Metamorphic Rocks

Metamorphic RocksWhen high temperature and pressure combine and change texture, mineral composition, or chemical composition of a rock without melting it, a metamorphic rock forms

High temperatures required come from Earth’s internal heat, either through deep burial or nearby igneous intrusions

High pressures come from deep burial or compression during mountain building

Gneiss

Classifying Metamorphic RockGeologists use metamorphic textures and mineral composition to identify metamorphic rocks

Two groups: foliated and non-foliated

Foliated TextureFoliated Rocks - metamorphic rocks with layers and bands of minerals

High pressure during metamorphism causes minerals with flat or needlelike crystals to form with their long axes perpendicular to pressure

Parallel alignment of minerals creates layers observed

Gneiss

Non-foliated TextureNon-foliated Rocks - metamorphic rocks composed mainly of minerals that form with blocky crystal shapes

Fossils are very rarely preserved in metamorphic rocks

Porphyroblast - large crystals that form in solid rock through reorganization of atoms during metamorphism

AmphibolitePorphyroblast

Grades of MetamorphismDifferent combinations of temperature and pressure result in different grades of metamorphism

Low, medium, and high grade metamorphism

Types of Metamorphism Three main types of metamorphism: regional, contact, and hydrothermal

Regional MetamorphismRegional Metamorphism - occurs when high temperature and pressure affect large regions of Earth’s crust (when plates collide)

Results include changes in minerals and rock types, foliation, and folding and deforming of rock layers that make up area

Contact Metamorphism - occurs when molten material, such as that in an igneous intrusion, comes in contact with solid rock

Because temperature decreases with distance from an intrusion, metamorphic effects also decrease with distance

Contact Metamorphism

Hydrothermal Metamorphism - occurs when very hot water reacts with rock and alters its chemical and mineral composition

Chemical changes are common during contact metamorphism near igneous intrusions and active volcanoes

Valuable ore deposits of gold, copper, zinc, tungsten, and lead are formed in this manner

Hydrothermal Metamorphism

The Rock CycleRock Cycle - shows how rocks are continually being changed above and beneath Earth’s surface

Any rock can be changed into any other type of rock