magma formation (3 ways)volcanoes.notebook 3 april 23, 2013 apr 20 3:56 pm formation of volcanoes...
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Volcanoes.notebook
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April 23, 2013
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Volcanoes
Cleveland Volcano, Alaska, May 2006
Taken from the ISS
Apr 202:28 PM
The term volcano can refer to two things:
1. the opening in Earth's crust through which molten rock, gases, and ash erupt
2. the landform that develops around this opening
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Apr 202:28 PM
Volcanoes can be classified as either:
1. Active: has erupted in recorded history (including those currently erupting)
2. Dormant: has not recently erupted, but is considered to be likely to do so in the future
3. Extinct (Dead): has not recently erupted and is not likely to in the future.
Apr 202:37 PM
Magma Formation (3 ways)
• A decrease in pressure can lower the melting temperatures of materials in the asthenosphere. Such a decrease takes place along the rift valley at a midocean ridge, where the lithosphere is thinner and exerts less pressure.
• An increase in temperature can cause materials in the asthenosphere to melt. Such an increase occurs at a hot spot.
• An increase in the amount of water in the asthenosphere can lower the melting temperatures of materials there. Such an increase occurs at subduction boundaries.
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Formation of VolcanoesSince volcanoes are a product of magma, they form under the same circumstances as magma:
1. at subduction boundaries2. at divergent boundaries3. over hot spots
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Subduction Boundaries
• Magma chambers are producedby the melting lithosphere (waterin the subducted rock lowers themelting temperature)
• Stresses in the less dense crust produces cracks.
• Since magma tends to be lessdense than the material around it,the magma rises through the cracks to the surface.
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Divergent Boundaries
• As two plates move apart, the pressure at the surfacedecreases, which in turn lowers the melting temperature.
• The material which rises below a rift rises from deeper,hotter regions within Earth.
• This combination causes large amountsof magma to form. So much, thatmost of the magma that reaches the surface does soat divergent boundaries.
Apr 204:20 PM
Hot Spots• This image (not to scale) shows the location of extinct, dormant, and active volcanoes in the Pacific. Can you see a pattern?
• Within this pattern, the volcanoes are older and extinct the further northwest you go.
• These volcanoes were formed as the plate moved over "hot spots" in the mantle.
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Types of MagmaThe type of molten rock contained in the lava has a large effect on the type of volcano generated. Silica, the principal ingredient in all magmas, determines the magma's viscosity (resistance to flow).
The more silica the magma contains, the more resistant to flow it is.
Basaltic Magma Andesitic Magma Rhoylitic Magma
Silica Content Least Intermediate Most
Gas Content Least Intermediate Most
ViscosityLeast viscous
(ie. flows easily)Intermediate
Most viscous
(ie. flows poorly)
Type of eruption Rarely explosive Intermediate Usually explosive
Melting Temperature Highest Intermediate Lowest
LocationRifts
Oceanic hot spotsSubduction boundaries
Continental hot spots
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Types of Volcanoes
Fissure Eruptions (Lava Plateau)> Flow of magma between cracks in
the lithosphere (like between plates).> This is the cause of the rock
responsible for sea floor spreading.
Shield Volcanoes> Built from magma that is low is silicas,
it flows out easily in many layers.> This creates volcanoes that are very
wide and very tall with a gentle slope.
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Types of VolcanoesComposite Volcano
> These develop when layers ofmaterials from successive eruptionsaccumulate around a vent.
> They tend to have violent eruptions,spaced between periods of relativequiet.
Cinder Cones> The simplest type of volcano, forms
when lava is thrown from a vent into the air. The lava hardens before hitting the ground and accumulates.
> These tend to be smaller than other types of volcanoes, and typically form in groups and on the sides of larger volcanoes
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CalderaSometimes magma beneath a volcano is released, and the top of the volcano can collapse, forming a large cratershaped basin. This basin is called a caldera, and in some cases fills with water to form a lake. Many active volcanoes have calderas at their summit.
Apr 228:50 PM
Homework:
1. "Iron Catastrophe" and Scale Diagram no later than Wed. morning
2. Epicentre Lab Thursday (suggest you complete in MSIP)
3. Chapters 9 and 10 from the text are fair game for quizzes and tests, so take some time and read them!