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Note: Volcanoes
Nature of Eruptions
•Eruptions can be violent and explosive (e.g. Mnt. St. Helens) or gentle
(e.g.Kilauea volcano, Hawaii)
Explosiveness depends on:
–Viscosity of lava
–Quantity of dissolved gas in magma
Eruption Column:
Lava fountain:
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Lava Flows:
–90% of lava on Earth is basaltic - less viscous; can flow 100s of km from vent
Pahoehoe Flows:
–Fluid basaltic lava partially congeals and has wrinkled skin
Lava Tubes:
–Pahoehoe lava hardens on outside, but lava flows beneath leaving cavelike
tubes.
Aa Flows:
–Basaltic lava
–Surface of flow has rough, jagged edges and projections
•Block Lavas:
–Andesitic & rhyolitic lava (high viscosity)
–Slow, move as a solid block
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Gasses:
–Gasses are dissolved under high pressure (like CO2 in colas)
–Mostly water; other gasses:
3.Pyroclasitc Materials:
–Magma that is thrown from volcano
–Large pieces fall close to vent, smaller dust & ash can, go further
Volcano Types
1.Shield Volcanoes:
–Broad, dome shape
–Basaltic (mafic) lavas
–Gentle slopes
2.Cinder Cones:
–Made of emitted pyroclastic material
–Smallest type
–Steep slope angle = 30 - 40 degrees (angle of repose)
- made entirely of…
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3.Composite Volcanoes/ Stratovolcanoes:
–Most dangerous
–Bigger than cinder cones; less symmetrical
4.Calderas
–Not a type of volcano, but a volcanic structure
–A depression formed from a depleted & collapsed magma chamber
•Crater Lake, Oregon
Plate Tectonics and Igneous Activity
1. Convergent Plate Boundaries
Island Arc:
•Two oceanic plates collide
•One plate subducts under the other
Continental Volcanic Arc:
–Oceanic plate collides with continental plate
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–Andesitic magma is produced and rises
2. Divergent plate boundaries
Oceanic Ridge:
Two oceanic plates separate
Magma rises to fill the crack --> new oceanic crust -->
Continental Rifting:
Continental plates separate
Magma rises to fill the rift
Creates vast outpouring of basaltic lavas; some large composite volcanoes
3. Intraplate Volcanism
Volcanism occurring in interior of a plate
Mantle plume =
Lithosphere rock melts and basaltic magma erupts as a volcano
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As the plate moves over this hot spot,