Metamorphic RocksThe Greek word meta means “change” and morph meaning “shape”.

Metamorphic rocks were previously either sedimentary or igneous rocks, but they have been subjected to very high temperature and pressure. (Below the surface of the Earth)

Metamorphic RocksThe temperature was not high enough to make the rock melt (Metamorphic changes occur as the rock is in solid-state), but it was high enough to allow some crystals to grow, and for the minerals to begin to re-crystallize and thus form a new rock.

As temperature rises, crystal lattices are broken down and reformed with different combinations of atoms. New minerals are formed.

Metamorphic RocksThe types of metamorphism

Regional metamorphism

Contact metamorphism

Regional metamorphismContact metamorphism

Metamorphic RocksContact metamorphism

In the case of contact metamorphism heat comes from contact with molten magma.

-This type of metamorphism has a limited and local effect.

Metamorphic RocksRegional metamorphism

When rocks are forced toward the mantle during the formation of a mountain range and/or other tectonic activity, regional metamorphism occurs.

-Large volumes of rock are altered in this way.

Metamorphic RocksMetamorphism does not take place on the Earth’s surface.

Rocks under the Earth’s surface are under great pressure from overlying rock layers.

Deep burial- as depth increases, in the Earth’s crust, the temperature also increases.

Tectonic forces in the Earth may apply lateral pressure to large volumes of rock.

Metamorphic Rocks

Metamorphic Rocks

Metamorphic Rocks

Metamorphic rocks have been exposed to the surface of the Earth because erosion has striped away overlying rocks.

So, when we see a large area of metamorphic rocks we know we are looking at the core of an ancient mountain range.

Changes During Metamorphism

Parent rock

(protolith)

Low temperature

Low pressureMedium temperature

Medium pressure

High temperature

High pressureHigher temp.

Higher pressure

shale slate phyllite schist gneiss

MELTING

quartz sandstone

quartzite

limestone marble

basalt amphibolite or schist

granite gneiss

Metamorphic RocksChange in metamorphic grade with depth

Increasing Directed Pressure and increasing Temps

Metamorphic RocksFoliated metamorphic rock

Foliation forms when pressure squeezes the flat or elongate minerals within a rock so they become aligned. These rocks develop a platy or sheet-like structure that reflects the direction that pressure was applied in.

Foliation: minerals have been rearranged into visible bands.

Metamorphic Rocks

Increasing Directed Pressure and increasing Temps

Metamorphic RocksDirected Pressure causes rocks to become folded, and minerals to reorient

perpendicular to the stress: “foliation”

Schist

Gneiss

Slate

Metamorphic RocksNon-foliated metamorphic rock

Non-foliated metamorphic rocks do not have a platy or sheet-like structure.

Metamorphic rock that does not show bands.

Marble

Quartzite