Volcanoes: eruptive style and associated landforms

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Volcanoes eruptive style and associated landforms
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Viscosity

• Resistance to flow

Which test tube contains the fluid with high
viscosity? Left? Right?
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Viscosity

• Which eruption was produced by high viscosity
  lava? What are the clues?

Eruption A
Eruption B
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Why does one type of lava have a higher viscosity
than the other?
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Why does one type of lava have a higher viscosity
than the other?

• Tectonic setting
• Source of lava
• Composition

Basalt asthenosphere and oceanic crust
Andesite sediments, water, oceanic crust and
continental crust
Lower percentages of silicon and oxygen
Intermediate composition
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The Silicon Tetrahedron

• Acts as a thickening agent
• Building block to all rock forming minerals
• Higher percentage higher viscosity

Rhyolite gt 65
Andesite 55-65
Basalt lt 55
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Rhyolite is the lava type with the highest
percentages of silicon and oxygen

• Most violent eruptions

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Hot spot under continental crust
Notice the direction of plate movement
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Andesite

• Intermediate composition lava

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Landforms associated with viscous lava
Andesitic lava produces stratovolcanoes
Rhyolitic or dacitic lava produces plugs.
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Mt. Rainier
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Mt. St. Helens before the 1980 eruption
Bulge plug that is pushed out by magma within
the conduit.
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Mt. St. Helens after the eruption
Plug dome
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Mt. St. Helens dome plug
The plug is nearly the height of the Washington
Monument and the width of four football fields.
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Plug dome andesitic to rhyolitic in composition
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Lassen Peak

• Lassen Peak is a plug dome volcanic landform
• Built from felsic lava
• One of the largest on Earth
• Carved by glaciers during the Ice Age

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Crater Lake volcanic caldera
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• Caldera formation and subsequent plug
• Volcanic eruption
• Large volume of material extruded
• Magma chamber empties
• Volcano collapses into the empty magma chamber

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Yellowstone hot spot under continental crust

• Three large eruptions in the last 2 million, 1.3
  million and 600,000 years ago

Calderas formed when felsic lava produced
enormous eruptions.
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Yellowstone caldera formation
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Long Valley Caldera

• An enormous eruption 760,000 years ago, forming a
  caldera

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Landforms associated with low viscosity lavas
Basaltic lava flows produce shield volcanoes and
lava plains or flood basalts.
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Shield volcano Mauna Loa is 9 miles high Built
over a long period of time Associated with
basaltic lava
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Modoc Plateau, northeastern California (extension)

• Medicine Lake volcanic field
• Mt. Shasta is in the background
• Tectonic setting?

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Basaltic lava flows from fissures Layer upon
layer of lava flows Covers continental crust
Columbia River Basalts
14-16 million years old
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What happened in Iceland?

• Eyjafjallajokull's eruption creates an ash cloud
  that closed Europes airports for weeks
• Shield volcano eruption under a layer of ice

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Size comparison
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Cinder cones found in most setting
Hawaii

• Short lived events
• made of cinders
• generally about 1000 feet high

Mojave Desert
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Composition,Viscosity and Eruptive Style
Composition
Basalt
Andesite
Rhyolite
Fluid
Pasty
Viscosity
Eruptive Style
Violent
Quiet
Cool
Temperature
Hot
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The three Vs
Viscosity
Strombolian
Icelandic
Volatiles
Volume
Plinian
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Volcanic material

• Pyroclastic debris

• Lava flow

• Pieces of older rock and magma
• Ash size to bombs

• Smooth or chuncky

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Volcanic Explosivity Index

• Volume of material
• How high the eruption column reached
• How long the main eruption occurred