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 111Volcanoes
Dr Ken Sprenke
Explosive eruptions are favored by high gas content and high viscosity (andesitic to rhyolitic magmas).
Bursting of bubbles fragments the magma into pyroclastsandtephra (ash).
Cloud of gas and tephra rises above volcano to produce an eruption column that can rise up to 45 km into the atmosphere.
Tephra that falls from the eruption column produces a tephra fall deposit.
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.
After the lateral blast on Mt St.Helens, pyroclastic flows of hot gas and ash raced down the mountain.
Pyroclastic flows from Mt Vesuvius instantly killed and entombed the citizens of Pompei and Herculaneum in 79 AD.
Lateral blasts and debris avalanches occur when gas is released suddenly by a large landslide taking out part of the volcano.
Mauna Loa: A shield volcano.
An extinct shield volcano
Fissure Eruptions - An eruption that occurs along a narrow crack or fissure in the Earth's surface.
Most flows in the Columbia Plateau erupted from fissures.
aa ---- rough
pahoehoe --smooth ropy texture
Hexagonal columns form as lava shrinks. The columns grow perpendicular to the cooling surface.
Pressure ridges: buckling of the lava surface due to stress
while the lava is cooling.
Lava tube: A tunnel below the solid surface of a flow where lava continued to flow.
Spatter cone: small steep-sided cone formed from molten material thrown into air that falls back to surface and adheres together.
200 m high cinder cone in California.
Eldfell: a 300 ft high cinder cone that appeared suddenly in 1973.
Stratovolcano (composite volcano) - a volcano built of interbedded lava flows and pyroclastic material.
Composite (strato-) volcano
Composite volcano in the Phillipines.
St.Helens before it blew up: a stratovolcano..
Crater - a depression caused by explosive ejection of magma or gas
A typical summit crater on a cinder cone.
Caldera - a depression caused by collapse of a volcano into the cavity once occupied by magma
Formation of Crater Lake.
Is Crater Lake a crater?
No! Crater Lake occupies a caldera.
Mt Lassen--world’s largest lava dome.
Some Volcanic Hazards
Nuee ardente--dense cloud of hot pyroclastic materials and gases.
Lahar: Mudflow composed
of volcanic materials
Volcanic gas emissions.
Most volcanoes occur at convergent and divergent plate margins.
Some volcanoes such as Yellowstone and Hawaii are caused
by mantle hot spots.
Next Class: Case Study Pinatubo.