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VOLCANISM. Mount St. Helens, 1980 eruption. Where are volcanoes located?. Volcanoes are located over divergent plate boundaries (mid-oceanic ridges). Laki fissure, Iceland. Volcanoes are located over convergent plate boundaries (subduction zones). Mount Rainier, Tacoma, Washington.
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VOLCANISM Mount St. Helens, 1980 eruption
Volcanoes are located over divergent plate boundaries (mid-oceanic ridges)
Volcanoes are located over convergent plate boundaries (subduction zones)
Gentle-slopes (Shield Volcano) Non-explosive eruptions Basaltic flows Low-viscosity Lava Low SiO2 content Mafic composition
steep-slopes (Cinder-cone Volcano) Pyroclastic material High SiO2 content More Felsic composition
steep-slopes (Composite Volcano) Alternating layers of Basaltic flowsand pyroclastic material Mafic-felsic composition
Volcanic domes Felsic lava Very high viscosity Lava accumulates over the vent
Evolution of Crater Lake Caldera
Hot-spot volcanoes are composed of lavas that… • Are rich in silica, viscous and explosive • Are rich in silica, fluid and explosive • Are poor in silica, fluid and not as explosive as convergent margin volcanoes