VOLCANISM Mount St Helens 1980 eruption Where are

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 Si. O 2 content Mafic composition

steep-slopes (Cinder-cone Volcano)

steep-slopes (Cinder-cone Volcano) Pyroclastic material High Si. O 2 content More Felsic composition

steep-slopes (Composite Volcano)

steep-slopes (Composite Volcano) Alternating layers of Basaltic flows and pyroclastic material Mafic-felsic composition

Volcanic domes

Volcanic domes Felsic lava Very high viscosity Lava accumulates over the vent

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 are the most dangerous