VolcanismVolcanism is a Planetary Thermal regulatory MechanismInterior temperature is much higher than T at surfaceHeat left over from planetary accretionRadioactive decay of long lived isotopes (40K, 235U, 238U, 232Th) gives off heatEarth and all planets lose heat to their external environmentUpward movement of magmas is a very effective mechanism for moving heat toward the surfaceDistribution of Active Volcanoes on EarthOceanic VolcanismMid-ocean ridge basalt (MORB) volcanismMore or less continuous volcanism along the mid-ocean ridgesMainly quiescent because of low viscosity and low gas content of low-K tholeiiteOccasionally above sea level, as in IcelandIsland arc volcanismCaused by ocean-ocean subductionSome high-Al basalts, butMostly andesitesAlso some dacites and rhyolitesHigher in SiO2 and viscosity, and more explosive than MORBsCommon in the West PacificOceanic island volcanismRelated to hot-spots or hot localized plumes of rising mantleHawaii and other oceanic islandsPacific Ocean Floor TopographyLinear Age/Distance Relation indicates ~constant rate of motion of Pacific Plate over the Hawaiian Hot SpotThe Hawaiian IslandsFive Volcanoes on the ‘Big Island’ of Hawaii show earliest stagesClassic Shield Volcanoes, 5-10o slopeStages in the History of a Typical Hawaiian VolcanoMauna Loa and Kilauea show shield building stage, subaerial substage (2c)Kilauea Caldera from the airRecent eruptions of KilaueaA’a lava flows (A’a is blocky, rough, jagged, with a spiny surface)Pahoehoe Flows (smooth, billowy, ropy surface)Maps and Photos of Recent Kilauea EruptionsMauna Kea represents the Capping Stage (3)Review Volcanic HistoryThe Hawaiian IslandsHaleakala Volcano – East Maui renewed volcanism stage (5) after erosional stage (4)Rocks at Haleakala Summit (renewed volcanism - Stage 5)Continental VolcanismContinental Arc VolcanismContinental Margin VolcanismWhen oceanic crust is subducted beneath continental crustContinental Margin VolcanismStratovolcanoes or Composite VolcanoesInterlayered Pyroclastics & LavasTypical Platy AndesiteContinental VolcanismContinental Flood BasaltsTholeiitesColumbia River Basalts (CRBs) - MioceneDeccan Flood Basalts (NW India, Bombay) – Cretaceous-EoceneContinental Rift Basalts (East African Rift)Alkali BasaltsColumbia River BasaltsDistribution of Active Volcanoes on EarthEruptive Styles: PlinianLarge explosive events that form enormous dark columns of tephra and gas high into the stratosphere, described by Pliny the younger in relation to the disastrous 79 A.D. eruption of VesuviusEruptive Styles: Phreatic EruptionsPhreatic eruptions are steam-driven explosions that occur when water beneath the ground or on the surface is heated by magma, causing water to boil and flash to steam, thereby generating an explosion of steam, water, ash, blocks and bombsNuée Ardente, ash flow or pyroclastic flow, Peléean eruptionA ground-hugging avalanche of hot ash, pumice, rock fragments, and volcanic gas that rushes down the side of a volcano as fast as 100 km/hr; temperature within the flow may be greater than 500CEruptive Styles: StrombolianIntermittent explosion or fountaining of basaltic lava from a single vent or crater, produces cinder cones like Stromboli in the Mediterranean (lower photo)Controls of EruptionComposition and viscositySiO2 and to a lesser extent Al2O3, control viscosity of magmas, because they form polymers in magmabasalt viscosity at eruption T – 102-104 pascal-sec.andesite viscosity at eruption T – 104-106 pa-srhyolite viscosity at eruption T ~ 1010 pa-s (up to one-hundred million x higher than basalt)Dissolved magmatic gasesH2O>>>CO2>>F2,Cl2, N2, SO2 (or H2S)Solubility of gases decreases with decreasing PressureBoiling (formation of bubbles) caused by magma rise and crystallizationBoiling of MagmaBoiling is separation of gas from a liquidEvolution of gas produces bubbles which rapidly expand and rise to the top of the chamberVolume expansion of bubbles fragments the surrounding liquid into tiny fragments and clasts, and provides the driving force for eruptionRapid and continuous magma ascent – Plinian eruptionContinuing gas evolution and magma fragmentation at low to moderate ascent rates – Cinder conesStagnation and gradual outgassing of magma below a vent plug – Strombolian eruptionGeol 285 - Petrology, Dr. Helen M. Lang, West Virginia University, Spring 2011VolcanismVolcanism is a Planetary Thermal regulatory Mechanism• Interior temperature is much higher than T at surface– Heat left over from planetary accretion– Radioactive decay of long lived isotopes (40K, 235U, 238U, 232Th) gives off heat• Earth and all planets lose heat to their external environment• Upward movement of magmas is a very effective mechanism for movingheat toward the surfaceDistribution of Active Volcanoes on Earthsee Simkin, et al., 2006, This Dynamic Planet, USGS, SmithsonianInstitution (interactive map)http://baird.si.edu/minsci/tdpmap/viewer.htmOceanic Volcanism• Mid-ocean ridge basalt (MORB) volcanism– More or less continuous volcanism along the mid-ocean ridges– Mainly quiescent because of low viscosity and low gas content of low-Ktholeiite– Occasionally above sea level, as in Iceland• Island arc volcanism– Caused by ocean-ocean subduction– Some high-Al basalts, but – Mostly andesites– Also some dacites and rhyolites– Higher in SiO2 and viscosity, and more explosive than MORBs– Common in the West Pacific• Oceanic island volcanism– Related to hot-spots or hot localized plumes of rising mantle– Hawaii and other oceanic islandsPacific Ocean Floor TopographyLinear Age/Distance Relation indicates ~constant rate of motion of Pacific Plate over the Hawaiian Hot SpotThe Hawaiian IslandsFive Volcanoes on the ‘Big Island’ of Hawaii show earliest stages Classic Shield Volcanoes, 5-10o slopeStages in the History of a Typical Hawaiian Volcano1. 2. 3. 4. 5. 6. 7. Mauna Loa and Kilauea show shield building stage,subaerial substage (2c)Kilauea Caldera from the airRecent eruptions of KilaueaA’a lava flows(A’a is blocky, rough, jagged, with a spiny surface) Pahoehoe Flows(smooth, billowy, ropy surface)Maps and Photos of Recent Kilauea Eruptions1983 – present Pu’u O’o eruptions are most voluminous from the East Rift of Kilauea in 500 yearsMauna Kea represents the Capping Stage (3)Review Volcanic HistoryThe Hawaiian IslandsHaleakala Volcano – East Mauirenewed volcanism stage (5) after erosional stage