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Welcome ! Geology 111 Volcanoes. Dr Ken Sprenke www.mines.uidaho.edu/~sprenke. Explosive eruptions are favored by high gas content and high viscosity (andesitic to rhyolitic magmas). Expansion of gas bubbles is resisted by high viscosity of magma - results in building of pressure

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Welcome ! Geology 111 Volcanoes

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Welcome geology 111 volcanoes l.jpg

Welcome !Geology 111Volcanoes

Dr Ken Sprenke


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Explosive eruptions are favored by high gas content and high viscosity (andesitic to rhyolitic magmas).

  • Expansion of gas bubbles is resisted by high viscosity of magma - results in building of pressure

  • High pressure in gas bubbles causes the bubbles to burst when reaching the low pressure at the Earth's surface.

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Bursting of bubbles fragments the magma into pyroclastsandtephra (ash).

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Cloud of gas and tephra rises above volcano to produce an eruption column that can rise up to 45 km into the atmosphere.

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Tephra that falls from the eruption column produces a tephra fall deposit.

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If eruption column collapses a pyroclastic flow  may occur, wherein gas and tephra rush down the flanks of the volcano at high speed.  This is the most dangerous type of volcanic eruption. 

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After the lateral blast on Mt St.Helens, pyroclastic flows of hot gas and ash raced down the mountain.

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Pyroclastic flows from Mt Vesuvius instantly killed and entombed the citizens of Pompei and Herculaneum in 79 AD.

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Lateral blasts and debris avalanches occur when gas is released suddenly by a large landslide taking out part of the volcano.

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Shield volcano.

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Mauna Loa: A shield volcano.

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An extinct shield volcano

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Fissure Eruptions - An eruption that occurs along a narrow crack or fissure in the Earth's surface.

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Most flows in the Columbia Plateau erupted from fissures.

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Flow surfaces:

aa ---- rough

pahoehoe --smooth ropy texture

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Hexagonal columns form as lava shrinks. The columns grow perpendicular to the cooling surface.

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Pressure ridges: buckling of the lava surface due to stress

while the lava is cooling.

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Lava tube: A tunnel below the solid surface of a flow where lava continued to flow.

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Spatter cone: small steep-sided cone formed from molten material thrown into air that falls back to surface and adheres together.

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200 m high cinder cone in California.

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Eldfell: a 300 ft high cinder cone that appeared suddenly in 1973.

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Stratovolcano (composite volcano) - a volcano built of interbedded lava flows and pyroclastic material.

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Composite (strato-) volcano

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Composite volcano in the Phillipines.

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St.Helens before it blew up: a stratovolcano..

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Crater - a depression caused by explosive ejection of magma or gas

A typical summit crater on a cinder cone.

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Caldera - a depression caused by collapse of a volcano into the cavity once occupied by magma

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Formation of Crater Lake.

Is Crater Lake a crater?

No! Crater Lake occupies a caldera.

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Mt Lassen--world’s largest lava dome.

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Some Volcanic Hazards

Nuee ardente--dense cloud of hot pyroclastic materials and gases.

Lahar: Mudflow composed

of volcanic materials

Volcanic gas emissions.

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Most volcanoes occur at convergent and divergent plate margins.

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Some volcanoes such as Yellowstone and Hawaii are caused

by mantle hot spots.

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Next Class: Case Study Pinatubo.

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