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• Metamorphic rocks are formed under heat, pressure or both deep in the Earth. • Look very different (most of the time) from parent rock Ig Sed Met Met FORMATION OF METAMORPHIC ROCKS • Most metamorphic changes= elevated temperatures and pressures • Occur few kilometers below Earth’s surface and into upper mantle Two locations for metamorphism: ① Contact: magma intrudes rock ① Regional: large areas of rock are subjected to extreme pressures and temperatures FORMATION: CONTACT METAMORPHISM • When magma intrudes solid rock • Produces low-grade metamorphism; minor changes • High temperatures & moderate-to-low pressure • Temperature decreases with distance from intrusion • Metamorphic effects also decrease with distance • Limited to thin zones because lava cools quickly • Example: marble forms when magma intrudes a limestone body FORMATION: REGIONAL METAMORPHISM • When high temperature and pressure affect large areas of Earth’s crust belts of regional metamorphism • Range in grade: low high • Ex: mountain building = high-grade metamorphism • Folding and deforming of layers in the area REGIONAL METAMORPHISM AGENTS OF METAMORPHISM • During metamorphism, rocks are usually subjected to 3 agents (at the same time): ①Heat: provides energy needed to drive chemical reactions ②Pressure (stress): causes spaces between mineral grains to close ①Hydrothermal solutions: surrounds mineral grains and help recrystallization process AGENTS: HEAT • Most important agent provides energy needed to drive chemical reactions cause existing minerals to recrystallize or new minerals to form • Heat originates from magma & change in temperature with depth • Upper crust increase averages between 20°C and 30°C per kilometer! • Different minerals recrystallize at different temperatures • Example: 8 km 150°C-200°C • Clay minerals: become unstable and recrystallize to form new stable minerals (chlorite) • Silicate minerals: stable @ these temps. requires higher temps. to change silicates AGENTS: PRESSURE (STRESS) • Pressure increases with depth • Applied in all directions • Causes spaces between mineral grain to close • Results in more compact rock greater density • May cause minerals to recrystallize new minerals • Cause rocks to flow versus fracture grains flatten & elongate AGENTS: HYDROTHERMAL SOLUTIONS • • When very hot water reacts with rock and alters chemical and mineral composition Hot fluids migrate in and out of the rock original mineral composition and texture changes • Example: gold lodes = gold deposit fills fissure in rock usually in vein formation A view of an undeveloped lode gold deposit. The gold is in the white veins in the photo. Andes Mtns CLASSIFICATION OF METAMORPHIC ROCKS • Classified by texture and composition Gneiss • Texture can be foliated or nonfoliated: ①Foliated: layered or banded appearance Contact metamorphism more compact more dense Pressure causes microscopic minerals to compact and align in similar direction Certain minerals can recrystallize in extreme conditions flat needle like crystals form perpendicular to direction of force Example: gneiss CLASSIFICATION OF METAMORPHIC ROCKS (CONT’D) ② Nonfoliated: does not have banded texture • most contain only one mineral & blocky crystal shapes • Example: marble – nonfoliated contact metamorphic rock made of calcite; parent rock (limestone) metamorphosed, calcite crystals combine larger Marble interlocking crystals seen in marble MIGHTY MORPHING ROCKS! Aureole = site of contact meta. • A metamorphic aureole in the Henry Mountains, Utah. • Top = greyish rock igneous intrusion, consisting of porphyritic granodiorite • Bottom = pinkish rock on the bottom is the sedimentary country rock, a siltstone. • Middle = metamorphosed siltstone is visible as both the dark layer (~5cm thick) and the pale layer below it.