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 1)   along each convergent plate boundaries where plates move toward each other and one sinks beneath the other.    2)    along divergent plate boundaries, where plates move away from each other and a new seafloor is created.  3)   areas within the plates proper that are not associated with any plate boundary.
 * Plate Tectonics and Igneous Activity **
 * More than 800 active volcanoes that have been identified, most are located along the margins of the ocean basins
 * This group of volcanoes consists mainly of composite cones that emite volatile- rich magma having an intermediate composition that occasionally produces awe-inspiring eruption.
 * The volcanoes comprising a second group emit very fluid basaltic lavas and are confined to the deep ocean basins including well know examples on Hawaii and Iceland.
 * The group contains many active submarine volcanoes that dot the ocean floor
 * Particularly notable are the innumerable small seamounts that lay algon the axis of the mid-ocean ridge
 * A third group includes those volcanic structures that are irregularly distributed in the interior of the continents.
 * Africa is notable because it has many potentially active volcanoes including Mount Kilimanjaro the highest point on the continent.
 * Volcanism on continents is the most diverse ranging from eruptions of very fluid basaltic lavas, like those generated the Columbia Plateau to explosive eruptions of silic-rich rhyolitic magma as occurred in Yellowstone
 * Until the late 1960s geologist had no explanation for the apparently haphazard distribution of continental volcanoes nor were they able to account for the almost continuous chain of volcanoes that circles the margin of the pacific basin.
 * Three zones of igneous activity and their relationship to plate boundaries.
 * Volcanism at convergent plate margin results in the development of a linear or slightly curved chain of volcanoes called a volcanic arc.
 * The early stage of island arc volcanism is typically dominated by the eruption of fluid basalts that build numerous shield like Structures on the ocean floor.
 * Volcanism associated with convergent plate boundaries may also develops where slabs of oceanic lithosphere are subducted under continental lithosphere to produce a continental volcanic arc.
 * Intraplate volcanism "within the plate" Canary Island, Yellowstone , and several volcanic centers.
 * Hot Spot: Localized volcanic region a few hundred kilometers across.
 * About 10 percent of earth population lives in the vicinity of active volcano.
 * Cities like: Seattle, Mexico city, Tokyo , Naples , and Quite are located near a volcano.