Volcanoes and volcanism

GoalsTo 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.

Honeymore viscous

Olive oilless 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 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 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 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