VolcanoesSlide 2Locations of Volcanic Activity and Volcanic HazardsMap of the World’s Active VolcanoesMagma and the Driving Force Behind EruptionsViscosityGas ContentSteps to a Volcanic EruptionSteps to a Volcanic Eruption: Two PossibilitiesVolcanic Explosivity IndexSlide 11Slide 12Bottom line: The world’s most dangerous volcanoes are those at convergent plate boundaries!Types of Volcanic Hazards and ProductsLava FlowsSlide 16Slide 17Diverting Lava Flows Refer to page 165.Ash FallsTephraVolcanic AshSlide 22Volcanic ash can be hazardous for a variety of reasons...Slide 24Pyroclastic FlowsPyroclastic Flows…..The Eruption of Mount Saint HelensThe Eruption of Mount Pelee Refer to page 179-181.The Eruption of Mount Vesuvius Refer to pages 165-167, 181-182.Mudflows or LaharsSlide 31The Eruption of Mount PinatuboSlide 33The Risk of Lahars of Mount Rainier Refer to page 165.LandslidesVolcanic landslides can trigger volcanic explosions. Refer to page 174-178.Poisonous GasesSlide 38Types of VolcanoesShield Volcanoes Refer to pages 135 – 139.Shield VolcanoHawaiiSlide 43Composite or Stratovolcanoes Refer to page 140.Stratovolcano HazardsMap of the world’s active volcanoes, showing that the majority of above sea active volcanoes (about 66%) are stratovolcanoes produced by subduction in the Pacific Ring of Fire.The Volcanoes of the Cascade Range Refer to pages 168-174.Stratovolcano EruptionsMt. St. HelensLava DomesCinder ConesSlide 53CalderaSupervolcanoes – Giant Continental Calderas Refer to page 141, 146-147.Will Yellowstone Erupt Again?Mitigation and Prediction of Volcanic HazardsHazard MapsPrediction of Volcanic Hazards Refer to pages 164-165.Slide 60Slide 611VolcanoesCHAPTER 6: MATERIALS, HAZARDS, AND ERUPTIVE MECHANISMSCHAPTER 7: TYPES, BEHAVIOR, AND RISKS2Volcanoes•Volcanism. 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.Volcanic activity is controlled by plate tectonics, because plate movements relate to where sources of magma originate inside the earth. Nearly all active volcanoes are located in one of three plate tectonic settings:•Subduction zones at convergent plate boundariesExample: Volcanoes lining the trenches of the Pacific Ocean, forming the Pacific “Ring of Fire”. •Rifting and sea floor spreading at divergent plate boundaries, Example: Volcanic eruptions at mid-ocean ridges, and in some rift zones on the continents, like the East African Rift Valley. •Hot spotsExample: The Hawaiian Islands and the Galapagos Islands.3Locations of Volcanic Activity and Volcanic HazardsMap of the World’s Active VolcanoesNote that the majority of above sea active volcanoes (about 66%) occur in the Pacific Ring of Fire. 45Magma and the Driving Force Behind Eruptions•Magma may be ejected onto the earth’s surface as:–Lava–Pyroclastics 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. Refer to page 125.Above: Gigantic eruption cloud of tephraAbove: aa lavaRight: pahoehoe lavaViscosityViscosity 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).1.Mafic or Basaltic magma – low silicon and oxygen, high iron and magnesium. Simple silicate minerals. Dark magmas. Low Viscosity.2.Intermediate or andesitic magma - intermediate silicon and oxygen, intermediate iron and magnesium. High Viscosity.3.Felsic or rhyolitic magma - high silicon and oxygen, low iron and magnesium. Complex silicate minerals. Pale magmas. High Viscosity.Refer to pages 126-127.6Left: The basic building block of all silicate minerals - the silica tetrahedron. Four oxygen atoms surrounding a single atom of siliconGas ContentThe gas content of a magma also relates to its behavior. A magma with low gas content will tend to flow out of a volcano as relatively quiet lava. A magma with high gas content will tend to blow apart violently upon erupting. The higher the gas content, the more violent the eruption. The composition of the gases in magma are:Water vapor Carbon DioxideMinor amounts of sulfur dioxide, hydrogen sulfide, chlorine, and flourine gases.But it is the amount of dissolved water that typically inspires a volcano to violence. Refer to pages 127-128.78Steps to a Volcanic Eruption•Volcanoes form wherever rock melts at depth and the magma can rise to erupt at the surface. Rock deep in the earth may melt by–Increasing its temperature–Decreasing its pressure–Adding water (to lower the melting temperature).Refer to page 125.•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. Refer to pages 127-128.9Steps to a Volcanic Eruption:Two Possibilities•If 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.Refer to pages 127-128.Volcanic Explosivity IndexThe violence and size of a volcano’s eruption is expressed by the Volcanic Explosivity Index (VEI). Values for the VEI range from 0 to 8, and are based on:1. the volume of material (lava and particles) erupted2. the height of the eruption column3. how long the eruption lasts The larger the VEI value, the larger the eruption. Refer to pages 129, 133.10This figure plots VEI values versus
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