3Metamorphism means "changed form". Agents of MetamorphismChanges occur because of:HeatPressureChemical fluids*Rocks adjust to become more stable under new, higher temperatures and pressures.
4HEAT There are several sources of heat for metamorphism. a. Geothermal gradient Temperature increases with depth at a rate of 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.
5HEATIntrusions of hot magma can bake rocks as it intrudes them. Lava flows can also bake rocks on the ground surface.
62. Lava or magma in contact with other rock causes Contact Metamorphism. Hornfels is a commoncontact metamorphic rock.
8Example 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.
9PressureBurial Pressure. Pressure increases with depth due to the weight of the overlying rocks.A cubic foot of granite weighs pounds. Increase of pressure and temperature with depth causes Regional Metamorphism. Regional metamorphism occurs at depths of km.
10Buried 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.
11Pressure2. 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.
13Chemical FluidsIn 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
14Chemical FluidsBlack smokers - Sea water percolates through newly formed oceanic crust, dissolving out metallic sulfide minerals.
15The 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.
16Black smokersSulfide minerals (such as pyrite, sphalerite, and galena) and copper precipitate when the hot water comes in contact with cold sea water.
17How do rocks change? Metamorphism causes changes in: Texture MineralogyTextureThe processes of compaction and recrystallization change the texture of rocks during metamorphism.
21clay to shale to slate(sediment) (sedimentary rock) (metamorphic)
22Recrystallization 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.
23The 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.
24Metamorphic TexturesFoliation is a broad term referring to the alignment of sheet-like minerals. Types of foliation:
25Metamorphic TexturesSchistosity - 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.
27Slate - 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.
29Phyllite - 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.)
31Gneiss - (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.
33Schist - metamorphic rock containing abundant obvious micas, several millimeters across. Several types of schist may be recognized, based on minerals which may be present:mica schistgarnet schistchlorite schistkyanite schisttalc schist
34Non-foliatedNon-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.
35Non-foliatedLimestone to Marble Sandstone to Quartzite
36Marble - fizzes in acid because its dominant minerals is calcite (or dolomite). The parent rock is limestone (or dolostone).
37Quartzite - 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.
38Mineral 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.
39Mineral 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.
41Metamorphic 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 phyllitechloritemuscovitebiotite
42Metamorphism 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.
43Greenschist –Olivine, pyroxene, and plagioclase in an original basalt change to amphiboles and chlorite (both commonly green)
44Amphibolite -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.
45Granulite -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.