San Francisco Volcanic Field - CoconinoHighSchool三藩火山区- coconinohighschool.ppt
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1、San Francisco Volcanic Field,Arizonas Hotspot,Continental Hotspot Volcanism: Hotspots are thought to be caused by a narrow stream of hot mantle convecting up from the Earths core-mantle boundary called a mantle plume The San Francisco Volcanic Field is comprised of more than 600 volcanoes-most of th
2、em relatively small cinder cones-scattered over an area of about 1,800 square miles.,Most hotspot volcanoes are basaltic because they erupt through oceanic lithosphere (e.g., Hawaii, Tahiti). As a result, they are less explosive than subduction zone volcanoes, in which water is trapped under the ove
3、rriding plate. Where hotspots occur under continental crust, basaltic magma is trapped in the less dense continental crust, which is heated and melts to form rhyolites. These rhyolites can be quite hot and form violent eruptions, despite their low water content.,Arizonas Hotspot,Geologic Background:
4、,The San Francisco volcanic field ranges in composition from basalt to rhyolite and began activity about 6,000,000.,Sunset Crater, northeast of San Francisco Mountain, is the youngest feature in the volcanic field.,Volcanoes and Types of Magma,Most of the more than 600 volcanoes in the San Francisco
5、 Volcanic Field are basalt cinder cones. Basalt has the lowest viscosity of all common magmas. Cinder cones are are built when gas-charged frothy blobs of basalt magma are erupted as an upward spray, or lava fountain.,Once sufficient gas pressure has been released from the supply of magma, lava ooze
6、s quietly out to form a lava flow. This lava typically squeezes out from the base of the cone and tends to flow away for a substantial distance because of its low viscosity.,Volcanoes and Types of Magma,Stratovolcanoes,Stratovolcanoes have moderately steep slopes and form by the accumulation of laye
7、r upon layer of intermediate-viscosity (andesite) lava flows, cinders, and ash, interspersed with deposits from volcanic mudflows (lahars) at lower elevations.,San Francisco Mountain is the only stratovolcano in the San Francisco Volcanic Field and was built by eruptions between about 1 and 0.4 mill
8、ion years ago.,Stratovolcanoes,Lava Domes,The San Francisco Volcanic Field also includes several lava domes. Lava domes are formed by dacite and rhyolite magmas, which have high silica contents. Dacite and rhyolite are so viscous that they tend to pile up and form very steep-sided bulbous masses (do
9、mes) at the site of eruption.,Elden Mountain, at the eastern outskirts of Flagstaff, is an excellent example of an exogenous dacite dome and consists of several overlapping lobes of lava.,Lava Domes,Dacite lava consists of about 63 to 68 percent silica (SiO2). It is one of the most common rock types
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