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Rock melts when the temperature within the earth (geotherm) exceeds the
melting point (solidus) of rock.
This happens for different reasons at (1) subduction zone volcanoes, (2)
mid-ocean ridge volcanoes, and (3) hotspot volcanoes.
Normally the geotherm does not cross the solidus, so there is no melting.
(THE MANTLE IS SOLID!!!!)
BUT, it is very close at about 100-250 km in depth ( Asthenosphere).
What are the 4 main forms of
volcanoes?
1. Seafloor Subduction
Subduction Zones:
“wet” melting
Mt. Fujiyama
Cotopaxi Volcano, Equador
Cotopaxi, by Frederic Church, 1862
Stratovolcano
(Composite Cone)
Cerro Negro, Nicaragua
Cerro Negro, Nicaragua
Paricutin, Mexico (1946)
Mayon, Philippines (1984)
Pacaya, Guatemala
Pacaya, Guatemala (Agua volcano in background)
Pacaya, Guatemala (2004)
Pyroclastic flow
sweeps down the side
of Mayon Volcano,
Philippines, 1984.
Mayon, Philippines (1984)
Mayon, Philippines (1984)
Mayon, Philippines (2000)
Mt. Pinatubo,
Philippines, 1991.
Pinatubo, Philippines (1991)
Pinatubo, Philippines (1991)
Pinatubo, Philippines (1991)
A small lahar triggered by rainfall in Guatemala, 1989.
Pinatubo, Philippines (1991)
Pinatubo, Philippines (1991)
Mt. Pinatubo: So
much ash into the
atmosphere that
Earth’s temperature
dropped, and sunsets
were redder.
What are the 4 main forms of
volcanoes?
2. Mid-Ocean Seafloor Spreading
Ridges: “pressure
release” melting
Figure 4-15b
Ocean Crust Layers
What are the 4 main forms of
volcanoes?
3. Continental
Rifting
Continental Rifting leaves a complex structure beneath
passive margins like the east coast of North America
What are the 4 main forms of
volcanoes?
4. Hotspot Mantle Plumes
Hawaii rises more than 5 miles above the seafloor.
Hawaii
Kilauea, Hawaii
Mauna Loa, Hawaii
Figure 4-7b
Kilauea, Hawaii
Figure 3-1
Pahoehoe lava, Hawaii
“Aa” lava flow, Kilauea, Hawaii
Figure 4-10b
Figure 4-10c
Mt. St. Helens: Giant Eruption May 18, 1980
Mt. Adams
Mt. St. Helens
Mt. St. Helens:
Before
May, 1980
After
Phase 1: Small earthquakes and puffs of steam indicate
that magma is rising. Bulge develops in North face.
Phase 2: A magnitude 5.1 earthquake shakes mountain,
dislodging bulge which slides down mountain. Decreased
pressure on magma starts lateral blast.
Phase 3: Eruption causes a second block to break free,
exposing more magma and initiating an eruption column.
Lateral blast goes at 300 mph, covers 230 square miles.
Phase 4: The Eruption Column reaches 80,000 feet in less
than 15 minutes.
Mt. St. Helens Earthquakes: 1995-2005
The Dome is Growing Again
The center of the
Yellowstone Caldera
is rising up at 7
cm/year!
Yellowstone Plume
Finished 15 minutes early again.
Not many questions. Got to
Chemical weathering in the next
lecture.