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Strato volcano. Mt St Helens, 1982. Global Volcano Distribution (active & dormant). Volcanism on the Earth: I. Some important Terms: magma : molten (or partially molten) rock and dissolved gas volcanoes: vents in the crust where magma reaches the surface
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Strato volcano Mt St Helens, 1982
Global Volcano Distribution (active & dormant)
Volcanism on the Earth: I • Some important Terms: • magma: molten (or partially molten) rock and dissolved gas • volcanoes: vents in the crust where magma reaches the surface • lava: magma and condensed H2O ejected from a volcano as liquid • pyroclastics: magma ejected as solids – ash, pumice, bombs, cinders…
Types of Volcanic Eruptions • pyroclastic falls: airborne ash and rock fragments • pyroclastic flows: avalanches of heated rock and gas mixtures • lahars: mudflows made of ash and rainwater • lava flow compositions: basalt, andesite, rhyolite • basaltic flow styles : pahoehoe (billowy) & aa (blocky) • strato volcano • shield volcano • fissure eruption • pyroclastic cone Types of Volcano Edifices
Historically Important Eruptions • Vesuviusce79 - ash fall destroyed Pompeii • 1815 Tambora -- pyroclastic flow ~ 300,000 fatalities • 1883 Krakatau -- pyroclastic flow + tsunami • 1902 Pelee -- famous pyroclastic flow • 1980 St. Helens • 1991 Pinatubo -- largest eruption of 20th century; global climate effects; forecasted
Historical Example by VEI VEI Volume (km^3) Eruption 0 Effusive Masaya (Nicaragua), 1570 >0.00001 Poás (Costa Rica), 1991 2 >0.001 Ruapehu (New Zealand), 1971 3 >0.01 Nevado del Ruiz (Colombia), 1985 4 >0.1 Pelée (West Indies), 1902 >1 Mount St. Helens (United States), 1980 >10 Krakatau (Indonesia), 1883 >100 Tambora (Indonesia), 1815 >1000 Yellowstone (United States), Pleistocene
Caldera with pyroclastic cones: Haleakala, Maui (last eruption: 1700 ce)
Basaltic Lava Flows, Kilauea eruption temperature ~ 1200 C Low silica, low viscosity low crystal content relatively low gas content (not explosive) pahoehoe (gas poor) aa (somewhat higher gas content)
Pyroclastic Flows Ash clouds rise above a pyroclastic flow traveling down the flank of Mayon Volcano, Philippines, September 12, 1984
Pyroclastic Flows Pyroclastic flows in 1997 sweep across the flank of Soufrière Volcano on Montserrat.
Pyroclastic Fall: Erosional dissection of an ash deposit from the 1991 Pinatubo eruption, Philippines
Lahars: volcanic mudflows St Helens Pinatubo
Famous Active Volcanoes I Stromboli Vesuvius (1944) Etna (2002)
Explosive Volcanoes, Eruption Columns and Plumes • Eruption Column Anatomy • Plume Types • Iceland Eruption • Historical Catastrophic Eruptions • Volcanic Aerosols and Climate
Anatomy of an Explosive Volcanic Eruption Neutral buoyancy level Umbrella Turbulent Gravity Current Turbulent convective plume Pumice & ash fallout Jet phase Surface vent conduit Fragmentation & ex-solution Magma chamber Silicate liquids & solids + volatiles (CO2, H2O, SO2…)
Famous Active Volcanoes II Hekla (2000) Krakatoa Arenal Paricutin (1947)
Melting in the Mantle magma genesis by decompression (pressure reduction) melting Decompression melting
Subduction Zone - Island Arc Volcanism • Volcanoes located ~ 200 km behind deep sea trench • Andesite (silica-rich) composition • Often explosive • Magma produced by subducted volatiles (melting point reduction) • Examples (abundant): Cascade Volcanoes, Aleutians, Andean & Central American Volcanoes…
Hotspots: long-lived mid-plate volcanic centers • tracks defined by island chains • regular age progression • tracks indicate plate motion • nearly stationary mantle heat source (thermal plume) • examples: Hawaii (best), Yellowstone, Iceland… • mostly basaltic volcanism • decompression melting starting ~ 100 km depth