• ~60 volcanoes erupt each year worldwide• 2-3 volcanoes erupt each year in Alaska• Kilauea (Hawaii) erupted continuously since 1983• Volcanoes differ greatly in style of eruption and associated hazards• Densely populated countries in volcanically active areas especially vulnerableVolcanoesTable 6.1 selected Historic Volcanic EventsSelect Historic Volcanic EruptionsTable 6.1 selected Historic Volcanic EventsDeadliest Historic Volcanic EruptionsDeaths Volcano When Major Cause of Death 92,000 Tambora, Indonesia 1815 Starvation 36,417 Krakatau, Indonesia 1883 Tsunami 29,025 Mt. Pelee, Martinique 1902 Ash flows 25,000 Ruiz, Colombia 1985 Mudflows 14,300 Unzen, Japan 1792 Volcano collapse, tsunami9,350 Laki, Iceland 1783 Starvation 5,110 Kelut, Indonesia 1919 Mudflows 4,011 Galunggung, Indonesia 1882 Mudflows 3,500 Vesuvius, Italy 1631 Mudflows, lava flows 3,360 Vesuvius, Italy 79 Ash flows and falls 2,957 Papandayan, Indonesia 1772 Ash flows 2,942 Lamington, Papua N.G. 1951 Ash flows 2,000 El Chichon, Mexico 1982 Ash flows 1,680 Soufriere, St Vincent 1902 Ash flows 1,475 Oshima, Japan 1741 Tsunami 1,377 Asama, Japan 1783 Ash flows, mudflows 1,335 Taal, Philippines 1911 Ash flows 1,200 Mayon, Philippines 1814 Mudflows 1,184 Agung, Indonesia 1963 Ash flows 1,000 Cotopaxi, Ecuador 1877 Mudflows 800 Pinatubo, Philippines 1991 Roof collapses and disease700 Komagatake, Japan 1640 Tsunami 700 Ruiz, Colombia 1845 Mudflows 500 Hibok-Hibok, Philippines 1951 Ash flows 1000s Santorini, Greece 1650 BC Ash flows, tsunami?Volcanism and Plate BoundariesVolcanism directly related to plate tectonicsVolcanoes located at plate boundaries or over hot spotsVolcanoes of the United StatesOver 40 different volcanoes have erupted in Alaska over last 100 years (subduction zone)Volcanoes also mark the Cascade subduction zoneHot spots include Yellowstone and HawaiiTectonic Settings of VolcanoesEach type of volcano has a characteristic style of activity determined by composition, viscosity, and dissolved gas content of its magma• Viscosity: liquid’s resistance to flow determined by silica content (composition) and temperature • Non-explosive eruptions: low viscosity and low dissolved gas contents (basaltic magma)• Explosive eruptions: high viscosity and high dissolved gas contents (andesitic and rhyolitic magma)Types of Volcanoes• Gently sloping, very broad volcanoes built up over time by successive lava flows• Characterized by gentle eruption of fluid basaltic lava that is low in silica and rich in Fe and MgShield VolcanoesMauna LoaHawaii• Stratovolcanoes: steep sided, conical shaped • Characterized by alternating periods of explosive activity and intermediate viscosity (andesitic) lava flows• Composed of alternating layers of pyroclastic (air –fall) deposits and lava flowsComposite VolcanoesMt. HoodOregon• Highly viscous rhyolitic magma with high silica content• Builds very steep sided dome prone to collapse• Activity mostly explosiveVolcanic DomesLassen PeakCalifornia• Frothy ejection of basaltic magma with high gas content• Steep, conical hill of volcanic fragments that accumulate aroundand downwind from the vent. • The rock fragments, called cinders or scoria, are glassy and contain numerous gas bubbles "frozen" into place as magma exploded into the air and then cooled quickly.Cinder ConesSunset CraterArizonaTable 6.2 Types of VolcanoesTypes of VolcanoesVents: openings from which lava is erupted• May be roughly circular openings (craters) or elongated cracks (fissures or rifts)Volcanic VentsLaki fissure row, IcelandSouth Sister summit crater, OregonCalderas: large circular depressions produced by rapid, often explosive ejection of magma and subsequent collapseVolcanic CalderasCrater Lake Caldera, OregonCalderas from when a large amount of magma is erupted over a short time. The upper portion of the volcano is left unsupported and collapses into the empty magma chamber.Volcanic Calderas• Caldera forming eruptions eject huge amounts of magma catastrophically• Recent caldera forming eruptions, though large, pale compared to eruptions in the PleistoceneHolocene calderas with eruptive volumes >25 km3• Tambora (Indonesia) 1815: 50 km3• Kuwae (Tropical Pacific) ~1453: 40 km3• Santorini (Thera) Greece ~3600 BP: 30 km3• Mazama (Crater Lake) 6845 BP: 60 km3Select Pleistocene Calderas• Toba (Indonesia) 74,000 BP: 3500 km3• Yellowstone Caldera 600,000 BP: 1000 km3• Yellowstone Caldera 2.1 million years ago: 2500 km3Volcanic CalderasThe Toba caldera, Sumatra, Indonesia, site of the second largest volcanic eruption ever discovered.The caldera is partly filled by Lake Toba. The flat area in the distance are pyroclastic deposits from the eruptionVolcanic CalderasThree major caldera systems are found in North America: Long Valley, California; Valles, New Mexico, and Yellowstone, WyomingLong Valley caldera formed 700,000 years ago with the eruption of 600 km3of magma to produce the caldera and its deposit, the Bishop Tuff. Eruptions have occurred as recently as 250 years ago at Mono Lake along a fissure extending through the western part of the caldera.Harmonic tremors (earthquake swarms, up to a magnitude of 5-6) occurred in the early 1980’s, indicating magma movement is still occurring.Volcanic CalderasShaded relief map of Long Valley CalderaVolcanic CalderasAbout 170 acres of trees have been killed on Mammoth Mountain since 1990 by high concentrations of carbon dioxide gas in the soil. The gas may be coming directly from magma that intruded beneath Mammoth Mountain during an earthquake swarm in 1989 or may be exsolved from limestone-rich rocks beneath Mammoth Mountain that were heated (contact metamorphism) by the hot magma.Volcanic CalderasThree caldera-forming eruptions have occurred at Yellowstone in the last two million years.The oldest event produced the Huckleberry Ridge Tuff, approximately 2,100,000 years ago. The second event occurred 1,300,000 years ago resulting in the Mesa Falls Tuff. The most recent cataclysmic explosion occurred a mere 600,000 years ago, producing the Lava Creek Tuff. The Yellowstone system is still active. Old Faithful and other geysers and hot springs indicate magma is still present beneath the caldera floor. Volcanic CalderasVolcanic CalderasVolcanic Hazards• Lava Flows• Pyroclastic flows• Ash Fall• Lahars• Poisonous gases• Climate ChangeLava flows: