GLG 112 1st EditionExam # 3 Study GuideMagma and ExplosivityMagma is liquid rock combined with volatiles (H20, CO2, SO2). They are liquid until they reach the surface because the pressure is so high. Most magma comes from the asthenosphere. There are three ways magma can form. 1. Decrease in pressure (decompression melting)-occur at hot spots and divergent plate boundaries-low melting temperature rocks rise first2. Addition of volatiles-water causes melting temperatures to lower-creates magma in subduction zones3. Increase in temperature-not a common way for magma to form-happens in the crust not the asthenosphere-hot magma in crustViscosity: resistance to flow. The higher the viscosity the less easily it flows.Magma is classified based on their silica content. Silica makes rocks and is a combination of silican and oxygen and oxygen.Mafic magma (basalt):- Lowest silica content- High temperature- Low viscosity (shampoo)- Max volatile content is low (from 0-2%)Intermediate magma (andesite):- About 60% silica- Medium temperature- Medium viscosity- Max volatile content about 4%Felsic magma (rhyolite):- Highest silica content- Low temperature- High viscosity (toothpaste)- Highest max volatile content (about 6%)Dacite is an extrusive igneous rock with a composition between rhyolite and andesite.Pumice is a vesical rich rhyolite.Volcanic Explosivity Index (VEI) scale from 0-8- 0 = non explosive (Kilauea, Hawaii)- 8 = extremely explosive (Yellowstone)Viscosity and volatile content control explosivity. Viscosity is dependent ontemperature and silica content. Volatile content is dependent on gases.The deadliest eruption of all time was Mt. Tambora in 1815. There were 92,000 deaths.Types of Volcanoes1. Shield volcanoesAppearance: gentle slopes, wider than it is tall, largest volcanoes, no obvious peakFormationMagma: maficVolatile content: poorEruption style: effusive, basalt oozes downhill from summit or fissuresLocations: divergent boundaries and oceanic hot spotsExamples: Mauna Loa, Mauna Kea, and KilaueaFissure eruptions: eruptions from bumps in the side rather than the summit2. Cinder ConesAppearance: circular base, large crater where magma erupts, steep slopes, black/dark/sometimes reddish because of oxidation, smooth slopes, fairly small, made of loose material called scoria.FormationMagma: basaltVolatile content: richEruption style: gas rich strombolian eruptions. Magma explodes up and falls down. It cools and hardens in the air. Lavaflows come after the gas is gone from the base. They only erupt once.Locations: everywhere but the bottom of the ocean. Commonly on shield volcanoes and commonly occur in clusters.Examples: SP Crater, Sunset Crater, Paricutin3. Volcanic DomesAppearance: no obvious peak, rough surface (lumpy, high viscosity magma), fairly small, can be smaller or larger than a cinder cone, gray color, high silica content, dome-shaped, steep slopes, uneven/irregular slopesFormation:Magma: daciteVolatile content: lowEruption style: pressure builds from trapped gas and eventually explodes. They are relatively effusive. Small volcanoes could take hours, but big ones could take thousands of years to erupt.Locations: on continents (hot spots and ocean/continent subduction zones), commonly on composite cones and calderas but can be isolatedExamples: in crater of Mt. St. Helens, Mt. Elden4. Composite conesAppearance: pretty tall (snow/tree line), fairly large, slope varies bottom-top, small crater possible, obvious peak, “classic volcano”Formation: Magma: intermediate (andesite)Volatile content: gas poor or gas richEruption style: effusive to explosive. There are lava flows and Plinian eruptions, which are huge ash clouds and are extremely explosive. Location: Subduction zonesExamples: San Francisco Peaks, Mt. St. Helens, Mt. Pinatubo5. CalderasDefinition: formed by collapseAppearance: big hole in the ground, anywhere from large to hugeFormation:Magma: felsicVolatile content: highEruption style: ultra-Plinian eruptions are super explosiveLocations: continental subduction zones and hot spotsExamples: Crater lake (subduction zone), Yellowstone (hot spot caldera)Volcanic Hazards1. Lava flowsEffusive Basalt lava flows are thinner and travel farther and fastest. (shield and cinder cones)Andesite lava flows are thicker and don’t travel farther than the base of the mountain and travel at a slower pace than basalt. (composite cones)Rhyolite lava flows are the thickest and barely make it out of the summit at the slowest speed. (volcanic domes)These are more harmful to property than people.2. Tephra fallAsh are small particles that shoot up higher and travel farther. Lapilli are stone/pebble sized and do not travel much farther than the mountain. Bombs are bigger piece of rock that you only have to worry about if you are near the peak. Basaltic tephra = cinder conesAndesitic tephra = composite conesRhyolitic tephra = calderasProximal hazards include bombs, asphyxiation, burial, and firesDistal hazards include airplane engine flameouts, ash inhalation that can cause asthma problems and eye irritation, visibility problems, drinking water contamination, crop failure, dead aquatic wildlife, flooding, transportation/shipping disrupted, air filters clog, turbines clog and block electricity, abrasive on surfaces, roof collapse is the #1 tephra related cause of death.3. Pyroclastic flowsThey are dense clouds of hot volcanic debris and gases. They flow down the sides of the volcano at 100mph. They can also travel across land and water. Andesite flows = composite conesRhyolite flows = calderas*the most lethal part of an eruption*Examples: Mt. Pelee and Mt. Vesuvius 4. LandslidesTriggered by loose material, earthquakes, and over steepened slopesAssociated volcano = composite conesExample: Mt. St. Helens in 1980They leave hummocky (bumpy) deposits5. Lahars Volcanic mudflows made of water and loose volcanic materialCome from explosive eruptionsAssociated volcano = composite conesHazards = high ground is safe. They are warm but not hot. They canlast for years.Examples = Nevado del Ruiz and Mt. RainierVideo Questions1. Where is Mt. Pinatubo located?Philippines2. What was the first sign that Mt. Pinatubo had “awoken”?Small steam explosions3. What U.S. agency sent geologists to monitor the eruption at Pinatubo?The United States Geological Survey4. What types of U.S. installations are located in the vicinity of Mt. Pinatubo?An Airforce Base and a Naval Base5. What geologic possibilities