Volcanism Gulp! Tectonic Settings of Igneous Activity Figure 5.11 The World’s Active Volcanoes Fig. 5.28 Volcanism Associated with Plate Tectonics Fig. 6.19 Material ejected from volcanoes Lava: Magma that has flowed on the surface of the Earth.
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Viscosity: Measure of a fluid’s ability to flow.
Higher viscosity = slower flow. (Ketchup has higher viscosity than water.)
Columbia River flood basalts in Washington and Oregon.
Pillow lava: Pillow-like blocks of basalt. ~1 m wide. Formed underwater. Blob of basalt extruded underwater (like toothpaste), skin cools quickly (“quenches”) to form glassy rind.
Not all lava flows have volcanoes.
When low-viscosity lava erupts from cracks in the Earth tens of kilometers long.
Make flood basalt provinces.
Laki fissure (Iceland) erupted in 1783 extruding the largest lava flow in human history (Fig. 6.13).
Low-viscosity lava flows (low silica, mafic).
Successive lava flows.
Gently sloping flanks (between 2 and 10 degrees)
Tend to be very large (many 10s of km in circumference)
Mauna Loa -- world’s largest structure -- 10 km above ocean base -- base diameter of 120 km -- took 1 million years to grow from successive lava flows
Is Mauna Loa about to erupt again?
Forms above a volcanic vent
Viscous lava — usually silica-rich (or cooler magma)
Associated with violent eruptions
Mark Hurd Aerial Surveys
A volcanic rock fragment ejected into the air during an eruption. Loss of gases due to pressure drop results in explosive eruption.
Classified according to size.
Volcanic ash <2 mm in diameter.
Volcanic bombs: Blobs of lava that cool as they fly trough the air. Can be as big as houses.
Gregory G. Dimijian/Photo Researchers
Science Source/Photo Researchers
Volcanic tuffs: Rocks created from smaller fragments.
Volcanic breccias: Rocks formed from larger fragments.
Doug Sokell/Visuals Unlimited
Gerals and Buff Corsi/Visuals Unlimited
Alternating pyroclastic layers and lava flows
Slopes intermediate in steepness
Intermittent eruptions over long time span
Circum-Pacific Belt (“Ring of Fire”), Mediterranean Belt
Raga/The Stock Market
An extremely explosive eruption that occurs when hot lava encounters cool seawater. Huge quantities of steam are released.
Phreatic eruption on a Pacific island south of Tokyo.
What is a hot spot?
A hot spot is the surface expression of a mantle plume.
What is a mantle plume?
A narrow, cylindrical jet of hot material, rising from deep within the Earth (perhaps the core-mantle boundary) that gives rise to surface volcanism.
Lava Flows: e.g. Hawaii, 1998
Gas: e.g. Lake Nyos (Cameroon), 1984
1700 people killed
Ash fall: e.g. Mt. Pinatubo, 1991
Pyroclastic flows: e.g. Mt. Pelee, 1902
Lahars (mudflows): e.g. Nevado del Ruiz, 1985
Tsunami: e.g. Krakatoa, 1883
Before May, 1980
Emil Muench/Photo Researchers
After May, 1980
David Weintraub/Photo Researchers
The only remaining buildings in Armero, Colombia, 72 km dowstream from Nevado del Ruiz volcano, destroyed and partially buried by lahars on November 13, 1985. Lahars reached Armero about 2.5 hours after an explosive eruption sent hot pyroclastic flows across the volcano's broad ice- and snow-covered summit area. Although flow depths in Armero ranged only from 2 to 5 m, three quarters of its 28,700 inhabitants perished.
Pyroclasticflow (nueé ardente)
Pyroclastic flows erupted by Mount Pinatubo on June 15, 1991, buried the Marella River valley (SW of Pinatubo) with pumice, ash, and other volcanic rocks to depths of between 50 and 200 m. This eruption was one of the largest in the 20th century, depositing about 5.5 km3 of rock debris over nearly 400 km2.