CH 7 - Metamorphism

phase change

transforms one mineral into another mineral (polymorph) with the same composition but a different crystal structure… rearrangment of atoms

what happens in a metamorphic reaction?

results in growth of new minerals that differ from those of the protolith. chemical reactions digest minerals of the protolith and cause new minerals to grow

pressure solution

surface of one mineral grain pushes against the surface of another, under conditions where a water film separates the grains. the grains dissolve at the surface of contact, and the resulting ions migrate away. the ions may precipitate on the sides of grains that aren’t being pushed together as tightly

what leads to solid state diffusion?

heat in a mineral grain causes atoms to vibrate rapidly, stretching and bending chemical bonds that lock the atoms to their neighbors. if bonds stretch too far and break, atoms detach from their original neighbors and form new bonds with other atoms. repetition of this process leads to solid state diffusion where atoms either rearrange within grains or migrate into and out of grains

temperature range of metamorphism

250-850

what are the two types of differential stress?

1. normal stress - pushes or pulls perpendicular to a surface

2. shear stress - moves one part of a material sideways relative to another part

what is a supercritical fluid?

substance that forms under high temperatures and pressures and has characteristics of both liquid and gas

metasomatism

hydrothermal fluids passing through a rock during metamorphism pick up some ions of one element and drop off ions of another, changing the overall chemical composition of the rock

migmatite

under high T in the presence of water, gneiss may begin to melt, producing felsic magma and leaving behind solid, relatively mafic metamorphic rock. if the melt freezes againt before flowing out of the source area, a mixture called migmatite, consisting of igneous rock and relict metm. rock

dynamic metamorphism

at greater depths, rock becomes warm enough to behave like soft plastic when shear happens along the fault. during this process, the minerals in the rock recrystallize… resulting rock is a mylonite

dynamothermal metamorphism

metamorphism that involves heat, compression and shearing (during mountain building)

hydrothermal metamorphism at MORs

• cold sea water leaks through cracks in crust and heats up and transforms into hydrothermal fluid

• water rises near the ridge and causes hydrothermal metamorphism of ocean floor basalt

how is foliation created in a rock?

when rock is compressed or sheared during metamorphism, causing mineral crystals to grow or rotate into alignment with each other

what is a shield?

broad region of long-lived, stable continental crust where sedimentary cover either hasn’t been deposited or has’t been eroded