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Chapter 4: Origin and Evolution of Igneous Rocks Caption. (Figure X.X) Fig. 4.20 OBJECTIVES • Describe how igneous rocks relate to the two other rock groups (sedimentary and metamorphic). • Describe how magma forms and the factors that influence magma’s ascent toward the surface and its cooling history. • Explain how magmas produce a variety of igneous rocks with textures that vary according to the environment of their formation. • Compare and contrast the different types of igneous rock and explain the basis of their classification. OBJECTIVES • Explain how the chemical composition and physical characteristics of magma evolve as magma cools. • Identify the processes that cause volcanic eruptions and the various types of volcanoes and volcanic rocks that eruptions produce. • Explain the role that plate tectonics plays in the formation and composition of magma and igneous rocks. Igneous Rocks and the Rock Cycle • Igneous rocks are one of the three main rock types (igneous, sedimentary, metamorphic). • Igneous rocks form through the solidification of molten or partially molten rock (lava on the surface or magma underground). • Magma and Lava: mixture of liquid rock, solid crystals, gases • Igneous rocks constitute one stage of the rock cycle. Igneous Rocks and the Rock Cycle Igneous rocks form when other rocks melt, or partially melt, and then solidify. Igneous rocks become sedimentary rocks through weathering, erosion, deposition, and lithification. Igneous rocks become metamorphic rocks through heating and/or compression. Fig. 4.1 Igneous Rocks and the Rock Cycle • Geologists study igneous rocks to better understand • Chemistry of igneous rocks • Cooling history • Composition of source rocks (and of Earth’s interior) • Transport of magma toward the surface • Volcanic eruptions • Plate tectonics Magma Formation and Transport Three Main Steps of Formation 1) Partial melting of source rock • Increase in temperature • Decrease in pressure • Addition of volatiles 2) Transport of magma • Magma less dense than surrounding rock • Rises through fractures • May “digest” wall rock • May cause collapse of surrounding rock 3) Crystallization of magma • Generally: decrease in temperature (cooling) • Sometimes: decrease in pressure (decompression) • Sometimes: increase in pressure • Sometimes: removal of volatiles Fig. 4.7 Textures of Igneous Rocks • Texture: size, shape, arrangement of crystals Figs. 4.4, 4.14 Coarse-grained Fine-grained Glassy Porphyritic (phenocrysts in groundmass) Textures of Igneous Rocks • Two main types of igneous rocks: • Intrusive (aka Plutonic): turn solid underground • Extrusive (aka Volcanic): turn solid on the surface • Texture is a result of melting, transport, and crystallization history Texture History Igneous Rock Type/ Environment Coarse-grained Generally slow crystallization Intrusive (plutonic) Fine-grained Generally fast crystallization Extrusive (volcanic) Glassy Turn solid almost instantaneously Extrusive (volcanic) Porphyritic Mixture of crystals that form slowly and those that form quickly Intrusive (plutonic) or extrusive (volcanic) Textures of Igneous Rocks Fig. 4.5 Igneous Rocks in the Field • Geologists study igneous rock formations in the field to better understand • Relative ages of formations • Partial melting and the source of igneous rocks • Magma transport • Crystallization history • Volcanic processes Igneous Rocks in the Field Fig. 4.10 Classifying Igneous Rocks • Classified based on • Composition • Relative abundance of various minerals • Relative amounts of oxides • Texture (coarse, fine, glassy, porphyritic) • Field relationships • Four main groups based on mineralogy/chemistry (SiO2, Fe, Mg) Group SiO2 content Fe, Mg content Color Felsic High Low Light-colored Intermediate Intermediate Intermediate Medium or speckled Mafic Low High Dark grey to black Ultramafic Very Low Very High Dark Green to Black Classifying Igneous Rocks Fig. 4.13 Evolution of Igneous Rocks • Magmas change over time. • Chemistry of the magma changes as crystals form. • Mineralogy changes as crystals react with magma. • Crystals settle out or separate from the magma (fractionation). • Magma mixes with other magmas. • Magma incorporates wall rock (assimilation). • Volatile content changes. • Temperature changes. • Viscosity changes. • Bowen’s Reaction Series • Different minerals are stable at different pressure and temperature conditions. • Crystals react with the remaining magma to form new minerals. Fig. 4.15 A magma may fractionate through the settling of crystals. Magmas of different compositions may mix together. Figs. 4.16, 4.18, 4.19 A magma can become more viscous as minerals with more complex structures become stable. Volcanic Eruptions • Eruption styles and volcanic structures are related to lava chemistry. • Mafic lavas • More fluid • Less explosive eruptions • Low shield volcanoes and small cinder cones • Ropy lava, blocky lava, fissure vents, lava tubes, lava fountains, pillow lavas • Intermediate and felsic lavas • More viscous • More explosive eruptions • Large, steep composite volcanoes (stratovolcanoes) • Blocks, bombs, lapilli, breccia, tuffs, pumice, pyroclastic flows, lahars Fig. 4.25 Volcanic Eruptions Explosive eruptions are associated with convergent boundaries. Effusive eruptions of basalt are associated with seafloor spreading centers and oceanic hot spots. Figs. 4.33, 4.24 Igneous Rocks and Plate Tectonics • Most volcanoes form along plate boundaries. • Tectonic environment affects magma composition and volcano type. Feature Boundary Type Lava Composition Volcano Types Mid-Ocean Ridge Divergent (oceanocean) Mafic Low linear shield Volcanic Arc Convergent (subduction zone) Intermediate Composite Rift Volcanoes Divergent (continental rift) Felsic to mafic Composite Ocean Island Intraplate (not along a boundary) Mafic Shield Igneous Rocks and Plate Tectonics Volcanism and Plate Boundaries: Igneous rocks are associated with divergent and convergent boundaries. SUMMARY • Igneous rocks are one of the three main groups of rocks. • Igneous rocks can evolve from or into metamorphic and sedimentary rocks. • Magma forms from the melting of existing rocks. Magma rises as a result of its low density compared to surrounding rocks. • Igneous rocks are classified based on composition, from ultramafic (silica poor and iron rich) to felsic (silica rich and iron poor). • Igneous rocks are also classified based on texture, from glassy (no crystals) to fine-grained (small crystals) to coarse-grained (large crystals) and porphyritic (large crystals within a fine-grained matrix). The texture of an igneous rock reflects its cooling history. • Magma evolves from mafic to silicic as minerals crystallize within it. SUMMARY • Magma changes in temperature, viscosity, volatile content, and composition as it is transported toward the surface, mixes with other magmas, assimilates surrounding rock, and fractionates. • Eruption styles and volcanic structures are related to lava chemistry. • Mafic lavas generally result in gentle eruptions and low-profile volcanic structures. Intermediate and felsic lavas generally result in violent eruptions and steep volcanoes. • Many igneous rock formations formed along divergent and convergent plate boundaries, though some form within plates. • The composition of an igneous rock reflects its tectonic environment. • Geologists study igneous rocks to better understand the composition of Earth’s interior, igneous processes, and plate tectonics.