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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
Volcanoes are located over Hot spots
What are the products of volcanism?
Welded tuff (ash & flow deposits)
Vesicular Basalt
Pyroclastic eruption at Arenal Volcano, Costa Rica
Volcanic Bombs
Pyroclastic flow at Mount Unzen
Volcanic Breccia
WHY DO DIFFERENT
VOLCANOES HAVE
DIFFERENT SHAPES?
Gentle-slopes (Shield Volcano)
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 (Cinder-cone Volcano)
steep-slopes (Composite Volcano)
Alternating layers of
Basaltic flows and
pyroclastic material
Mafic-felsic composition
steep-slopes (Composite Volcano)
Volcanic domes
Felsic lava
Very high viscosity
Lava accumulates over the vent
Volcanic domes
Evolution of
Crater Lake Caldera
Mount St. Helens (Before eruption)
Mount St. Helens (During eruption)
Mount St. Helens (After eruption)
Mount St. Helens (Before eruption)
Hot Spots and Plate movement
Hot-spot volcanoes are composed of lavas that…
1. Are rich in silica, viscous and explosive
2. Are rich in silica, fluid and explosive
3. Are poor in silica, fluid and not as explosive as convergent margin volcanoes
Volcanic Hazards
Pyroclastic Flows arethe most dangerous
