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Notes from section 6.3 Metamorphic Rocks pages 145-151 Metamorphic rocks form when preexisting rocks are exposed in increases in temperature (heat) and pressure. Temperature and pressure increase with depth in the Earth. When high temperature and pressure combine and change texture, mineral composition or chemical composition of a rock without melting it, a metamorphic rock forms. Metamorphism means changing form. During metamorphism, a rock changes form while remaining solid. The high temperatures come from the Earth’s internal heat, either from deep burial or igneous intrusions, and the pressure comes from deep burial or during mountain building. Metamorphic textures – metamorphic rocks are classified into two texture groups foliated metamorphic rocks are characterized by layers or bands of minerals that are perpendicular to the pressure applied on them. Nonfoliated rock are composed mainly of minerals that form with blocky crystal shapes. Metamorphic rocks come in 3 grades: low grade has low pressure and low heat High grade has high pressure and high heat Intermediate grade is in between low and high Types of metamorphism: Regional metamorphism is when high temperatures and pressure affect large regions of Earth’s crust. The grade of regional is from low to high. The result of this is changes in mineral and rock types, plus folding and deforming of the rock layers that make up the area. Contact metamorphism is when molten material, such as an igneous intrusion ( magma comes in contact with the surrounding rock), comes in contact with solid rock. This is from high temperatures and moderate to low pressure. Temperatures decrease with distance from the intrusion, therefore the metamorphic change also decrease with distance. Hydrothermal metamorphism is when very hot water reacts with rock an alters its chemical and mineral composition. As hot water flows in and out during metamorphism, the original mineral composition and texture can change. Theses changes are very common during contact metamorphism near igneous intrusions and active volcanoes. Valuable deposits of lead, gold, copper are formed this way.