Lecture Metamorphic rocks (2).pdf - EASC1402(2019/2020 Principle of Geology 1 Main Objectives Metamorphism METAMORPHIC ROCKS Metamorphic Minerals

Lecture Metamorphic rocks (2).pdf - EASC1402(2019/2020...

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8/10/2019 1 EASC1402 (2019/2020) Principle of Geology METAMORPHIC ROCKS Dr. MC Cheung Department of Earth Sciences, The University of Hong Kong Email: [email protected] 1 Main Objectives: Metamorphism Metamorphic Minerals Classification of Different Metamorphic Rocks 2 Metamorphic rocks Changed from one type of rock into another by heat, pressure, and chemical processes Mineralogical and textural / structural change in the solid state , but not melting ALL metamorphic rocks are formed from pre-existing igneous, sedimentary or metamorphic rocks. 3 Metamorphism Under new P / T conditions (fluids may be involved), Pre- existing minerals are unstable, which elements recombine to form new minerals Metamorphism occurs deep beneath the Earth’s surface, metamorphic rocks can only be seen when they are uplifted 4 Formation of Metamorphic Rock Metamorphism occurs under increased temperature and pressure, the pre- existing rocks become unstable and altered in composition, texture, or changing in mineral composition to become a new rock. Example of transformation of metamorphic rock types include Shale Slate Schist, and Granite Gneiss. Controlling Factors Heat, pressure and chemical processes 5 Metamorphism Controlling Factors - Heat Most important agent Provide energy to drive chemical reactions Heat from magmatic activities Geothermal Gradient An increase in temperature with depth (20 o 30 o C/km) Recrystallization of individual grains, either keeping the same minerals but coalesce to forming larger crystals, or forming new, stable minerals. 6
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8/10/2019 2 Metamorphism Controlling Factors - Pressure Confining Pressure pressure from surrounding rocks, increases with depth Rocks being squeezed equally in all directions, grain spaces close, produce more compact and dense rock. 7 Metamorphism Controlling Factors - Pressure Differential Stress stress adding on the rock bodies are uneven, greater in one direction Rocks being folded Rocks are shortened in the same direction with the greatest stress and thickened perpendicular to the greatest stress 8 Metamorphism Controlling Factors Chemical Processes Occurred with agent of fluids from seawater, volatiles from magma, hydrated minerals (clay and mica). Enhanced migration of ions leading to increase reaction rate of existing minerals Mineral grains dissolve along the high stress zone and precipitate in the low stress zone As a result, minerals tend to grow longer in the direction perpendicular to compressional stress. 9 Types of Metamorphism The three main types of metamorphism: Regional Metamorphism Contact Metamorphism Dynamic Metamorphism Other Metamorphism Burial Metamorphism Hydrothermal Metamorphism 10 Regional Metamorphism Metamorphic process over a large region, produce the wide range and greatest amount of metamorphic rocks Heat supplied from tectonic activity Associated with convergent plate margins, mountain building processes and
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