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Metamorphic Rocks Standards Classify matter in a variety of ways Describe the composition and structure of Earth’s materials, including: the major rock types (i.e., sedimentary, igneous, metamorphic) and their formation Properties of Metamorphic Rocks • Metamorphic means change in form. • Metamorphic rocks are formed when existing rocks undergo changes. • Sedimentary & igneous rocks can change into metamorphic rocks, and metamorphic rocks can change into different metamorphic rocks. Properties of Metamorphic Rocks • During the process of metamorphism, rocks and minerals change in response to changes in temperature, pressure or other environmental changes. • The mineralogy, texture and/or chemical composition of the rocks may alter during metamorphism. • Most metamorphic reactions (changes) occur while the rocks are in a solid state (if the rock becomes liquid, it is an igneous rock). Texture Changes • The average size of crystals in the rock usually increases. • The shape and arrangement of crystals in the rock change • Ex: limestone changes to marble: still made of the mineral calcite, but it is now dense and its texture is coarse with irregularly shaped, interlocking grains. Mineral Changes • In general, if the original (parent) rock contains one mineral, metamorphism produces a rock of the same mineral. Ex: limestone changes to marble, both are made of calcite • If multiple minerals are present in the parent rock, the new metamorphic rock may have different minerals. Ex: shale contains clay minerals, quartz & feldspar. During metamorphism, these minerals change to chlorite. Formation of Metamorphic Rocks • Metamorphic rocks are formed by high temperature and/or pressure Formation of Metamorphic Rocks 1. Temperature induced changes • Elevated temperatures cause metamorphism: Lower temperatures cause slow changes and produce less of a change. Higher temperatures cause faster changes and produce a greater change Formation of Metamorphic Rocks 2. Pressure induced changes • Chemical reactions are sensitive to pressure (measured in kilobars). • Greater amounts of pressure will produce greater changes. Formation of Metamorphic Rocks 2. Pressure induced changes • Deformation – change in the shape of rocks or minerals in response to mechanical forces Tectonic activity causes pressure that creates foliation in metamorphic rocks. Foliation is a set of flat or wavy planes in a metamorphic rock Formation of Metamorphic Rocks • Types of foliation: Slaty cleavage – rock splits into thin sheets Schistosity – wavy, parallel arrangements of minerals Gneissic – alternating layers, or bands, of light and dark minerals Types of Metamorphism 1. Regional metamorphism 2. Contact metamorphism 3. Metasomatism Regional Metamorphism • Caused by burial or strong tectonic forces that impose high temperatures and pressures on the rocks over a large area. • Pressure plays a greater role than heat • Most metamorphic rocks are of this type. • These rocks are foliated. Regional Metamorphism • Types of regional metamorphic rocks: Rock Names Parent Rock • Slate………………..shale • Phyllite……………..shale • Schist………………shale • Gneiss……………..shale, granite or conglomerate Slate Schist Phyllite Gneiss Contact Metamorphism • Caused by heat and pressure produced by a nearby igneous intrusion. • Does not cover large areas like in regional metamorphism. • Heat plays a greater role than pressure. • These rocks are called granoblastic and are non-foliated. Contact Metamorphism • Types of contact metamorphic rocks: Rock Names Parent Rock • Metaconglomerate……...conglomerate • Quartzite…………………sandstone • Marble……………………limestone Metaconglomerate Quartzite Marble Metasomatism • Super-heated water enters pore spaces or fractures in rocks and creates metamorphic changes. • Many deposits of valuable metal ores, such as copper, zinc and lead, are formed in this way.