M ETAMORPHIC R OCKS Chapter 4 Section 3

M ETAMORPHIC R OCKS Rocks that have changed because of changes in temperature and pressure or the presence of hot fluids are called metamorphic rocks. Metamorphic rocks can form from igneous, sedimentary, or other metamorphic rocks.

H EAT AND P RESSURE Temperature and pressure increase with depth in the Earth. In some areas, heat and pressure cause rocks to melt and magma to form. In other areas, melting doesn’t occur, and mineral grains can change by dissolving and recrystallizing. Minerals may exchange atoms with surrounding minerals and form new, bigger minerals. These processes form metamorphic rocks. One type of rock can change into several metamorphic rocks depending upon the amount of pressure and heat. Ex. Shale can change to slate, which can change to phyllite, then to schist, then to gneiss.

S LATE

H OT F LUIDS Water with dissolved elements and compounds can react chemically with a rock and change its composition, especially when fluids are hot. Rock surrounding a hot magma body reacts with hot fluids from the magma, forming metamorphic rock. This is called contact metamorphism.

F OLIATED R OCKS Metamorphic rocks are classified according to their texture and composition. When mineral grains line up in parallel layers, the metamorphic rock has foliated texture. Two examples of foliated rocks are slate, formed from the sedimentary rock shale, and gneiss, formed from the igneous rock granite. -Slate separates easily along the foliation layers, and the minerals in slate are pressed together so tightly that water can’t pass between them easily. -Foliation in gneiss shows up as alternation light and dark bands.

S LATE A GAIN !!

G NEISS

N ONFOLIATED R OCKS In a nonfoliated metamorphic rock, the mineral grains grow and rearrange, but they don’t form layers. Quartzite is a nonfoliated metamorphic rock formed from sedimentary rock sandstone. Marble is another nonfoliated rock formed from the sedimentary rock limestone.

M ARBLE