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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 l.jpg

Welcome !Geology 111Volcanoes

Dr Ken Sprenke

www.mines.uidaho.edu/~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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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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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. released suddenly by a large landslide taking out part of the volcano.


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Mauna Loa: A released suddenly by a large landslide taking out part of the volcano.shield volcano.


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An extinct shield volcano released suddenly by a large landslide taking out part of the volcano.


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Fissure Eruptions released suddenly by a large landslide taking out part of the volcano. - 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. released suddenly by a large landslide taking out part of the volcano.


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Flow surfaces: released suddenly by a large landslide taking out part of the volcano.

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: perpendicular to the cooling surface. buckling of the lava surface due to stress

while the lava is cooling.



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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. material thrown into air that falls back to surface and adheres together.



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Stratovolcano 1973. (composite volcano) - a volcano built of interbedded lava flows and pyroclastic material.





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Crater 1973. - a depression caused by explosive ejection of magma or gas

A typical summit crater on a cinder cone.


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Caldera 1973. - a depression caused by collapse of a volcano into the cavity once occupied by magma


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Formation of Crater Lake. 1973.

Is Crater Lake a crater?

No! Crater Lake occupies a caldera.



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Some Volcanic Hazards 1973.

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

Lahar: Mudflow composed

of volcanic materials

Volcanic gas emissions.





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