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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. - PowerPoint PPT Presentation
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Volcanoes and volcanism
GoalsTo examine the relationship between magma
composition, the kinds of volcanoes and volcanic processes that occur, and plate-tectonic setting.
Magma viscosityViscosity: Measure of resistance to flow. The
greater the viscosity, the harder it is to get a material to flow.
Honeymore viscous
Olive oilless viscous
Magma viscosityViscosity 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 semi-liquid magma flowing on Earth’s surface
Pyroclastic materials: Material blasted out of volcano during an explosive eruption
“Ropy” pahoehoe-type lavas: Lowest viscosity, always mafic.
“Blocky” a’ a’-type lavas: Low viscosity, colder mafic lavas and hot intermediate lavas
Lava flows
Pyroclastic materialsTephra: 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 materialsPyroclastic 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 materialsVolcanic 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 magmasShield 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 magmasFissure 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
areasSmall fissure eruption
15-m.y. old flood basalts in the western United States
Mafic magmasCinder cones: Small volcanic cones produced by
fountain eruptions• Often found on the flanks of larger volcanoes or
in continental rifts
Fountain eruption
Cinder cone in Arizona
Intermediate magmasComposite 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 magmasVolcanic domes: Mounds formed when lava is too
viscous to flow away from the vent
Lava Dome
Felsic magmasCaldera 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