Volcanoes

• Why do they form?

– Heat and pressure in the earth melts mantle rocks, or earth’s crust melts forming magma, which rises to the surface through cracks and weak spots in the crust

Magma rises to the surface because it is less dense than other mantle and rock material.

When magma reaches the surface it is called lava

Lava Types

• There are different types of magma

– The type depends on the kind on the minerals that make up the rock material that was melted to form the magma **(silicia).

– Three common types of magma are basaltic and granitic (andesitic, dacite and rhyolitic)

Basaltic magma

– Comes from rocks in the mantle– Lowest in silica– Least viscous, flow easily

Grantic Type #1 -Andesitic Magma

• Mostly from the melting of wet oceanic crustal rocks

• This occurs at subduction zones where an ocean plate goes beneath another ocean plate or a continent.

• Higher in silica, has more dissolved gasses, more viscous than basaltic lava

Granitic Type #2 -Rhyolitic Magma

• Forms from melting of continental crust rocks.

• High in silica, high viscosity• Often solidifies before it reaches the

surface.

Main Ideas about magma/lava

• The more silica it has, the higher the viscosity, the more explosive the volcano.

• Temperature can affect the viscosity of magma. The hotter the temperature the lower the viscosity so the faster it flows.

Where do volcanoes form?

• Divergent plate boundaries• Convergent ocean to ocean subduction

zone• Convergent ocean to continent

subduction zone• Hot spots

Divergent Boundaries

• Basaltic lava flows• Form large shield shaped volcanoes

from repeated layers of flowing lava

Convergent boundaries• Subduction zones result in melting

ocean and continental crust• Viscous lava and higher gas content

leads to explosive eruptions

Erupts large amounts of fragmented material (tephra), much of which falls back to earth forming large steep-sided volcanoes called stratovolcanoes

Hotspot

• Can form anywhere. • Magma rise from deep in the mantle• Hawaii – shield volcano• Yellowstone – explosive caldera volcano

Types of Volcanoes

• Shield• Cinder cone• Stratovolcano (composite)

Shield Volcano

• Layers of “quiet” basaltic lava flows• Divergent boundaries, oceanic hotspots

Stratovolcanoes (composite)• Andesitic/rhyolitc type magmas• Higher in silica, trapped gasses• Explosive eruptions and lava flows

Cinder cone volcanoes• Most common type. Form from eruptions that

are not superexplosive.• Cinders cool while they are thrown through

the air

• Occur on flanks of shield volcanoes stratovolcanoes, or in fields of smaller eruptions

• They could have lava flows that occur at the base or out of the flank but not the main vent.

• Smaller than other volcanoes usually

less than 250 m high and 500 m in diameter

What happens when a volcano erupts?

• Lava flows• Tephra, falling ash• Pyroclastic flows, glowing avalanches• Mud and Debris Flows (lahars)• Toxic Gasses• Tsunamis

Warning signs

• Ground deformation, magma collects in magma chamber causing swelling of volcanoes slopes

• Earthquakes and tremors, especially increased activity

• Gas and Steam Emissions – especially increased activity

Picture credits• Convergent boundary http://courses.unt.edu/hwilliams/GEOG_3350/examreviews/

tectonics.htm

• Shield Volcano

http://wapi.isu.edu/EnvGeo/EG6_volcano/volcanoes.htm

• Stratovolcano

http://wapi.isu.edu/EnvGeo/EG6_volcano/volcanoes.htm

• Kiauea

http://www.freewebs.com/kilauea_16/coolpictures.htm

• Mt. St. Helens

http://epod.usra.edu/archive/images/mt_st_helens_110.jpg

• Lava Fountain

http://volcano.und.nodak.edu/vwdocs/vwlessons/landforms/part8.html

• Cinder cone

http://encarta.msn.com/media_461547396_761570122_-1_1/Cinder_Cone.html