METAMORPHISM amp ROCKS - Kean University：变质amp；岩-基恩大学.ppt

METAMORPHISM ROCKS ? Marcia Anderson Metamorphic Rocks Metamorphic rocks are highly deformed. Usually caused by intruded igneous masses. Metamorphism: Transformation of rocks by heat, pressure chemically active fluids. Change is textural and mineralogical . Always occurs deep within the earth. Provides clues @ geologic processes w/in earths crust Must remain solid, otherwise becomes igneous. Rock becomes unstable under heat pressure gradually changes. Metamorphism: Occurs during mountain building; intense stress temperatures associated with large scale deformation. This Large scale deformation is regional metamorphism. Regional Metamorphism Occurs during mountain building. Crust is squeezed and deformed, shortened and thickened. Thickening forms mountains. Cores of many mountains consist of intrusive igneous bodies surrounded by high-grade metamorphic rocks. Regional Metamorphism Since they are formed under direct stress, metamorphic rocks are usually foliated. Shale ? slate ? mica schist ? feldspar/ hornblende ? gneiss Appalachian Mts.: Ohio--Cent. Pa --E. Pa.--NY --Vt../ N.H. Metamorphism Plate Tectonics Most metamorphism occurs at convergent zones. Compressional forces squeeze deform the converging plates accumulated sediments along the continental margins. As lithosphere descends, pressure increases faster than temp. Material is continually transported to great depths at convergent plate boundaries. At only a few km., clay becomes unstable, it recrystallizes into minerals: (muscovite). Shields Extensive areas of metamorphic rocks igneous plutons on every continent. Canadian shields forms the bedrock over much of no. Canada from Hudson Bay to No. Minnesota. Among oldest rocks on earth. Remnants of earlier mountain building. Contact Metamorphism Molten rock comes in contact with country rock Zone of alteration; aureole, or halo forms @ the magma body Near the magma body, high temp minerals form. le: garnet. Further away: low-grade minerals