VolcanoesVolcanoes

What are volcanoes?What are volcanoes?

A volcano is an opening in Earth that erupts gases, ash, and lava.

How do volcanoes form?How do volcanoes form?Magma is less dense than the rock

around it, so it is forced toward the surface

Vent: Opening that magma flows through

Crater: The steep-walled depression around a volcano’s vent

Where do volcanoes occur?Where do volcanoes occur?Divergent Plate Boundaries

– Iceland– Mid-Atlantic Ridge

Convergent Plate Boundaries– Oceanic Plate slides beneath Continental

PlateAndes mountains in South AmericaErupt more violently

– Oceanic Plate slides beneath Oceanic Plate

Where do volcanoes occur?Where do volcanoes occur?

Where do volcanoes occur?Where do volcanoes occur?Hot Spots

– Areas between Earth’s mantle and core that are unusually hot.

– Hot rock forces toward the crust where it melts partially to form a hot spot.

– Hawaiian IslandsPacific Plate moving over a stationary hot

spot.

Hot Spot Occurrence Hot Spot Occurrence

What controls eruptions?What controls eruptions?

Trapped Gases

Composition of Magma–Silica Poor–Silica Rich–Varying Silica Content

Liquid Hot Magma!!!!Liquid Hot Magma!!!!

What controls eruptions?What controls eruptions?– Silica Poor: quiet eruption

Basaltic Magma, Pahoehoe flows

– Silica Rich: explosive eruption Granitic Magma, Mount St. Helens,

WA– Intermediate Silica: Quiet / explosive

eruption. Andesitic Magma, Convergent Plate

Boundaries, Krakatau, South America

Forms of VolcanoesForms of VolcanoesShield Volcano

– Quiet eruptions of basaltic lava spread out in flat layers

– Layers buildup and form a broad volcano with gently sloping sides

– Hawaiian Islands

Forms of VolcanoesForms of VolcanoesCinder Cone

Volcanoes– Explosive

eruptions throw lava and rock high into the air

– When rock and ash land, it forms a steep-sided, loosely packed volcano

– Paricutín, Mexico

Forms of VolcanoesForms of VolcanoesComposite Volcano

– Varying explosions depend on amount of silica and trapped gases.

– Alternating layers of lava and ash form a volcano

– Mount Rainier, WA

Violent EruptionsViolent Eruptions1883: Krakatau,

Indonesia

1906: Vesuvius, Italy

1983: Kilauea, Hawaii

2000: Popocatepetl, Mexico

Volcanic Activity ReportsVolcanic Activity Reports

http://volcano.und.edu/vwdocs/current_volcs/current.html

http://www.volcano.si.edu/reports/usgs/