metamorphism

metamorphic rocks

produced from preexisting igneous, sedimentary or metamorphic rock

these things can alter the mineral compositions and crystalline textures

temperature, pressure, and the chemical environment of rocks

metamorphic changes

occur in the solid state, so there is no melting (partial melting = anatexis)

drivers of metamorphism

temperature, pressure, and fluids

temperature

• increases with depth

• can break and reform chemical bonds

• geothermal gradient varies (10 degrees - 60 degrees celsius per km)

pressure

• increases with depth

• force per unit; also called stress

• can influence rock chemistry, mineralogy, and texture

• this gradient is between 0.3-0.4 kbar per km

two types of stress

• confining stress

• differential stress

confining stress

force applied is equal in all directions (uniform)

differential stress

force applied is greater in one direction

fluids

• can cause chemical reactions

• create more vigorous reactions at higher temperatures

• can alter mineral composition w/o changing rock texture

metasomatism

the change in the composition of a rock by fluid transport of chemical substances into or out of the rock.

regional metamorphism

• directed pressures and high temperatures during mountain building

• produces the greatest volume of metamorphic rock

• most widespread form of metamorphism
  

Contact, or thermal, metamorphism

• occurs near a body of magma

• changes are driven by a rise in temperature

• high temperature no pressure

seawater metamorphism

• form of metasomatism

• basalt is altered into hydrous minerals

non foliated textures

• uniform stress

• contain equidimensional crystals

• resembles coarse grained igneous rock

foliation

• differential stress

• minerals are in parallel alignment

• alignment is perpendicular to the compressional force

Arabia mountain gneiss

foliated with signs of partial melting, such as dikes

porphyroblastic

large crystals set in fine grained matrix

examples of foliated rocks

• slate

• phyllite

• schist

• gneiss

• parent rock: shale, sandstone

examples of granoblastic (non foliated) rock

• hornfels

• quartzite

• argillite

• marble

• granulite

• Parent rocks: shale, quartz rich sandstone, limestone, dolomite, sale, mudstone, basalt

Examples of porphyroblastic rock

• slate to gneiss or schist

• partent rock: shale