Earth Science Midterm 2 Study Guide Ch 8 Plate tectonic Setting of Volcanoes Convection of heat in mantle leads to plate tectonics 90 volcanism on plate edges others over hot spots 80 magma happens at ocean spreading centers subduction causes tall volcano mountains at continent edge transform faults and continent continent collision zones little no volcanism o slide past lid on hot asthenosphere o extra thick masses deeply bury hot mantle rock difficult for magma to rise Oceanic peaceful subduction explosive and dangerous Classification errors often occur dormant extinct active o Well formed conical shape active pretty shape dangerous Chemical Composition of Magmas 8 elements 98 Earth s crust o oxygen and silicon most abundant Magma cools oxygen atoms bond w silicon o 4 oxygen 1 silicon Viscosity Temperature Water Content of Magmas liquids flow freely but vary in how flow viscosity measures fluidity internal resistance to flow o lower viscosity faster flow magma viscosity affected by o increase in temp lower viscosity atoms spread vibrate more bonds break increased fluidity o silicon and oxygen increase viscosity form tetrahedral that link in chains sheets networks more there are increase in flow difficulty o increase content of mineral crystals increase viscosity Volatiles dissolved gases solubility increases with pressure increase with temperature decrease 3 types of Magma 1 Basaltic 2 Andesitic 3 Rhyolitic a Low viscosity subduction zones peaceful eruption dark rock a medium viscosity medium rock medium explosiveness a High viscosity explosive light rocks Plate Tectonic Setting of Volcanoes Revisited Why spreading centers ideal for volcanism peaceful 1 Above high temperature asthenosphere 2 Asthenosphere rock has low of SiO2 3 Oceanic Plates pull apart hot asthenosphere rock to rise experience low pressure and change to magma that continues to rise Magma rise temp low SiO2 low volatile low viscosity Subduction o Magma generated rises to surface o Subducting plates stir mantle hot rock to rise and melt with low pressure o Subducting plate carries water melting and low temp to melt o Magma compsition depends on amount of crustal rock melted and put in rising magma o Magma temp lowers SiO2 increases water content increases viscosity increases more explosiveness How Volcanoes Erupt Heat at depth superheated rocks rise with low pressure magma generated by existing rock melting some change solid to liquid volume expansion increase explosiveness lower pressure decompression increasing water content o 2 most common o raising temps o not heat decompression melting melted caused by decrease in pressure largest reservoir of super hot ready to melt rock Asthenosphere Magma rises Pressure lowers gases come out of solution making bubbles increase w lowered pressure o Helps prople magma upward Role of water content with eruption styles Explosiveness depends on water content Need high concentration of volatiles Basalt magma can violently erupt Hawaii Rhyolite explosive bc water Viscosity Secondary Role Non explosive Icelandic and hawaiin types Explosive volcanian plinian Vocanic Materials Explosive high water high viscosity difficult gas escape o Break rocks tear apart magma and older rock into pyroclastic debri o Pyroclastic debris pyro fragments gas blasting in atmosphere takes chunks of magma and older rock o Airborne pyroclasts coarsest grains fall 1st clsoest to volcano and finer grains farther away Non explosive eruptions o Basaltic magma variety of textures lava flows o Pahoehoe smooth ropy texture of highly liquid cooled lava o Aa slower flowing more viscous lava rough blocky texture o Lava Flows low water low viscosity easy gas escape non explosive Obsidian o Cooled volcanic glass o Magma solidified so quickly that crystallization can t take place bc atoms can t arrange in ordered structure of minerals Pumice frothy glass full of holes left by former gas bubbles o Porous gas escapes quickly violently Viscosity Volatiles Volume Viscosity Controls if magma flows or piles up o low med high Volatile ooze out harmlessly or explosive o Low med high abundance Volume correlates with eruption intensity o Increase volume increase eruption Peaceful low volatile and low viscosity Shield Volcanoes Low viscosity low volatiles large volume Basalt rock Ex Mauna Loa Hawaii on hot spot great distances bc so fluid pancake batter basaltic flow cool to form gently dipping thin rock layer increase width lower height Hawaiian Type Eruptions Preceded by earthquakes as rocks fracture o Split ground lower pressure gas escapes Curtain of fire escaping gas form lines of lava fountains Great floods of basaltic lava flow down mountains long time periods Few live lost but damaged poverty increase Icelandic Type Eruptions Low viscosity low volatile Most peaceful Fissure eruptions lava pours out of linear vents or long fractures Curtain of fire Lava builds up and creates wide volcanic plateaus of horizontal volcanic rock layers Flood Basalts Low viscosity low volatile large volume Largest volcanic events Immense amounts of mass and energy Large amounts of magma in short time All continents and ocean floors Climate modifying volatiles globally Siberian flood basalt o Greatest mass extinction India dinosaur mass extinction Scoria Cones Medium viscosity medium volatile low volume Conical hills low height Basaltic to andesitic pyroclastic debris piled up next to volcanic vent Single eruptive interval Summit crater basin on top of cone Lava lake o during eruption after gas expelled lava drains emerges Strombolian Type Eruptions build scoria cones Stromboli SW Italy daily eruptions Gas builds under crust eruptions occur distinctly and as separate bursts o Each gives pyroclastic debris Eruptions to weak to break volcanic cone Mexico Strato Volcanoes High viscosity high volatiles high volume Steep sided symmetrical volcanic peaks Alternating layers of pyroclastic debris form protective caps Volcanian and plinian eruptions styles Mount Kilimanjaro Mount Fuji Volcanian Type Eruptions o Increase viscosity andesitic to rhyolitic lava flows over and solidifies to Alternate btwn thick high viscous lavas and masses of pyroclastic material Some more violent blasts bc more trapped gas o widespread Early phase of other eruptions clearing throat Plinian Type Vesuvius Throat now clear 2 per century Gas powered vertical eruption columns with pyrolclastic debris Heavy in pumice Submarine volcano that greatly grew Subduction of