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Volcanoes
Main topics to be covered:
• Magma
• Intrusive Activity
• Volcanoes
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MagmaHow does magma form?• Magma forms when temperatures are high
enough to melt rock.• Rocks usually melt between 800°C and
1200°C• Pressure can also effect the formation of
magma…greater pressure requires higher temperatures to melt rock. (p. 472 Fig. 18-1)
• Presence of water allows rock to melt at a lower temperatures!
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Types of Magma
• Basaltic Magma
• Andesitic magma
• Rhyolitic magma
Do these magma types have familiar names?
What makes these magma types different?
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Magma Types
What makes magma different?
1. Viscosity – resistance to flow. (temp + comp)
2. Gas content – percent of trapped gasses in the magma.
3. Silica content – percent of silica in magma.
4. Explosiveness – how explosive the magma is.
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Magma Typesviscosity Gas content Silica
contentexplosiveness Location of
magma
Basaltic
magma
low 1-2% ~50% least Oceanic and continental crust
Andesitic
magma
Intermediate 3-4% ~60% intermediate Subduction zone at continential margins
Rhyolitic
magma
high 4-6% ~70% greatest Continential crust
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Intrusive Activity
• Magma will rise upward in the Earth, coming in contact with or intruding into the overlying crust. Why does magma rise?
Because it is less dense than surrounding rock!
• Magma can cool inside the Earth forming plutons.
• Plutons are large areas of coarse-grained igneous rock.
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Types of Plutons
• Batholiths – largest type of pluton, irregular shaped mass of coarse-grained igneous rock. (greater than 100 km2)
• Stocks – similar to batholiths, but smaller.• Laccoliths – mushroomed shaped pluton, cause
by rocks bowing upward due to heat and pressure. (much smaller)
• Sill – pluton that forms when magma intrudes into parallel layers of rock. (Palisades Sill)
• Dike – pluton that cuts across existing rock.
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Intrusive Activity Diagram
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Palisades Sill
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Anatomy of a VolcanoVent – opening in the crust that allows magma
to come out on the surface.
Crater – bowl-shaped depression that connects the magma chamber to the vent, usually near the top of the volcano.
Magma chamber – large pocket of magma below the surface.
Caldera – large depressions that are formed when the summit of a volcano collapses into the magma chamber. (Figure 18-11)
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3-Main Types of Volcanoes1. Shield Volcano – a volcano with broad,
gently sloping sides and a nearly circular base.
2. Cinder-Cone Volcano – Volcano with steep sides as a result of material being ejected and being piled up around the vent.
3. Composite Volcano – volcano with alternating layers of lava and volcanic fragments.
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•Numerous layers of basaltic lava build up.
•Low viscosity and less explosive…lava flows for great distances.
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• Contains andesitic lava
• Magma is more viscous and contains more water and gasses.
• More explosive than shield volcanoes.
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•Contains mostly rhyolitic lava.
•Lava contains large amounts of silica, water and gasses.
•Violently explosive!
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Volcanic Material
Tephra – rock fragments thrown into the air by a volcano, classified by size below.
dust <0.25 mm dia. smallest
ash 0.26-2.00 mm dia. larger
lapilli 2.01-64.00 mm dia. larger
blocks >64 mm dia. biggest
(angular)
bombs >64 mm dia. Biggest
(rounded)
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Pyroclastic Flows
• Hot material that is ejected during violent eruptions.
• Contain poisonous gasses, dust, ash, and other material that move down hill at amazing speeds.
• Can reach speeds of 200 km/h
• Can reach temperatures of greater than 700°C
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Locations of Volcanoes• Mostly occur at plate boundaries.
• 80% occur at convergent boundaries.
• 15% at divergent boundaries.
• ~5% at other locations not at boundaries, such as hot spots.
• Most of Earth volcanoes occur in 2 main belts:
1. Circum-Pacific Belt
2. Mediterranean Belt.
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Hot Spots
• Volcanoes that are far away from plate boundaries are usually over hot spots.
• These are unusually hot area of the mantle, where molten material rises to the surface.
• The heat melts rock of the crust, and the magma rises upward…forming volcanoes.
• Hawaii is located over a mantle hot spot.• As the plates move over the hot spot,
volcanoes are formed.http://www.geo.wvu.edu/~donovan/geol101/animations/55.swf
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Anatomy of a Volcano
Video Clips:/www.cnn.com/interactive/nature/0201/volcano/volcano.swf
• http://dsc.discovery.com/convergence/pompeii/videogallery/videogallery.html