Metamorphic Rock

Rock Cycle

Metamorphism means "changed form". Agents of Metamorphism Changes occur because of: Heat Pressure Chemical fluids *Rocks adjust to become more stable under new, higher temperatures and pressures.

HEAT There are several sources of heat for metamorphism. a. Geothermal gradient Temperature increases with depth at a rate of 20 - 30 degrees C per km in the crust. Ultimate source of the heat? Radioactive decay. b. Increase of temperature and pressure with depth causes Regional Metamorphism *Heat may come from large bodies of molten rock rising under a wide geographic area.

HEAT Intrusions of hot magma can bake rocks as it intrudes them. Lava flows can also bake rocks on the ground surface.

2. Lava or magma in contact with other rock causes Contact Metamorphism. Hornfels is a common contact metamorphic rock.

Example of : Contact metamorphism along a narrow (approx. 1 meter wide) diabase dike in the Deep River Basin of North Carolina. Diabase weathers tan. Contact metamorphic hornfels are gray. Host rocks are red siltstones.

Pressure Burial Pressure. Pressure increases with depth due to the weight of the overlying rocks. A cubic foot of granite weighs 167.9 pounds. Increase of pressure and temperature with depth causes Regional Metamorphism. Regional metamorphism occurs at depths of 5 - 40 km.

Buried rocks are subjected to the force, or stress, Pressure (stress)as metamorphic agent exerted by the load above. This confining pressure is analogous to water pressure where the force is applied equally in all directions.

Pressure 2. Tectonic pressures associated with convergent plate boundaries and continental collision also cause Regional Metamorphism. Pressure along fault zones causes Dynamic Metamorphism, due to the crushing and ductile flow of rock.

Convergent plate boundary

Chemical Fluids In some metamorphic settings, new materials are introduced by the action of hydrothermal solutions (hot water with dissolved ions). Many metallic ore deposits form in this way. Hydrothermal solutions associated with magma bodies

Chemical Fluids Black smokers - Sea water percolates through newly formed oceanic crust, dissolving out metallic sulfide minerals.

The hot sea water rises along fractures and pours from vents in the seafloor as black clouds of dark mineral-rich water pour from the vent.

Black smokers Sulfide minerals (such as pyrite, sphalerite, and galena) and copper precipitate when the hot water comes in contact with cold sea water.

How do rocks change? Metamorphism causes changes in: Texture Mineralogy Texture The processes of compaction and recrystallization change the texture of rocks during metamorphism.

Classification of Metamorphic Rocks

Compaction The grains move closer together. The rock becomes more dense. Porosity is reduced. Example: clay to shale to slate

clay to shale to slate (sediment) (sedimentary rock) (metamorphic)

Recrystallization Growth of new crystals. No changes in overall chemistry. New crystals grow from the minerals already present. A preferred orientation of minerals commonly develops under applied pressure. Platy or sheet-like minerals such as muscovite and biotite become oriented perpendicular to the direction of force. This preferred orientation is called foliation.

The diagram illustrates the effect of foliation. A texture of this sort in a metamorphic rock is called FOLIATION and the rocks are said to be FOLIATED.

Metamorphic Textures Foliation is a broad term referring to the alignment of sheet-like minerals. Types of foliation:

Metamorphic Textures Schistosity - alignment of large mica flakes, as in a mica schist derived from the metamorphism of shale. Slaty cleavage - alignment of very fine-grained micas, as in a slate derived from the metamorphism of shale. Phyllitic structure - alignment of fine-grained micas, as in a phyllite. Gneissic banding - segregation of light and dark minerals into distinct layers in the rock, as in a gneiss.

Slaty cleavage

Slate - very fine grained rock. Resembles shale. Has slaty cleavage which may be at an angle to the original bedding. Relict bedding may be seen on cleavage planes. Often dark gray in color. "Rings" when you strike it. (Unlike shale, which makes a dull sound. Temperature about 200 degrees C; Depth of burial about 10 km.

Phyllitic structures

Phyllite - fine-grained metamorphic rock Phyllite - fine-grained metamorphic rock. Has a frosted sheen, resembling frosted eye shadow. This is no coincidence. Cosmetics commonly contain ground up muscovite (ground to a size similar to that occurring naturally in phyllite.)

Gneissic Banding

Gneiss - (pronounced "nice") - a banded or striped rock with alternating layers of dark and light minerals. The dark layers commonly contain biotite, and the light layers commonly contain quartz and feldspar.

Schistosity

Schist - metamorphic rock containing abundant obvious micas, several millimeters across. Several types of schist may be recognized, based on minerals which may be present: mica schist garnet schist chlorite schist kyanite schist talc schist

Non-foliated Non-foliated or granular metamorphic rocks are those which are composed of equidimensional grains such as quartz or calcite. There is no preferred orientation. The grains form a mosaic.

Non-foliated Limestone to Marble Sandstone to Quartzite

Marble - fizzes in acid because its dominant minerals is calcite (or dolomite). The parent rock is limestone (or dolostone).

Quartzite - interlocking grains of quartz. Scratches glass Quartzite - interlocking grains of quartz. Scratches glass. The rock fractures through the grains (rather than between the grains as it does in sandstone). The parent rock is quartz sandstone.

Mineral changes in metamorphic rocks Recrystallization - rearrangement of crystal structure of existing minerals. Commonly many small crystals merge to form larger crystals, such as the clay in shale becoming micas in slate, phyllite, and schist.

Mineral changes in metamorphic rocks Formation of new minerals - there are a number of metamorphic minerals which form during metamorphism and are found exclusively (or almost exclusively) in metamorphic rocks: Garnet - dark red dodecahedrons (12 sides) Staurolite - brown lozenge-shaped minerals, commonly twinned to form "fairy crosses". State mineral of Georgia.

Metamorphic index minerals In regional metamorphic terranes, the temperature and pressure regime is indicated by the distribution of metamorphic minerals across a large area. Low metamorphic grade (low temperatures and pressures) - about 200 degrees C Slate and phyllite chlorite muscovite biotite

Metamorphism of Basalts and Gabbros Greenschist - Olivine, pyroxene, and plagioclase in an original basalt change to amphiboles and chlorite (both commonly green) as water in the pore spaces reacts with the original minerals at temperatures and pressures of low grade metamorphism. Amphibolite - As pressure and temperature increase to intermediate grades of metamorphism, only dark colored amphiboles and plagioclase survive and the resulting rock is called an amphibolite. Granulite - At the highest grade of metamorphism the amphiboles are replaced by pyroxenes and garnets, the foliation is lost and a granulite that has a granulitic texture is produced.

Greenschist – Olivine, pyroxene, and plagioclase in an original basalt change to amphiboles and chlorite (both commonly green)

Amphibolite - As pressure and temperature increase to intermediate grades of metamorphism, only dark colored amphiboles and plagioclase survive and the resulting rock is called an amphibolite.

                                                          Granulite - At the highest grade of metamorphism the amphiboles are replaced by pyroxenes and garnets, the foliation is lost and a granulite that has a granulitic texture is produced.