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Metamorphic Rocks Rocks that are formed by heat and/or pressure (but not completely melted) 5/1/2017 Metamorphic rocks are formed by: • Regional metamorphism –Involves mostly pressure • Contact metamorphism –Involves mostly heat 5/1/2017 Metamorphism by Pressure What do you think causes enough pressure to metamorphose miles of rock? 5/1/2017 Causes flattening of mineral crystals 5/1/2017 5/1/2017 Metamorphism by Pressure • Pressure often causes realignment of intergrown mineral crystals –Called banding –Or foliation foliation banding 5/1/2017 Banding 5/1/2017 Foliation 5/1/2017 5/1/2017 Regional Metamorphism • Mountain building activity • Involves large areas 5/1/2017 Large-scale metamorphism 5/1/2017 Metamorphism by Heat Known as contact metamorphism 5/1/2017 Contact Metamorphism • Also known as thermal metamorphism –Creates baked rock • ex. hornfels 5/1/2017 5/1/2017 5/1/2017 Contact Metamorphism • Igneous intrusion –Where molten rock is forced between existing rock –Tends to bake (metamorphose) part of the existing rock 5/1/2017 Increasing degrees of metamorphism Rocks melt in this range 5/1/2017 5/1/2017 Composition of metamorphic rock is determined by: • Conditions of metamorphism • Mineralogy of parent (original) rock 5/1/2017 Great heat and pressure limestone (small crystals) marble (large crystals) In this example the mineral composition stays the same 5/1/2017 shale slate In this example the minerals in shale actually change into other minerals during metamorphosis 5/1/2017 • Some metamorphic rocks can form from several different parent rocks –Ex. Gneiss may form from sandstone, shale or granite 5/1/2017 5/1/2017 Where can you find this image? When sandstone is subjected to heat and or pressure it becomes what? Is marble a foliated or non-foliated metamorphic rock? Name three minerals in schist What is the only metamorphic rock in your ESRT that exhibits banding? 5/1/2017 How to identify rocks Interlocking crystals? Igneous/metamorphic banding/foliation? metamorphic 5/1/2017 igneous sedimentary Rocks • All rocks are classified on the basis of how they formed –Sedimentary rocks form from bits and pieces of existing rock –Igneous rocks form from molten minerals –Metamorphic rocks form from existing rocks that are changed by heat and/or pressure 5/1/2017 Rock Cycle 5/1/2017 Rock Cycle • Nearly all rocks are formed from other rocks • Exceptions include – coal – Fossil limestone • Some rocks exist for thousands to billions of years before undergoing change 5/1/2017 5/1/2017 5/1/2017