Module 5: Metamorphic Minerals, Rocks & Processes

Metamorphism

changes in a rock’s mineralogy or texture due to heat, pressure, and/or fluids without complete melting

metamorphic rock

rock formed by the alteration of a pre-existing rock through metamorphism

protolith

original rock before metamorphism

metamorphic grade

degree of metamorphism, generally reflecting increasing temperature and pressure

foliation

planar alignment or layering of minerals in a metamorphic rock

metamorphic fabric

arrangement and orientation of minerals within a metamorphic rock

slaty cleavage

very fine foliation that allows rock to split into thin, flat sheets

phyllitic foliation

fine-grained, often wavy foliation with a silky sheen

schistosity

foliation formed by aligned, visible platy or elongated minerals

gneissic banding

alternating light and dark mineral-rich bands

porphyroblast

large metamorphic crystal grown within a finer-grained rock

index mineral

mineral indicating particular metamorphic pressure-temperature conditions

isograd

boundary marking the first appearance of a particular index mineral

metamorphic facies

group of mineral assemblages associated with particular pressure-temperature conditions

regional metamorphism

metamorphism affecting large areas, commonly during mountain-building

contact metamorphism

metamorphism caused primarily by heat from a nearby magma intrusion

subduction zone metamorphism

high-pressure, relatively low-temperature metamorphism association with subduction

seafloor metamorphism

alteration of rocks by circulating hot, ion-rich fluids

diagenesis

low-temperature, low-pressure changes during sediment burial, before significant metamorphism

contact aureole

zone of metamorphosed rock surrounding an igneous intrusion

slate

fine-grained foliated rock with slaty cleavage, commonly derived from shale

phyllite

fine-grained foliated rock with a silky sheen

schist

medium- to coarse- grained foliated rock with visible aligned minerals

gneiss

coarse-grained metamorphic rock with light and dark mineral banding

quartzite

hard, quartz-rich metamorphic rock, commonly formed from sandstone

marble

metamorphic carbonate rock, commonly formed from limestone or dolostone

hornfels

very hard, fine-grained metamorphic rock commonly formed by contact metamorphism

amphibolite

metamorphic rock composed mainly of amphibole and plagioclase

granulite

high-grade metamorphic rock, commonly containing feldspar, pyroxene, and garnet

serpentinite

greenish metamorphic rock rich in serpentine minerals, derived form mafic or ultramafic rock

soapstone

soft metamorphic rock composed mainly of talc

metaconglomerate

metamorphosed conglomerate, sometimes containing stretched pebbles

anthracite coal

hard, shiny, high-carbon coal produced by metamorphism of lower-grade coal

migmatite

rock containing both metamorphic and partially melted igneous components