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Intro to volcanoes


Steve Daughtrey

on 12 November 2010

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Transcript of Intro to volcanoes

volcanoes Getting their energy from deep in the earth there are three ways volcanoes are formed. Subduction zones are where many volcanoes are formed. This is where one tectonic plate moves under another. This is commonly what happens when dense oceanic crust meets continental crust. The result is oceanic crust moving below the continent where water is infused into the rock lowering the melting point to form magma. A subdution zone is formed and the less dense magma heads toward the surface forming a volcano. Midocean ridges form from an upwelling of magma whch pushes two plates apart. This isthe scond type of volcanic activity. HotSpots -- areas where volcanoes form over a rising mantle plum existing far below in the tectonic plate boundaries. Magma- exists below the surface
Lava - exists above the surface Volcano - opening (vent) in earth’s surface where lava, ash, and gasses are expelled Volcano -- the vent where magma, ash and gas are expelled onto the surface of the Earth. Two types of Magma
Mafic (MAF ik) also called Basaltic magma-- Dark colored -- Rich in magnesium and iron
Makes up oceanic crust and continental
Runny often resulting in quiet eruptions

Felsic (FEL sik) also called Granitic magma-- Light Colored -- Rich in silicates
Makes up continental crust
sticky, often resulting in explosive eruptions. Eruptions can be quiet or explosive. Why? Quiet Erupions -- Small amount of trapped gasses occur in magma. Associated with malfic magma and shield volcanoes.
Example: Hawaii -- Mt. Kileaua Explosive Eruptions -- Large amounts of trapped gasses occur in magma. Often associated with Felsic Magma and cinder cone or composite volcanoes.
Example: Mt. St. Helens Pyroclastic material (tephra)-- The rock that formes during an eruption.
Classified by the size of particles Types of pyroclastic material:

Largest -- Volcanic blocks
64mm or less -- lapilli (luh PIL ie)
2mm or less -- Volcanic ash
0.25mm or less -- Volcanic dust What does the volcano look like? Depends on the cone

There are three types of cone to look for: Calderas -- This large depression in the ground is formed when the magma chamber partially empties collapsing the earth above. Shield -- Broad, wide area formed from quiet eruptions where lava flows have layered the land. These types are often formed over Hot Spots. Example -- Hawaii Composite -- Forms the largest volcanic mountains. Sometimes called stratovolcanoes.
Formed from alternating layers of pyroclastic material (tephra) and lava flows. These volcanoes are often formed over Convergent boundaries. Cinder -- Steep sides with a concave top. Formed from explosive eruptions and made mostly from pyroclastic material (tephra). These volcanoes are often formed over Divergent boundaries. Backup -- Remember lava forms Igneous rock. Igneous rock that forms below
ground is intrusive, igneous rock that is formed above ground is extrusive. most are dormant (not active)
-over 600 are active
-most are located on the “ring of fire”
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