Metamorphic Rocks

How rocks are transformed: Heat (Temperature): Increases with depth in Earth. Heat drives chemical reactions and recrystallization. Higher temperatures create higher-grade metamorphic rocks. Pressure: Increases with depth. Can be: - Confining Pressure: Equal in all directions (like being underwater) - Directed Pressure: Stronger in one direction (like being squeezed) Chemically Active Fluids: Hot water and other fluids can dissolve and transport material, changing rock composition. Important in some types of metamorphism. Time: Metamorphic processes operate over millions of years. Longer time allows more complete transformation. No Melting: Metamorphism occurs below the melting point. If rock melts, it becomes igneous, not metamorphic. The boundary is sometimes blurred (migmatites are partially melted). Protolith: The original rock before metamorphism. Can be igneous, sedimentary, or another metamorphic rock. Protolith composition affects what metamorphic rock forms.

Different metamorphic environments: Regional Metamorphism: Large-scale metamorphism affecting vast areas. Associated with mountain building (orogeny). Creates most metamorphic rocks. Operates over millions of years. Contact Metamorphism: Occurs when hot magma intrudes cooler rock. Creates a "baked" zone (aureole) around the intrusion. Localized but can produce interesting minerals. Hydrothermal Metamorphism: Hot, mineral-rich water alters rocks. Can create ore deposits and interesting minerals. Often associated with igneous activity. Burial Metamorphism: Occurs when rocks are buried deeply. Lower temperature than regional metamorphism. Creates low-grade metamorphic rocks. Dynamic Metamorphism: Occurs along fault zones. High pressure but low temperature. Creates fault breccias and other distinctive rocks. Shock Metamorphism: Caused by meteorite impacts. Extreme pressure creates unusual minerals (coesite, stishovite). Rare but interesting. Each Type: Creates characteristic rocks and minerals. Understanding the type helps interpret geological history and predict what minerals might be present.

Intensity of metamorphism: Low Grade: Mild transformation, original features still visible: - Slate: Fine-grained, splits into thin sheets. Formed from shale. Low temperature, low pressure. - Phyllite: Slightly higher grade than slate. Has a sheen from tiny mica crystals. Medium Grade: Moderate transformation: - Schist: Coarse-grained, foliated (layered). Mica crystals visible. Can contain interesting minerals like garnet, staurolite, kyanite. - Amphibolite: Composed of amphibole and plagioclase. Formed from mafic rocks. High Grade: Strong transformation: - Gneiss: Coarse-grained, banded. Alternating light and dark layers. High temperature and pressure. - Granulite: Very high grade. Coarse-grained, often lacks hydrous minerals. Very High Grade: Extreme conditions: - Migmatite: Partially melted. Shows both metamorphic and igneous features. Highest grade before complete melting. Index Minerals: Minerals that form at specific grades: - Low: Chlorite, muscovite - Medium: Biotite, garnet, staurolite - High: Sillimanite, orthoclase For Rockhounds: Higher grade rocks often have larger, better-formed crystals. Schists and gneisses can contain beautiful minerals.

Rocks with layered or banded appearance: Foliation: Parallel alignment of minerals, creating layers or bands. Results from directed pressure during metamorphism. Slate: Fine-grained, perfect cleavage. Splits into thin, flat sheets. Used for roofing and flooring. Formed from shale at low grade. Phyllite: Between slate and schist. Has a silky sheen from tiny mica crystals. Slightly coarser than slate. Schist: Coarse-grained, foliated. Mica crystals visible, often sparkly. Can contain porphyroblasts (large crystals) of garnet, staurolite, or other minerals. Formed at medium grade. Gneiss: Coarse-grained, banded. Alternating light (felsic) and dark (mafic) layers. High-grade metamorphism. Can be very beautiful. Migmatite: Partially melted. Shows both foliated (metamorphic) and igneous features. Highest grade before complete melting. Formation: Foliation forms perpendicular to directed pressure. Minerals align to minimize stress. Creates the layered appearance. Rockhounding Value: Foliated rocks often contain interesting minerals. Schists can have beautiful garnets, staurolite crosses, and other crystals. Gneisses can be very attractive.

Metamorphic rocks without foliation: Marble: Metamorphosed limestone or dolomite. Composed of calcite or dolomite. Can be very pure white or contain impurities creating colors. Takes polish well. Valued for sculpture and building. Quartzite: Metamorphosed sandstone. Very hard, composed of interlocking quartz grains. Resistant to weathering. Often white or light-colored but can be other colors. Hornfels: Fine-grained, formed by contact metamorphism. Hard, dense. No foliation because formed under confining pressure (not directed pressure). Amphibolite: Composed of amphibole and plagioclase. Formed from mafic rocks. Can be foliated or non-foliated depending on conditions. Serpentinite: Metamorphosed ultramafic rock. Often green, can be attractive. Sometimes contains asbestos (handle with care). Anthracite: High-grade coal. Shiny, hard. Highest carbon content of coals. Formation: Non-foliated rocks form when there's no directed pressure, or when minerals don't align (like in marble and quartzite where grains recrystallize but don't align). Rockhounding Value: Marble can be beautiful and is valued for carving. Quartzite is hard and durable. Some non-foliated rocks contain interesting minerals.

Characteristic features of metamorphic rocks: Index Minerals: Minerals that form at specific metamorphic grades. Their presence indicates the grade of metamorphism: - Chlorite: Low grade, green - Garnet: Medium to high grade, often red - Staurolite: Medium grade, often forms crosses - Kyanite: Medium to high grade, blue, bladed - Sillimanite: High grade, needle-like Porphyroblasts: Large crystals that grow during metamorphism. Stand out from finer-grained matrix. Common in schists. Can be garnet, staurolite, kyanite, or other minerals. Foliation Types: - Slaty Cleavage: Perfect parallel planes (slate) - Schistosity: Parallel alignment of platy minerals (schist) - Gneissic Banding: Alternating light and dark bands (gneiss) Textures: - Granoblastic: Equant grains, no preferred orientation (marble, quartzite) - Porphyroblastic: Large crystals in fine matrix (many schists) - Blastoporphyritic: Preserved igneous texture in low-grade metamorphism Reaction Rims: Zones around minerals showing chemical reactions. Indicate changing conditions during metamorphism. Rockhounding Value: Metamorphic minerals are often well-formed and beautiful. Schists with porphyroblasts are highly collectible. Index minerals help identify metamorphic grade and can be interesting specimens.