CHAPTER 8 Metamorphism A Process of Change Metamorphism Metamorphism refers to the changes in a rock that result in the formation of metamorphic assemblage or a metamorphic foliation in response to change in temperature or pressure to the application of differential stress and to interaction with hydrothermal uids metamorphism is induced by heat pressure differential stress heat can be added by burial depth or contact with hot ground water pressure is added with depth differential stress arises at fault zones and over broad regions during orogenesis chemical weathering and low temperature diagenesis are process of chemical alteration but are not included within metamorphism Metamorphism involves recrystallization neocrystallization phase changes pressure solution or plastic deformation If hot water solutions bring in or remove elements we say that metasomatism has occurred 1 How are metamorphic rocks different from igneous and sedimentary rocks Igneous Rocks formed by the cooling of magma molten rock inside the Earth or on the surface Sedimentary Rocks formed from the products of weathering and erosion on the Earths surface when sediment is deposited out of air ice wind gravity or water ows carrying particles in suspension Metamorphic Rocks formed from a preexisting rock that undergoes a solid state change by temperature and pressure changes inside the Earth Migmatite is a mixture of igneous and metamorphic rock that forms under conditions where partial melting begins Protoliths affected by metamorphism can be of any type igneous sedimentary or metamorphic Protolith the original unmetamorphosed rock from which a given metamorphic rock is formed 2 What two features characterize most metamorphic rocks Foliation Texture Metamorphic rocks can be separated into two classes depending on whether the rock contains foliation Foliated Rocks Non Foliated Rocks quartzite marble 3 What is metamorphic foliation and how does it form Metamorphic foliation is the development of planar fabric It can be de ned either by preferred mineral orientation aligned inequant crystals or preferred mineral associations composition banding Preferred mineral orientation develops where differential stress causes the squashing and shearing of a rock so that its inequant grains align parallel with each other 4 How is a slate different from a phyllite How does a phyllite differ from a schist How does a schist differ from a gneiss Slate further metamorphosed nest grain foliated rock contains slatty cleavage thin sheets Phyllite ne grained orientation of white mica Schist medium to coarse grained large mica akes schistosity Gneiss compositionally layered rock with a stripped appearance Mica crystals within schist are larger than within phyllite gneiss is generally more coarsely crystalline and has color banding schist smells bad 5 How does prograde metamorphism differ from retrograde metamorphism Prograde metamorphism alteration of metamorphics to produce higher grade rocks temp and pressure progressively increase Retrograde metamorphism alteration of metamorphics to produce lower grade rocks temp and pressure progressively decrease 6 Describe the geologic settings where thermal dynamic and dynamothermal regional metamorphism take place Dynamic fault zone shear zone dynamically metamorphosed rocks form along faults where rocks are only sheared under metamorphic conditions shearing along plate boundaries causes dynamic metamorphism minerals in the rock recrystallize requires no change in pressure or temperature Thermal contact occurs in an aureole surrounding an igneous intrusion because there is no shearing involved nonfoliated rocks form in contact aureoles igneous plutons in rifts cause thermal metamorphism it develops in response to heat without a change in pressure and without differential stress Dynamothermal regional results when rocks are buried deeply during mountain building the process of mountain building in convergent zones causes dynamo thermal metamorphism includes heat and compression and shearing effects a large region forms only at accretionary prism of subduction zones 7 Why does metamorphism happen at the site of meteor impacts and along mid ocean ridges when a meteorite slams into the Earth the heat pressure from it are de nitely suf cient to cause suf cient to cause metamorphic changes to the rock cold seawater sinks through cracks along ridges heats up and transforms into hydrothermal uid uid then rises through crust causing hydrothermal metamorphism strike causes massive instant increase in heat pressure at impact sight and causes shock metamorphism 8 How does plate tectonics explain the peculiar combination of low temperature but high pressure minerals found in a blueschist Blueschists unusual metamorphic rocks base of prism experiences high pressure because of weight above it but subjected oceanic crust underneath is cool 9 Where would you go if you wanted to nd exposed metamorphic rocks How did such rocks return to the surface of the Earth after being at depth in the crust Found at mountain ranges and continental shields results from exhumation rock caught between two convergent plates is squeezed upward as mountain range grows crust beneath it warms up and becomes soft and weak range begins to collapse under its own weight upper crust stretches horizontal thinning bringing lower crust to the surface Metamorphic Grade High grade formed at high temperatures and pressures Low grade formed at lower temperatures and pressures Intermediate grade rocks develop under intermediate conditions Different assemblages of minerals form at different grades CHAPTER 10 FAULTS Fault A fracture on which sliding occurs Hypocenter Focus the place where rock breaks and earthquake energy is released Epicenter the point on the ground directly above the hypocenter Active faults are faults on which movement is currently taking place displacement on active faults that intersect the ground surface may yield a fault scarp fault scarp a small step or offset on the ground surface where one side of a fault has moved vertically with respect to the other Inactive faults ceased being active long ago but still can be recognized because of the displacement across them Fault trace the intersection of the fault with the ground 1 Compare normal reverse and strike slip faults We can distinguish between normal reverse thrust and strike slip faults on the basis of the relative motion of rock across the fault the amount of movement
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