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Volcanoes The Nature of Volcanic Eruptions • Viscosity (resistance to flow) determines the “violence” or explosiveness of a volcanic eruption • Factors which determine viscosity • Composition of the magma • Temperature of the magma • Dissolved gases in the magma Temperature - Cooler magmas are more viscous • A volcano’s eruptions may get more explosive over time, as magma in chamber cools down • Example: Crater Lake (formerly Mt. Mazama) Magma Composition and Viscosity • Granitic/andesitic lavas have greater silica (SiO2) content and are more viscous – Convergent plate volcanism • Basaltic lavas have less(SiO2) content and are less viscous – Divergent plate volcanism – Intraplate (hot spot) volcanism Dissolved Gases Content and Viscosity • Gases expand within a magma as it nears the Earth’s surface due to decreasing pressure • The violence of an eruption is related to how easily gases escape from magma • “Wet” magma (oceanic subduction) has significant gas content Materials extruded from a volcano • Lava Flows – Pahoehoe lava (resembles a twisted or ropey texture) – Aa lava (rough, jagged blocky texture) Figure 4.5a Figure 4.3 Materials extruded from a volcano • Dissolved Gases: Mainly water vapor and carbon dioxide • Pyroclastic materials – “Fire fragments” – Ash and dust - fine, glassy fragments – Cinders – slightly larger than ash – Pumice - porous rock from “frothy” lava – Blocks and bombs – larger discrete pieces of lava Shield volcanos • Broad, slightly domed-shaped • Composed primarily of basaltic lava • Generally cover large areas • Produced by mild eruptions of large volumes of lava • Mauna Loa on Hawaii is a good example Cinder Cones – Built from ejected lava (mainly cinder-sized) fragments – Steep slope angle – Rather small size – Frequently occur in groups Composite cone (Stratovolcano) – Most are located adjacent to the Pacific Ocean (e.g., Fujiyama, Mt. St. Helens). – Large, classic-shaped volcano (1000’s of ft. high & several miles wide at base). – Composed of lava flows alternating with large quantities of pyroclastic flow deposits. Figure 4.1a Figure 4.1b A composite volcano Figure 4.7 Size comparison of volcano types Formation of Crater Lake Partial Melting and Magma Formation • Formation of Basaltic magmas • Most originate from partial melting of ultramafic rock in the mantle • Basaltic magmas form at mid-ocean ridges by decompression melting or at subduction zones • Formation of Granitic magmas • Basaltic magma pools beneath granitic continental rock and melts it, forming granitic magma • Granitic magma often does not reach the surface, but instead forms intrusive rocks at depth. How Magma Rises Formation of Plutons from Granitic Magma • Formation of Granitic magmas • Basaltic magma pools beneath granitic continental rock and melts it, forming granitic magma • Granitic magma often does not reach the surface, but instead forms intrusive rocks at depth. • Pluton – a large mass of intrusive rock • Most plutons are granitic in composition • Granitic magma forms at base of continental crust and rises up because it is less dense than the solid crust Forming Igneous Features and Landforms Fig. 8-15, p.179 Fig. 8-16, p.180 Figure 4.24 Figure 4.26 Figure 4.20 Figure 4.21 Plate Tectonics and Magma Generation Figure 4.27 Tectonic Settings and Volcanic Activity