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Metamorphic Rocks and Rock Cycle Shmulik Marco Three Basic Types of Rocks 1. Igneous Rocks formed by solidification of molten rock (magma). Examples: basalt, granite. 2. Sedimentary Rocks formed as layers of sediments accumulate. Examples: sandstone, limestone. 3. Metamorphic Rocks Formed by transformation of preexisting rocks in the solid state under high pressures and temperatures Differential Pressure Lithostatic Pressure Metamorphism: Processes of generating metamorphic rocks. Rocks undergo mineralogical and physical changes in solid state Metamorphic Textures • Foliation – set of flat or wavy parallel surfaces produced by deformation – define a preferred orientation, usually perpendicular to directed pressure and parallel to shear – often expressed in orientation of “platy” minerals such as micas Schist – Preferred Orientation Foliation Foliated Rock: Schist 5 cm 10 cm Metamorphic Textures: Foliation and Cleavage • Foliation – set of flat or wavy parallel surfaces produced by deformation – define a preferred orientation, usually perpendicular to directed pressure and parallel to shear – often expressed in orientation of “platy” minerals such as micas and chlorite Granite – Equidimensional Crystals, No Preferred Orientation Gneiss with Lineation Gneiss – Preferred Orientation Lineation Metamorphic Textures Lineation Ductile Deformation: Folds Quartzite No preferred orientation Metamorphic Minerals Grow in rocks only during metamorphism (solid state!) Indicative of pressure and temperature Do not form from cooling magma Contact Metamorphism Contact Metamorphism: The main metamorphic agent is heat. Pressure & Temperature Regimes Pressure & Temperature Regimes •Pressure – Increases with depth at about 1/3 kilobar per kilometer – Does not vary much with geographic position or platetectonic setting Pressure & Temperature Regimes •Temperature: Increases with depth at a rate that varies substantially with plate-tectonic setting •20ºC per kilometer in stable continental crust •40ºC per kilometer in orogenic belts •60ºC per kilometer in volcanic regions Pressure & Temperature Regimes •Deformation stress – Direction varies substantially with plate-tectonic setting – Magnitude limited by strength of rocks to be less than 1 kilobar Metamorphic Grade •Geologists determine pressure and temperature from mineral assemblages –geothermometers –geobarometers 60º/km 40º/km 20º/km Types of Metamorphism •Regional metamorphism – caused by large-scale lithospheric movements •Contact metamorphism – caused by high temperatures near magmatic intrusions •Deformation metamorphism – caused by faulting, tight folding, and shearing •Hydrothermal metamorphism – caused by circulation of hot fluids •Burial metamorphism – caused by subsidence and burial of sediments PTt Path 1. Original rock forms: t0 2. Burial, pressure and temperature increase, metamorphism, : t1 - t3 3. Exhumation: t4 - t5 t0 t5 t1 P t4 t2 t3 T Rock Cycle סלעים מגמתיים התכה אירוזיה מטמורפוזה התכה סלעים מטמורפיים אירוזיה מטמורפוזה סלעים סדימנטריים Rock Cycle