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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:

High silica contentfelsic

High amount of dissolved gasses

low silica contentmafic

high flow-rate

More explosive

Less explosive

Low flow-rate

Low amount of dissolved gasses

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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 Types1.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 Activity1. Convergent Plate BoundariesIsland Arc:•Two oceanic plates collide•One plate subducts under the other

Continental Volcanic Arc:

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–Oceanic plate collides with continental plate–Andesitic magma is produced and rises

2. Divergent plate boundaries Oceanic Ridge:Two oceanic plates separateMagma rises to fill the crack --> new oceanic crust -->

Continental Rifting:Continental plates separateMagma rises to fill the riftCreates vast outpouring of basaltic lavas; some large composite volcanoes

3. Intraplate VolcanismVolcanism occurring in interior of a plateMantle plume =

Lithosphere rock melts and basaltic magma erupts as a volcano

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As the plate moves over this hot spot,