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Volcanoes
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Anatomy of a Volcano Magma Chamber –
the source of the volcano’s magma.
Conduit – A tubelike structure connecting the magma chamber to the vent.
Vent – the opening at the top
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Crater vs. Caldera
Small Explosion outward
Large Collapse of the chamber
inward
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Types of Volcanoes Composite Cones
Steep Slopes due to vertical eruptions
Composed of alternating layers of ash and lava
Explosive eruptions Generally found at
subduction zones where oceanic crust is pulled under continental crust
Andesitic magma
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Types (continued) Shield Cones
Flat, broad slopes due to passive oozing lava
Composed of layered lava flows
Passive eruptions Found at rift valleys
and hot spots in the ocean
Basaltic magma Hawaiian Dangers
Pa’hoe’hoe flows A’a’ flows
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Pa’hoe’hoe Fluid
Oozing
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A’a’ Rocky
Jagged
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Types (continued) Cinder Cones
Small cones Steep slopes Small, explosive
eruptions Found on the flanks of
composite and shield cones
Composed of tephra (ash)
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Warning Signs of a Composite Cone Eruption
Strange Animal Behavior Presence of sulfur gas Increased earthquake activity Increased avalanches and landslides Swelling of the mountain Minor small eruptions
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Dangers of a Composite Cone Eruption Pyroclastic flow –
dense, fast moving currents of ash and rock 450 mph 1800 degrees
farenheit
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Dangers (continued) Lahar – a mudflow
mixture of ash and water Can be 400 feet deep Can cover 100 square
miles
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Dangers (continued) Tephra – Fragments
of volcanic rock blasted from an eruption As small as ash As large as boulders
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Intrusive Volcanism
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Formation of Magma Comes from the Upper Mantle when it melts
Forms in 3 general areas Subduction Zones Spreading Centers Mantle Plumes
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Subduction Zone The friction of the plate
subducting adds heat that melts the rock
The seawater lowers the melting point allowing it to melt compared to the rock around it
A decrease in pressure also helps rock melt
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Spreading Center When the plates split
apart, the asthenosphere moves upwards and the loss of pressure helps melt the rock
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Mantle Plumes Mantle rock rises
beneath the lithosphere and the loss of pressure helps melt the rock.
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Factors that affect the magma Amount of silica present
Silicates form in long chains and cause the magma to slow
High Viscosity (resistance to flow)
Amount of water present Magma cools when it rises helping it solidify but
the decreasing pressure also helps keep it liquid. Dropping pressure tends to override the cooling
effect unless there is a lot of water present. In this case the water is lost as steam and helps
the magma solidify faster
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Igneous Rocks Igneous rocks are rocks that have formed from
the direct cooling and crystallizing of magma.
They can be classified based upon two things: The type of magma The rate of cooling
There are 2 types of magma that form most igneous rock
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Andesitic Magma The type of magma that is
explosive
High silica
Is sticky and doesn’t flow well.
Creates lighter colored rocks
Has more water content because it is trapped by the thicker magma
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Basaltic Magma Magma that is very fluid
Low silica
Flows very well
Produces darker colored rocks
Low water content because it escapes as steam
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Cooling Rates Igneous rocks that are
formed at or near the Earth’s surface cool very quickly. These are called
extrusive rocks
Igneous rocks that are formed deep below Earth’s surface cool very slowly. These are called
intrusive rocks
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Extrusive Rocks These rocks cool so
quickly that the minerals do not have the time to develop Small crystal size
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Intrusive Rocks These rocks cool so
slowly that the minerals take a long time to develop Large crystal size
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Other Properties Some igneous rocks
have holes in the surface
This is from the escape of gases as the rock cools
This rock is considered vesicular when this occurs.