volcanoes. chapter 9 pages: 247-266 questions: 1-7, 12, 19, 21- 24
TRANSCRIPT
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