Lecture 9Questions to considerOutlinePowerPoint PresentationRemember: Volcanic rocks are recognized by their textureRelationship of composition to volcanic rock typePyroclastic materialSlide 8Slide 9Slide 10Slide 11Slide 12Other volcanic productsSlide 14Other volcanic products:Volcanoes release Volatiles (Gases)Slide 17Volcanoes also release liquidsLava is generally very slow moving, especially when unconfinedLava can often travel in “Tubes”Viscosity of the Lava is a factor in VelocityStructures built by lava flowsColumnar jointingCooling lava contracts to make “columns”Pillow structuresPillow lavas can be formed at mid ocean ridgesConvergent zones can expose seafloor pillow structures on landFlood basalts and Basalt PlateausSlide 29Columbia River flood basaltsFlood basalts in northwest USSlide 32The relationship of Volcanism to Plate TectonicsMost of earth’s volcanism occurs at mid ocean ridgesSlide 35Hot SpotsHawaiian Hot spotSlide 38Yellowstone Hot spotHot spots around the globeSlide 41Slide 42Slide 43Slide 44Slide 45Slide 46Lecture 9Volcanic Rocks and Extrusive ProcessesQuestions to consider•What are the range of volcanic products and their respective hazards?•Why is lava composition directly related to the hazards it may or may not present?•What geologic scenarios are volcanoes found?Outline•Volcanism–Volcanic Products–Importance of lava composition–Volcanic structures–Relationship of volcanoes to Plate tectonics–Hot spots• Volcano types•Volcanic hazardsVolcanoes release a combination of Solid, Gas and Liquid material at earth’s surfaceRemember: Volcanic rocks are recognized by their texture•Aphanitic texture – small, interlocking crystals not visible to unaided eye•Volcanism produces Rocks such as Basalt, Andesite, and Rhyolite, but also other “products”–Ash, volcanic glass, pyroclastic material,pumice, scoria•Magma also contains gases that are released by volcanismRelationship of composition to volcanic rock typePyroclastic material1) Blocks – pre-existing country rocks or already solidified volcanic rock ejected into the air during eruption2) Tephra – Molten material that solidifies in the air before falling back to earth–Classified according to size•Ash < 2mm•Lapilli 2 – 64 mm•Bombs > 64 mmTephraBomb Lapilli AshVolcanic BombAsh•Effect depends upon the volume of material ejected but can be potentially produce a large impact on a global scaleAsh and aerosols in upper atmosphere from Mount PinatuboAsh and aerosols in upper atmosphere from Mount PinatuboProduced vibrant sunsets and resulted in minor global T decreaseProduced vibrant sunsets and resulted in minor global T decreaseAsh clouds from 1992 explosive eruptions of Mount Spurr volcano, Alaska, significantly disrupted air traffic across the United States and Canada.Other volcanic products•Volcanic Glass = Obsidian–Results from very rapid cooling – i.e. crystals haven’t formedOther volcanic products•Tuff (often called welded tuff)–Ash and glass fragments “welded” together from intense heatOther volcanic products:•Pumice – “frothy” volcanic glass•ScoriaScoriaPumiceVolcanoes release Volatiles (Gases)Gas content of a magma:Water vaporCarbon dioxideCarbon monoxideNitrogenSulfur dioxideHydrogen sulfideChlorineSulfurLake Nyos, Cameroon, Africa – 19861700+ fatalities due to CO2 cloud released from magma chamber below lakeExtent of tree kill from CO2 gases near Mammoth Mountain, CAVolcanoes also release liquids•Lava – the liquid, molten material extruded to the surfaceLava is generally very slow moving, especially when unconfinedLava can often travel in “Tubes”Quickly congealed outer rinds form pipes in which lava can rapidly travelLava tubesViscosity of the Lava is a factor in Velocity•Mafic lavas – less viscous; very “runny”•Felsic lavas - viscous and resistant to flowViscosity of lava also controls Viscosity of lava also controls the type of volcanism, the explosivity the type of volcanism, the explosivity and the type structures that are builtand the type structures that are builtStructures built by lava flows•Lava tubes (already discussed)•Columnar jointing•Pillow structures•Flood basalts and Basalt plateausColumnar jointing•Usually a product of vast outpourings of Basaltic lava (But can form from other products)Devil’s Postpile, CACooling lava contracts to make “columns”Pillow structures•In undersea lava flows, rapidly quenched lava forms “pillow” like formsPillow lavas can be formed at mid ocean ridgesConvergent zones can expose seafloor pillow structures on landFlood basalts and Basalt Plateaus•Vast outpourings of sequences of lava can build very large (100’s – 100,000’s of miles2) broad, flat plateaus•Called “Flood” basalts because they flooded the surface with low viscosity molten rock that rapidly spread.•Resultant plateaus evident on land and in oceans•Can attain thicknesses up to a mile!Columbia River flood basaltsFlood basalts in northwest USFlood basalts are widely distributed.Most occur on the seafloorThe relationship of Volcanism to Plate Tectonics•Volcanoes associated with–Divergent Zones•Mid ocean ridges•Some continental rifts–Convergent Zones•Oceanic to oceanic subduction•Oceanic to continental subduction–Hot spots or Mantle plumesMost of earth’s volcanism occurs at mid ocean ridgesDivergent ZonesHot Spots•Not related to Plate Boundaries•Stationary mantle plumes melt crust of plate that slowly drifts over the plumeHawaiian Hot spotHawaiian Hot spotThis is yet another way we know that lithospheric plates moveWe can also calculate the relative rates of plate movement byDividing the age of rocks by their distance from the Hot SpotYellowstone Hot spotHot spots around the globeVolcanoes can be divided into two broad categories:•Fissure eruptions–Lava and/or pyroclastic material flow outward from a large cracks or “fissures”–Flood basalts and basalt plateaus are the result•Pyroclastic floods can occur, too•Volcanic “cones”–Lava and pyroclastic material emanates from a central vent–Generally build conical peaks–Several varieties of volcanic cones, the type dependent upon the material producedFissure eruptionsFissuresFissuresFlood basaltsActive fissuresPyroclastic flows(ash sheet deposits)Central vent volcanoes•Many different landforms as a result of the magma composition, explosivity, etc.•The morphology (shape) of