150Lec13 - Lecture #13: Intermediate and Felsic Volcanism...

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Lecture #13: Intermediate and Felsic Volcanism (Abbott, pp. 194-199, 201-214, 219-244, 308-311) Formation of Felsic and Intermediate Magmas Intermediate and felsic magmas often form in subduction zones. Felsic, and perhaps some intermediate, magmas are also associated with hot spots within continental areas. In continent-ocean convergence boundaries (such as along the west coast of South America), water from the subducting oceanic plate partially melts overlying mantle materials to produce mafic magmas (Figure 8.6, p. 189). As the rising mafic magmas pass through the overlying continental crust, they partially melt the felsic and intermediate materials to produce intermediate and felsic magmas (Figure 8.6, p. 189; Figure 9.6, p. 221; Figure 8.11, p. 194). Similarly, in continental hot spots (such as at Yellowstone National Park in Wyoming ), mafic magmas extensively melt continental materials to form felsic and perhaps some intermediate magmas as they pass through thick continental crusts. Besides forming in continent-ocean subduction zones and some continental hot spots, intermediate magmas may also occur in ocean-ocean convergence zones, such as the Aleutian Islands of Alaska, the Philippines, and Indonesia. Water released from the heated subducting oceanic plate rises into the overlying mantle (Figure 3.15, p. 61). The water partially melts the mantle materials to form mafic magmas. As the mafic magmas rise into the overlying oceanic crust, they partially melt the crust to form some andesitic (intermediate) magmas (Figure 8.11, p. 194). Formation of Felsic and Intermediate Magmas in Subduction Zones and Hot Spots: Felsic and Intermediate Volcanoes Some intermediate and mafic volcanoes are cinder cones (also called scoria cones; Figure 8.25, p. 202). Cinder cones are entirely composed of pyroclastics (Lectures #3 and #11) and contain little or no lava flows. Cinder cones may contain considerable amounts of ash, blocks and bombs, and not just cinders. As expected, the larger bombs and blocks are located closer to the summit of the cinder cone, whereas at least some of the ash may be blown farther away from the eruption site. Cinder cones are usually no
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more than 200 meters high and often just erupt once. Figures 8.10 (p. 193) and 8.25 (p. 202) show the eruption of pyroclastics from Paricutín in Mexico, which is a cinder cone of mafic and intermediate composition. Because of their greater viscosity, felsic and intermediate extrusives tend to form volcanic domes and composite (stratovolcano) cones rather than shield volcanoes. Stratovolcanoes (also called composite volcanoes) are cones that consist of alternating layers of pyroclastic and lava flows (p. 203; Figure 8.26, p. 203). Each layer in a stratovolcano may represent more than one eruption. For example, a stratovolcano may have two or more pyroclastic eruptions before a lava flow eruption occurs, and vice- versa . Mt. St. Helens, Mt. Shasta in northern California, and Mt. Rainer are stratovolcanoes (Figure 9.7, p. 221; Figure 9.10, p. 223; Figure 9.18, p. 228; Figure 9.31,
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This note was uploaded on 07/14/2011 for the course GLY 150 taught by Professor Henke during the Spring '08 term at Kentucky.

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150Lec13 - Lecture #13: Intermediate and Felsic Volcanism...

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