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?
What do you remember about oceanic and
continental crust?
• Oceanic crust:
– Lower in silicon and
oxygen
– Higher in
magnesium and iron
• Continental crust:
– Higher in Si and O
– Lower in Mg and Fe
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 Subduction zones: Ring of
and hot spots under
Fire (Cascadia subduction
oceanic crust (Hawaii)
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.
Lava fountains
Hawaii?
Basalt
Aa flows: blocky, jagged, slow
moving flows
Shield volcanoes are
associated with basaltic lava
flows.
Type of lava produced?
• Basalt
• Fluid- low viscosity
What other tectonic setting would produce
basaltic lava flows?
Subduction zone: stratovolcanoes
Steep at summit,
less steep at base
Composite Volcano: subduction zones
Built over a long period of time
Can reach over 14000 feet
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