (Rock that actually melts under heat is called igneous rock upon cooling.) For example, limestone can be metamorphosed by heat into marble, and shale by pressure into slate. The term was coined in 1833 by Scottish geologist Charles Lyell. Metamorphism is part of the rock cycle, the gradual formation, change, and re-formation of rocks over millions of years.

The mineral assemblage present in a metamorphic rock depends on the composition of the starting material (which may be sedimentary or igneous) and the temperature and pressure conditions to which it is subjected. For example, a clay rich in sediment might become a slate when metamorphosed at low temperature and pressure, a mica-schist at a higher temperature and pressure, or a gneiss if temperature and pressure are very high.

Thermal metamorphism, or contact metamorphism, involves mainly heat changes as in rocks adjacent to an igneous body; dynamic metamorphism occurs with changes in stress as in a fault. Regional metamorphism involves both heat and pressure and is associated with rock deformation taking place at convergent plate boundaries (see plate tectonics). Most metamorphism involves little change in bulk chemistry except for loss or gain of water and other volatiles; chemical changes may, however, occur owing to the action of fluids. Very high-grade metamorphism can cause a rock to melt, and some granites appear to have been formed in this way.