Melt composition changes as a result Fe Mg Ca are removed Igneous Environments

Melt composition changes as a result fe mg ca are

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Melt composition changes as a result Fe, Mg, Ca are removed Igneous Environments o Two major categories Extrusive settin—cool at or near the surface Cool rapidly Chill too fast to grow big rystals Intrusive settings—cool at depth Lose heat slowly Crystals often grow large o Extrusive rocks are mostly mafic o Intrusive rock are mostly felsic Extrusive Settings o Lava flows cool as blankets that often stack vertically o Lava flows exit volcano vents and spread outward Intrusive Settings o Magma invades colder wall rock, initiating: Thermal (heat) metamorphism and melting Inflation of fractures, wedging wall rock apart Detachment of large wall rock blocks (stopping), and Incorporation of wall rock fragments (xenoliths).
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o Magma that doesn’t reach the surface freezes slowly o Geologists categorize intrusions by shape Tabular (sheet)—planar with uniform thickness Blister-shaped—a sill that domes upward Balloon-shaped—blobs of melted rock Tabular intrusions Have two major subdivisions o Sill —injected parallels to rock layering o Dike —cuts across rock layering o Dikes and sills: Cause the rock to expand and inflate Thermally alter surrounding rock o Both dikes and sills exhibit wide variability in: Size Thickness (or width). Lateral continuity Intrusions… o Dikes: Cut across preexisting layering (bedding or foliation) Spread rocks sideways Dominate in extensional settings o Sills: Are injected parallel to preexisting layering Are usually intruded close to the surface Plutonic Activity o Immense volumes of intrusives o Form above subduction zones o May add magma for tens of Ma. o Batholiths mark former subduction Intrusive and Extrusive o Intrusive and extrusive rocks commonly co-occur o Magma chambers feed overlying volcanoes o Magma chambers may cool to become plutons o Many igneous geometries are possible Influence on landscape o Continued uplift and erosion exposes the pluton Intrusive rocks are usually more resistant to erosion Often stand high on the landscape Describing Igneous Rock o Igneous rock is used extensively as building stone Office building Kitchens
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o Why? Durable (Hard) Beautiful o Often called “granite”; it is not always true granite o The size, shape, and arrangement of the minerals Interlocking—mineral crystals fit like jigsaw pieces Fragmental—pieces of preexisting rocks Glassy—made of solid glass shards o Texture directly reflects magma history Crystalline Igneous Textures o Texture reveals cooling history Aphanitic (fine-grained). Rapid cooling Crystals do not have time to grow Extrusive Phaneritic (course-grained) Slow cooling Crystals have a long time to grow Intrusive Crystalline Textures o Porphyritic texture—a mixture of coarse and fine crystals Indicates a two-stage cooling history Initial slow cooling Subsequent eruption cools remaining magma more rapidly Fragmental Textures o Preexisting rocks that were shattered by eruption o
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