Volcanoes: eruptive style and associated landforms

Viscosity

• Resistance to flow

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

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

Viscosity

Eruption A Eruption B

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

• 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

Hot spot under continental crust

Notice the direction of plate movement

Andesite

• Intermediate composition lava

Landforms associated with viscous lava

Andesitic lava produces stratovolcanoes

Rhyolitic or dacitic lava produces plugs.

Mt. Rainier

Mt. St. Helens: before the 1980 eruption

Bulge: plug that is pushed out by magma within the conduit.

Mt. St. Helens: after the eruption

Plug dome

Mt. St. Helens:

dome plug

The plug is nearly the height of the Washington Monument and the width of four football fields.

Plug dome: andesitic to rhyolitic in composition

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

Crater Lake: volcanic caldera

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.

Yellowstone caldera formation

Long Valley Caldera

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

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

Size comparison

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

The three Vs

Viscosity

Volatiles

Volume

Icelandic

Strombolian

Plinian

Volcanic material

Pyroclastic debris

• Pieces of older rock and magma

• Ash size to bombs

Lava flow

• Smooth or chuncky

Volcanic Explosivity Index

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