SPRING 2014GEOL 285:PETROLOGYDr. Helen LangDept. of Geology & GeographyWest Virginia UniversityVolcanismOne of the few petrologic processes that we can observe directlyVolcanism 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 moving heat toward Earth’s surfaceDistribution of Active Volcanoes on EarthSimkin, et al., 2006, This Dynamic Planet, USGS, Smithsonian InstitutionExplore the Interactive Map at “This Dynamic Planet”http://baird.si.edu/minsci/tdpmap/viewer.htmSmithsonian, USGS, US Naval Research LabWorld Map of Volcanoes, Earthquakes, Impact Craters, and Plate TectonicsActive Volcanoes and Plate BoundariesBlatt, Tracy, Owens, 2006, Petrology: Igneous, Sedimentary, MetamorphicNote Mid-oceanic ridges and IcelandOceanic 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-K tholeiite– Occasionally above sea level, as in IcelandOceanic Volcanism• Island arc volcanism– Caused by ocean-ocean subduction– Calc-alkaline magmas– Some high-Al basalts, but – Mostly andesites– Also some dacites and rhyolites– Higher in SiO2and viscosity, and more explosive than MORBs– Common in the West PacificClose-up of West PacificSimkin, et al., 2006Oceanic Volcanism• Oceanic island volcanism– Related to hot-spots or hot localized plumes of rising mantle– Hawaii and other oceanic islandsPacific Ocean Floor TopographyEmperor-Hawaiian Chainbend~40 Ma8.6 cm/yrLinear Age/Distance Relation indicates ~constant rate of motion of Pacific Plateover the Hawaiian Hot SpotThe Hawaiian IslandsJ.E. Robinson, USGS(Peterson & Moore, 1987, in Volc. in Hawaii, USGS PP 1350)History of a typical Hawaiian VolcanoMauna Loa, KilaueaMauna KeaHaleakala, MauiFive Volcanoes on the ‘Big Island’ of Hawaiishow the earliest stages KohalaHualalaiMauna LoaKilaueaMauna KeaLoihi124356Classic Shield Volcanoes, 5-10oslopeMauna Loa - 13,679 feetMauna Kea - 13,796 feetSummits are about 30,000 feet above the seafloor!!Mauna Loa and Kilauea show shield building stage,subaerial substage (2c)KILAUEAMauna Ulu Flows1969-1974Pu’u O’o Flows1983-pres.Kilauea CalderaKilauea Caldera from the airRecent eruptions of KilaueaMauna Ulu Flows1969-1974Pu’u O’o Flows1983-presentA’a lava flows(A’a is blocky, rough, jagged, with a spiny surface) A’a over pahoehoe on Kilauea plainKelby Hicks in Hawaii 2004Goodbye Kelby, deceasedApril 17, 2013, on Colima volcano in Mexico.Pahoehoe Flows(smooth, billowy, ropy surface)Fresh Pahoehoe –Apr. 2004, K. HicksOctober 2006 mapUSGS – HVOJanuary 13, 2011 map1983-present eruption is most the voluminous from Kilauea East rift in 500 yearsMostly July 21, 2007USGS – HVORecent activityhttp://hvo.wr.usgs.gov/kilauea/update/maps.htmlMauna Kea represents the Capping Stage (3)Cinder cones areprobably alkali olivine basaltThe Hawaiian IslandsMaui shows later stages than Hawaii –erosional stage (4) and renewed volcanism (5) stageHaleakala Volcano – East Mauirenewed volcanism stage (5) after erosional stage (4)Rocks at Haleakala Summit (renewed volcanism - Stage 5)Note: steep-sided and blockyReview Volcanic History(Peterson & Moore, 1987, in Volc. in Hawaii, USGS PP 1350)Continental Volcanism• Continental Arc Volcanism– Continental Margin Volcanism– When oceanic crust is subducted beneath continental crust– Calc-alkaline magmas– Composite volcanoes with lava flows and pyroclastic material– Steep-sided stratovolcanoes– Much more explosive than Hawaiian volcanismContinental Margin VolcanismSimkin, et al., 2006Stratovolcanoes or Composite VolcanoesMt. Rainier and Tacoma, 1984More later…..Interlayered Pyroclastics & LavasProximal pyroclastic breccia Pyroclastics plus lava flowsTypical Platy AndesiteContinental Volcanism• Continental Flood Basalts – Tholeiites– Low viscosity, thin flat flows– Columbia River Basalts (CRBs) - Miocene– Deccan Flood Basalts (NW India, Bombay) –Cretaceous-Eocene • Continental Rift Basalts (East African Rift)– Alkali BasaltsColumbia River BasaltsColumbia River BasaltsCh. Heubeck, Berlin, GermanyContinental Rift Volcanism - East African RiftSimkin, et al., 2006Alkalic rocks-nephelinites and alkali basalts to phonolites, trachytesDistribution of Active Volcanoes on EarthSimkin, et al., 2006Eruptive Styles: Plinian• Large 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 VesuviusMt. Spurr, AK, 1992USGS Photo GlossaryEruptive Styles: Phreatic Eruptions• Phreatic 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 bombsMount St. Helens, WA, 1980USGS Photo GlossaryNuée Ardente, ash flow or pyroclastic flow,Peléean eruption• A 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CPyroclastic flow on Mayon Volcano, Philippines, 1984, USGS Photo GlossaryEruptive Styles: Strombolian• Intermittent explosion or fountaining of basaltic lava from a single vent or crater, produces cinder cones like Stromboli in the Mediterranean (lower photo)USGS Photo GlossaryControls of Eruption• Composition and viscosity–SiO2and to a lesser extent Al2O3, control viscosity of magmas, because they form polymers in magma– basalt viscosity at eruption T – 102-104pascal-sec.– andesite viscosity at eruption T – 104-106pa-s– rhyolite viscosity at eruption T ~ 1010pa-s (up to one-hundred million times higher than basalt)• Dissolved magmatic gases–H2O>>>CO2>>F2,Cl2, N2, SO2(or H2S)– Solubility of gases decreases with decreasing Pressure– Boiling (formation of bubbles) is caused by magma rise and crystallizationBoiling of Magma• Boiling is separation of gas from a liquid• Evolution of gas produces bubbles which rapidly expand and rise to the top of the