Transcript VOLCANOES form where molten rock is vented at Earth`s surface.

VOLCANOES
form where molten rock is vented at Earth’s
surface.
BIG Questions
Where do volcanoes form in the
context of plate tectonics?
Volcanoes aren’t equally dangerous....
how do their hazards differ, and why?
What comes out of a volcano?
Ash
What comes out of a volcano?
Gas
Most common:
H2O
CO2
SO2
HCl
What comes out of a volcano?
Lava
Mauna Loa (Hawai’i): A typical shield volcano
Mt. St. Helens: A typical composite volcano
Mt. St. Helens after its 1980 eruption
Composite Volcano
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Cinder Cone
Built from ejected lava fragments
Steep slope angle
Rather small size
Sunset Crater, near Flagstaff, AZ
A size comparison of the
three types of volcanoes
Volcanologists recognize many types of
volcanoes, but we only need to worry about
two:
Explosive
(“Clogged”)
Andesitic composition
Composite volcanoes
Non-explosive
Basaltic composition
Shield volcanoes
(“Runny”)
Volcanic eruptions
Factors that determine the violence of an eruption
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Composition of the magma
Temperature of the magma
Dissolved gases in the magma
Viscosiy of magma
Viscosity is a measure of a material's
resistance to flow
Volcanic eruptions
Viscosity of magma – factors
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Temperature (hotter magmas
are less viscous)
Composition [Si02 (silica)
content]
- High silica – high viscosity (e.g.,
rhyolitic lava)
- Low silica – more fluid (e.g.,
basaltic lava)
Dissolved gases
- Mainly water vapor and
carbon dioxide –
- Gases expand near the
surface
More silica = more linkage
= more viscous
Magma compositions affect properties
Volcanic Hazards
Lava flows
Ash fall
Pyroclastic flows
Mudflows
Volcanic Gases
Tsunami
Plate-tectonic setting of volcanism
Explosive (andesitic) volcanoes form at subduction
zones.
Plate-tectonic setting of volcanism
At spreading centers, low pressure triggers
mantle melting—fluid basaltic magma rises.
Plate-tectonic setting of volcanism
Within plates, rising plumes of hotter mantle feed
hot spots; varied volcanoes result (basaltic on Hawaii).