Volcanoes

Chapter 9

• Pages: 247-266• Questions: 1-7, 12, 19,

21- 24

Volcanoes are associated with plate boundaries and hot spots

What do you remember about oceanic and continental crust?

• Oceanic crust:– Composition?

• Continental crust:– Composition?

• Oceanic crust:– Lower in silicon and

oxygen– Higher in

magnesium and iron

• Continental crust:– Higher in Si and O– Lower in Mg and Fe

What do you remember about oceanic and continental crust?

Who cares about the composition?

Composition determines the explosivity of a volcano

Volcanic rock names are based on the composition

Basalt Andesite Rhyolite

Lowest % of silicon and oxygen

Intermediate % of silicon and oxygen

Highest % of silicon and oxygen

Oceanic crust Mixture of oceanic and continental crust

Continental crust

Divergent plate boundaries and hot spots under oceanic crust (Hawaii)

Subduction zones: Ring of Fire (Cascadia subduction zone)

Hot spot under continental crust (Yellowstone)

Magma or lava?

Within the crust On the Earth’s surface

The more silicate tetrahedrons in a lava, the more explosive the

volcano.

Which lava contains the higher percentage silicon and oxygen?

Rhyolitic or andesitic lavas

Why?

Viscosity

• Resistance to flow

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

Viscosity: resistance to flow

• Lavas with higher percentage of silicon and oxygen have a high viscosity-pasty

• Lavas with a lower percentage of silicon and oxygen have a low viscosity-fluid

Rhyolitic lava is associated with continental crust

Rhyolite is cooled from

pasty or thick lavas.

Andesitic lava is a combination of oceanic and continental crusts

Andesite is cooled from

pasty or thick lavas.

Rhyolitic and andesitic lavas produce pyroclastic debris

Basaltic lava is associated with oceanic crust

Basalt is cooled from fluid lava.

Basaltic eruptions produce lava flows

Pahoehoe flows: smooth, ropey surface

What type of lava is produced at each plate boundary or hot spot?

Hot Spots

• The type of lava produced depends on hot spot location

• Under oceanic crust?• Under continental crust?

Tectonic setting of Hawaii?

Hot spot under oceanic crust

Hot spot under oceanic crust

• Stationary plume under oceanic crust

• Hot plume acts like a Bunsen burner

• Melts oceanic crust• Material from the

mantle is also added

As the Pacific plate moves over the hot spot, a chain of islands form.

Hawaii?

Aa flows: blocky, jagged, slow moving flows

Shield volcanoes are associated with basaltic lava flows.

Lava fountains

Basalt

Type of lava produced?

• Basalt• Fluid- low viscosity

What other tectonic setting would produce basaltic lava flows?

Subduction zone: stratovolcanoes

Composite Volcano: subduction zones

Built over a long period of time

Can reach over 14000 feet

Steep at summit, less steep at base

Mt. St. Helens versus Kiliauea

• Subduction zone• Stratovolcano• Andesitic lava (pasty)• Violent eruptions– Pyroclastic debris

• Hot spot under oceanic crust

• Shield volcano• Basaltic lava (fluid)– Lava flows

Mt. St. Helens: before the 1980 eruption

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

Kilauea eruption

Caldera Formation: enormous

eruption in a short period of

time

Rhyolitic lava composition

Size Comparison of Volcanic Landforms

How is magma produced?

Decompression Melting

Decompression Melting

Decrease pressure, lower melting temperature

Addition of volatiles lowers melting temperature

Adding water, lowers the rock’s melting temperature.

Gases drive volcanic eruptions