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150Lec11 - Beginning of Material on Exam#3 Lecture#11 Mafic...

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Beginning of Material on Exam #3: Lecture #11: Mafic Volcanism (Abbott, pp. 192-202, 211) Properties of Magmas and Lavas As discussed in Lecture #3, mafic magmas and lavas tend to be hotter, runnier (less viscous ) and drier than felsic melts (Figure 8.9, p. 193; Figure 8.14, p. 195; Table 8.5, p. 192). The greater viscosity of felsic melts results from them being cooler (often partially solidified) and an abundance of solidifying minerals with three-dimensional silicate crystalline structures. Summary Table of the Properties of Mafic and Felsic Magmas and Lavas (Also see Table 8.5, p. 192) KNOW! Felsic Mafic Temperature Relatively cool (600-900 o C) Very hot (1000-1300 o C) Moisture May be "wet" Relatively dry unless in contact with surface and near-surface water Viscosity High (sticky) When dry, relatively low ("runny") Chemistry Silica-rich Rich in iron and magnesium Table of Igneous Rock Types ( KNOW! ) Mafic Intermediate Felsic Extrusive (Volcanic) Basalt Andesite Dacite Rhyolite Intrusive (Plutonic) Gabbro Diorite Granodiorite Granite Origins of Mafic Magmas Mafic magmas primarily form from the partial melting of peridotite , an ultramafic rock in the mantle. (Ultramafic means that the rock contains even less silica and more magnesium and iron than a mafic rock.) Mafic magmas and their resulting rocks
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(gabbros and basalts) are often associated with hot spots, spreading zones and some subduction zones. As discussed in Lecture #5, hot spot mafic magmas originate from the deep mantle near the boundary with the outer core. In subduction zones, water from subducting ocean sediments and rocks volatilizes and partially melts overlying mantle materials (Figure 8.6, p. 189). In ocean-continent boundaries and hot spots in continents, the chemistry of magmas may change from mafic to intermediate or even felsic as the mafic magmas melt and incorporate surrounding intermediate and felsic continental materials (Figure 8.11, p. 194; Figure 8.6, p. 189; p. 193, 211). In spreading zones, ductile peridotite in the asthenosphere rises. As the hot plastic rock rises, there's less overlying material and pressures on the rock decrease. Decreasing pressure on the hot rocks causes them to partially melt and produce mafic magmas even without any increase in temperature ( decompression melting , p. 194). The magmas may eventually cool in the subsurface to form gabbros or erupt as mafic lavas and cool to basalts along spreading zones, especially on the ocean floors (Figure 8.6, p. 189).
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