VOLCANOESCHAPTER 6: MATERIALS, HAZARDS, AND ERUPTIVE MECHANISMSCHAPTER 7: TYPES, BEHAVIOR, AND RISKSVolcanoesVolcanism. Refers to the rise of magma onto the earth’s surface.Magma - a mixture of liquid rock, mineral crystals, and dissolved gases.Volcanoes are conical or dome shaped landforms built by the emission of magma and its contained gasses onto the earth’s surface.Magma and the Driving Force Behind EruptionsMagma may be ejected onto the earth’s surface as:LavaPyroclastics or tephra- flour sized to boulder sized particles which are thrown in the air due to the built up pressure of gasses.The violence of a volcanic eruption depends on the magma’s viscosity and gas content. The more viscous (thick) and more gaseous the magma, the more explosive the eruption.Viscosity is a measure of a fluid’s resistance to flow. The main factor that determines the viscosity of magma is its silica (SiO2) content. The more silica in the magma, the more viscous it is. The more viscous (silica-rich) the magma, the more violent the eruption.We are concerned, then, with three types determined by the chemical composition of the magma (Pgs. 146 -148).Mafic or Basaltic magma – low silicon and oxygen, high iron and magnesium. Simple silicate minerals. Dark magmas. Low Viscosity.Intermediate or andesitic magma - intermediate silicon and oxygen, intermediate iron and magnesium. High Viscosity.Felsic or rhyolitic magma - high silicon and oxygen, low iron and magnesium. Complex silicate minerals. Pale magmas. High Viscosity.Steps to a Volcanic EruptionVolcanoes form wherever rock melts at depth and the magma can rise to erupt at the surface. Rock deep in the earth may melt byIncreasing its temperatureDecreasing its pressureAdding water (to lower the melting temperature).Magmas that are generated deep within the Earth begin to rise because they are less dense than the surrounding solid rock.As they rise they may encounter a depth (or pressure) where the dissolved gas no longer can be held in solution in the magma, and the gas begins to form a separate phase (makes bubbles) and will continue to grow in size as pressure is reduced.Two PossibilitiesIf the magma has a low viscosity, the gas will easily expand to atmospheric pressure at the earth’s surface and simply burst, and a non-violent eruption will occur, usually as a lava flow.If the magma has a high viscosity the gas will not be able to expand very easily creating a high pressure inside which will cause them to burst explosively on reaching atmospheric pressure. This will cause and explosive volcanic eruption.Volcanic Explosivity Indexthe volume of material (lava and particles) eruptedthe height of the eruption columnhow long the eruption lastsThe larger the VEI value, the larger the eruption.Two main factors determine the size and explosivity of a volcano’s eruption:The amount of time that has passed since the last eruption.The viscosity (thickness) and gas content of the magma. Both relate to plate tectonic setting:Divergent boundaries (MOR) and hot spots both draw their magmas from the upper mantle. This magma is called MAFIC (basaltic) MAGMA, and is characterized by relatively low silica and low gas content. (the eruptions have low VEI).The magma at convergent boundaries comes from melting of subducted oceanic plates. Subduction forms FELSIC (rhyolitic) to INTERMEDIATE (andesitic) MAGMA, and is characterized by relatively high silica and high gas content. (the eruptions have high VEI).Bottom line: The world’s most dangerous volcanoes are those at convergent plate boundaries!Types of Volcanic Hazards and Products are:lava flowsash fallspyroclastic flowslaharsgas emissionsLava FlowsLava is molten magma that flows out and onto the Earth’s surface.Lava flows are typically formed from low viscosity mafic magma that erupts at divergent boundaries and hot spots.The lower silica content (and therefore low viscosity) of mafic magma allows the lava to run down slopes easily.Lava flow eruptions are fairly gentle and quiet. They may cause property damage, but rarely fatalities.Lava flows destroy whatever they overrun. Everything in the path of an advancing lava flow will be knocked over, surrounded, buried, or ignited by the extremely hot temperature of lava.Ash FallsAsh falls form when an eruption column of tephra and gas is blown into the air by an explosive eruption. The eruption column can rise up more than 20 km into the atmosphere.Tephra is a general term for any size of fragmental material blown out of a volcano.Large-sized tephra typically falls back to the ground on or close to the volcano.Volcanic ash, the smallest tephra fragments, can travel hundreds to thousands of kilometers downwind from a volcano. Volcanic ash can be hazardous for a variety of reasons...Ash spreads in upper atmosphere around the globe. The suspended ash can decrease the insolation from the sun and lower global temperatures!Daylight turns into darknessRoofs may collapse from added weight.Machinery and vehicles will be abraded and engines may seize.Farmland will be covered.Roads will be slippery or blocked.Waste-water systems may clog.Pyroclastic FlowsPyroclastic flows (Ash Flows) are avalanches of a very hot (1300-1800F) mixture of hot rock particles and hot gas that are blown out of the vent of the volcano as an eruption column which subsequently collapses and moves very rapidly down the flanks of the volcano at speeds from 50 to over 200 km per hour and can travel for 10’s of kms burning, burying and suffocating everything in their path. Pyroclastic flowsMudflows or LaharsLahars have caused more fatalities than any other volcanic hazard because they are more common, and they can occur at any time.Volcanic Mudflows or Lahars form by mixing water with loose volcanic ash and debris on the flanks of a volcano. As the mud moves downslope, it gathers rocks of all sizes accelerating as it goes.The water can come in several ways including:A major rainstorm.An eruption melts large amounts of snow and ice on the flanks of the volcano.When moving, a lahar looks like a mass of wet concrete that carries rock debris ranging in size from clay to boulders. Most lahars travel much too fast for people to outrun.LandslidesLandslides are large masses of rock and soil that fall, slide, or flow very rapidly under the force of gravity. A number of factors can trigger a landslide:intrusion of magma into a volcanoexplosive eruptionslarge earthquakeintense rainfall that saturates
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