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Volcanoes and
volcanism
Goals
To examine the relationship between magma
composition, the kinds of volcanoes and volcanic
processes that occur, and plate-tectonic setting.
Magma viscosity
Viscosity: Measure of resistance to flow. The
greater the viscosity, the harder it is to get a
material to flow.
Honey
more viscous
Olive oil
less viscous
Magma viscosity
Viscosity of magma is controlled by two things:
temperature and composition
– Higher T = lower viscosity
– Higher silica content = higher viscosity. Felsic
magmas don’t like to flow.
Magma gas content
• All magmas contain 1–9% dissolved gases—
mostly CO2 and H2O.
• These gases behave like the carbonation in a
soft-drink or a beer.
Eruptive materials: What comes out of
volcanoes
Lava flows: Liquid or
Pyroclastic materials:
semi-liquid magma
flowing on Earth’s
surface
Material blasted out of
volcano during an
explosive eruption
Lava flows
“Ropy” pahoehoe-type lavas:
Lowest viscosity, always mafic.
“Blocky” a’ a’-type lavas:
Low viscosity, colder mafic
lavas and hot intermediate
lavas
Pyroclastic materials
Tephra: Magma blasted into the air to form very
hot (500° C) solid particles
Volcanic dust: Tephra
particles less than 1/1000
mm in diameter.
Volcanic ash: Tephra particles
between 1/1000 mm to 2mm
in diameter.
Volcanic bombs: Tephra
chunks >64mm in diameter
Volcanic dust can stay suspended in the upper
atmosphere for years
• Large eruptions can alter global climate
• Really awesome sunsets
Volcanic ash falls like snow for 100’s of miles
down-wind from eruption
Pyroclastic materials
Pyroclastic flows a.k.a. Nuee Ardente: Billowing
clouds of super-heated gas and ash that flow
down the slope of the volcano
Pyroclastic flows travel at more than 50 mph
Pyroclastic materials
Volcanic mudflows a.k.a. Lahars: Eruption melts
snow, glacial ice, and/or frozen soil
• Mixture of water and debris flow rapidly down
the slope of a volcano
Types of Volcanoes—Dictated by
magma type
Most volcanoes have two basic elements: A
volcanic cone and a central crater
Crater
Mafic magmas
Shield volcanoes: Enormous, gently sloping
volcanic mounds
• Form over oceanic hotspots: localized zone of
hot mantle upwelling
• Largest topographic features on Earth
Shield volcano—Big Island of Hawaii
Hot Spot Volcano Tracks
Hawaiian Islands are hot-spot volcanoes on
the Pacific plate
Which way is
the pacific
plate moving?
?
?
Hawaii
Mafic magmas
Fissure eruptions: Low-viscosity magma flows up
along km-long cracks in the crust
• Often form in continental rifts
• Form Flood basalts: Lava spreads out over large
areas
Small fissure eruption
15-m.y. old
flood basalts in
the western
United States
Mafic magmas
Cinder cones: Small volcanic cones produced by
fountain eruptions
• Often found on the flanks of larger volcanoes or
in continental rifts
Cinder cone in Arizona
Fountain eruption
Intermediate magmas
Composite volcanoes: Contain alternating layers of
pyroclastic deposits and lava flows
• Explosive eruptions producing ash clouds and
pyroclastic flows
• Found almost exclusively in volcanic arcs related
to subduction zones
Mt. Vesuvius, Italy
Mt. Fuji, Japan
Felsic magmas
Volcanic domes: Mounds formed when lava is too
viscous to flow away from the vent
Lava Dome
Felsic magmas
Caldera eruptions: Large magma chamber
collapses, causing enormous, explosive eruptions
• Form over continental hotspots and in continental
rifts
• Form Very large, steep-walled depressions called
calderas
Steps in the formation of a caldera
Yellowstone
National Park
is a giant
caldera
Caldera
The Yellowstone volcanic system has had 3
catastrophic eruptions in the past 2.1 m.y.
• 2.1 m.y. eruption
released 600 cubic
miles of tephra
• 640 k.y. eruption
released 240 cubic
miles of tephra
• 1.3 m.y. eruption
released 67 cubic
miles of tephra
Tectonic Settings and Volcanic Activity