Download Volcanoes

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
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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?
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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/
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Shield Volcano
tectonics.htm
http://wapi.isu.edu/EnvGeo/EG6_volcano/volcanoes.htm
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Stratovolcano
http://wapi.isu.edu/EnvGeo/EG6_volcano/volcanoes.htm
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Kiauea
http://www.freewebs.com/kilauea_16/coolpictures.htm
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Mt. St. Helens
http://epod.usra.edu/archive/images/mt_st_helens_110.jpg
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Lava Fountain
http://volcano.und.nodak.edu/vwdocs/vwlessons/landforms/part8.html
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Cinder cone
http://encarta.msn.com/media_461547396_761570122_-1_1/Cinder_Cone.html