volcanoes: eruptive style and associated landforms
TRANSCRIPT
• Which eruption was produced by high viscosity lava? What are the clues?
Viscosity
Eruption A Eruption B
• Tectonic setting
• Source of lava
• Composition
Why does one type of lava have a higher viscosity than the other?
Andesite: sediments, water, oceanic crust and continental crust
Intermediate composition
Basalt: asthenosphere and oceanic crust
Lower percentages of silicon and oxygen
The Silicon Tetrahedron
• Acts as a thickening agent• Building block to all rock forming minerals• Higher percentage = higher viscosity
Basalt < 55%
Andesite = 55-65 %Rhyolite > 65%
Rhyolite is the lava type with the highest percentages of silicon and
oxygen
• Most violent eruptions
Landforms associated with viscous lava
Andesitic lava produces stratovolcanoes
Rhyolitic or dacitic lava produces plugs.
Mt. St. Helens: before the 1980 eruption
Bulge: plug that is pushed out by magma within the conduit.
Mt. St. Helens:
dome plug
The plug is nearly the height of the Washington Monument and the width of four football fields.
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
Caldera formation and subsequent plug
1.Volcanic eruption
2. Large volume of material extruded
3. Magma chamber empties
4. Volcano collapses into the empty magma chamber
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.
Landforms associated with low viscosity lavas
Basaltic lava flows produce shield volcanoes and lava plains or flood basalts.
Shield volcano
Mauna Loa is 9 miles high
Built over a long period of time
Associated with basaltic lava
• Medicine Lake volcanic field
• Mt. Shasta is in the background– Tectonic setting?
Modoc Plateau, northeastern California (extension)
Basaltic lava flows from fissures
Layer upon layer of lava flows
Covers continental crust
14-16 million years old
Columbia River Basalts
What happened in Iceland?
• Eyjafjallajokull's eruption creates an ash cloud that closed Europe’s airports for weeks
• Shield volcano eruption under a layer of ice
Cinder cones: found in most setting
Hawaii
Mojave Desert
•Short lived events•made of cinders•generally about 1000 feet high
Composition,Viscosity and Eruptive Style
Basalt Andesite Rhyolite
Fluid PastyViscosity
Composition
Quiet ViolentEruptive Style
Hot CoolTemperature
Volcanic material
Pyroclastic debris
• Pieces of older rock and magma
• Ash size to bombs
Lava flow
• Smooth or chuncky