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Fig. 03.01 Fig. 03.02 Magma: molten rock + volatiles beneath the Earth’s surface. Lava: molten rock on the Earth’s surface that has lost at least some of its volatiles. Magma is usually silicate in composition: ~50-70 wt% SiO2 The rest is made up of Al2O3, Fe2O3, FeO, Na2O MgO, K2O, CaO, P2O5, TiO2 A continuous range of magma chemistries is observed. Partial Melting Rocks rarely melt to 100%. Rocks contain several minerals – the one with the lowest melting point melts first and that with the highest melts last. Liquid may be removed along grain boundaries. This can create a magma of a different composition than the source material. Fig. 03.06 Table 03.01 Felsic Intermediate Mafic Ultramafic ~70 wt% SiO2 ~60 wt% SiO2 ~50 wt% SiO2 <50 wt% SiO2 (Komatiite) Fig. 03.07a Fig. 03.07b Fig. 03.07c Fig. 03.07e Fig. 03.07f General Rules Felsic = light-colored (Si-, Al-, alkali-rich). Ultramafic and Mafic = dark-colored (Fe, Mg, Ca rich, ferromagnesian mineral-rich). Magmatic temperatures range from ~600˚C to >1,500˚C. The Exception that Proves the Rule - Obsidian Obsidian = felsic (quenched glass), but black. General Textures & Crystallization Rate of crystallization dictates mineral size: slow cooling = large crystals; fast cooling = small crystals or a quench glass. General Textures Extruded or erupted magmas or shallowly emplaced magma bodies cool quickly many small, interlocking crystals with some interstitial quench glass. Intrusions of magma deeper within the crust are insulated. Although the margin of the magma next to the wallrock is chilled (small grain size) and the wallrock is baked, the interior of the magma cools much more slowly = large grain size (phaneritic). Deep intrusive igneous rocks are give the generic term “plutonic”. Fig. 03.05a Plutonic Rock (Granite) Fig. 03.05b Plutonic Rock (Granite) in Thin Section Fig. 03.03 Plutonic Rock (Granite) in the Field Textural Terms APHANITIC: individual crystals are so small they cannot be discerned with the naked eye. Common in eruptive and shallow intrusive igneous rocks. Textural Terms PHANERITIC: crystals can be seen, common in interiors of thick flows and deeper intrusions. Textural Terms PORPHYRITIC: large crystal (PHENOCRYSTS) in a fine-grained groundmass. Reflects two cooling stages – slow cooling (in a magma chamber) to form the phenocrysts, followed by rapid cooling as would occur upon eruption. Textural Terms PEGMATITIC: contains very large (cm-size) crystals. Crystal Shapes EUHEDRAL: wellformed crystals with many crystal faces developed. Crystal Shapes SUBHEDRAL: crystals with only a few crystal faces developed. Crystal Shapes ANHEDRAL: crystals with no crystal faces developed. Magma Viscosity Magma viscosity is dependent upon composition and temperature: Mafic << Felsic Hot << Cool Silica affects viscosity because of polymerization or linking of SiO2 tetrahedra. The more silica-rich the more felsic the magma is and the stickier it is. Magma Viscosity All magmas contain gases, but felsic magmas are generally more gas-rich - explosive. When gases escape, can form vesicular lavas, e.g., vesicular basalt: Pumice = “froth” = felsic. Intrusive Rocks Intrude older rocks (generically known as “countyrock” – can be igneous, sedimentary, or metamorphic) and bake them = contact metamorphism. Intrusive Rocks Intrusive rocks recognized by: Coarse grain size, interlocking crystals, typically lacking a fabric (oriented texture). Baked contacts (country rock) and chill zone (finer grain size) at the edges of the intrusion. Inclusions of country rock = “xenoliths”. They cross-cut features in the country rock. Veins protrude outward into country rock. Fig. 03.04 Ship Rock, New Mexico Fig. 03.08b Fig. 03.13 Fig. 03.12 Fig. 03.14 Granite Batholith, Sierra Nevada Fig. 03.10 Intrusion Types How to distinguish a sill from a flow: • baked contacts above and below; • vesicles (gas bubbles) in flow often filled in with flows; • smaller dikes present above sill intruding the overlying countryrock. Changing a Magma Composition Fractional crystallization via crystal settling: Changing a Magma Composition Mixing of two compositionally distinct magmas: Changing a Magma Composition Assimilation of country rock: Changing a Magma Composition Partial melting of the source rock to different degrees. Plate Tectonics & Igneous Rocks Divergent Plate Margins – lithosphere and asthenosphere; pressure-release, almost all mafic magmatism. Plate Tectonics & Igneous Rocks Intraplate: Hawaii, Yellowstone, LIPs – mantle plume activity. Fig. 03.24 Convergent Plate Margins: more intermediate and felsic magmatism, especially if volcanoes are built on continents. Plate Tectonics & Igneous Rocks Fig. 03.26 Origin of Granite: magmatic underplating.