Chapter 7- Volcanic ActivityOverview50-60 volcanoes erupt each year worldwideVolcanic activity and volcanoes are directly related to plate tectonicsSpreading or sinking lithospheric plates interact with earth materials and create MAGMAOutside of volcano magma is called LAVAVolcano TypesShield volcanoesShape- Gentle slope (like a shield)Silica content of magma- Low (around 50%)Viscocity (Resistance to flow)- lowRock Type formed- BasaltEruption Type- lave flows, tephra ejection aka pyroclastic debris (volcanic debris)Example- Mauna Loa, HawaiiComposite volcano/ stratocvolcanoShape- Steep, cone shapedSilica content of magma- medium (around 60%)Viscosity- mediumRock type formed- AndesiteEruption type- combination of lava flows and explosive activityExample- Mt. Fuji, JapanVolcanic Dome-Shape- Dome ShapedSilica content of magma- High (around 70%)Viscosity- HighRock type formed- rhyoliteEruption type- Highly explosiveExample- Mt. Lassen, CACinder coneShape- cone shaped, steep sides, often with summit craterSilica content of magma- LowViscosity- lowRock type formed- basaltEruption type- tephra, (mostly ash) ejectionExample- Springerville, AZSomething to rememberMagma’s viscocity is determined by 2 things1. Silicone content (the higher the silicone content, the more viscous)2. Temperature (The higher the temperature the less viscous)Volcano OriginsSome Shield volcanoes occur by spreading mid-ocean ridgesMagma mixes with oceanic crust (basalt)These shield volcanoes are formed above hot spots located below the lithospheric platesComposite volcanoes associated with andesitic rocks and subduction zonesRising magma mixed with oceanic crust and continental crust is what creates andesitic rocksCaldera-forming eruptionsAssociated with rhyolitic rocksRhyolitic rocks are formed by magma mixing with continental crustFound usually in Dome volcanoesLocated inland of subduction zonesVolcanic FeaturesCraters, Calderas and VentsCraters- Depressions found at top of volcanoFormed by explosion or collapse of upper portion of volcanic coneFew km in diameterCalderas- Gigantic, often circular depressionsCome from explosive ejection of magma and collapse of upper portion of volcano coneCan be >20 km in diameterMay contain volcanic vents and hot springsHot springs and geysersGround water that comes in contact with hot rock and is heated and sometimes discharges t surface as hot springsCaldera eruptionsVery rare and very violent!Only 10 in last million years1,000 times more destructive than Mt. Saint Hellens!Long Valley Caldera near Mammoth lakesVolcanic HazardsPrimary Effects and Secondary EffectsPrimary Effects- Volcanic activity that are direct result of eruptionLava Flows, Pyroclastic activity (ash fall, ash flows, lateral blasts, and release of gasses)Lava flowsPahoehoe- fastest moving (1 meter per hour), hotter basaltic lava, consists of smooth ropy surface textureAA- Slower moving (few meters per day), cooler basaltic lava, blocky textureWays to control lava flowsHydraulic Chilling- Cooling lava with waterShown to be pretty effectiveWall construction- Build a wall to keep lava outPyroclastic ActivityAsh Fall- lots of rock fragments, natural glass fragments, and gasses blown into airCause health problems, vegetation problems, water contamination, structural damage to buildings, jetliner engines to ”flame out”Lateral Blasts- Gas and ash from the side of the volcano that destroy part of the mountainCan travel nearly the speed of soundand can be very destructiveAsh Flows- Avalanches of very hot pyroclastic materialCan travel 62 MPH and incinerate everything in its pathPoisinous Gasses- Gasses emitted are H2O, CO2, CO, SO2, and H2SCO2 and H2O make up 90% of emmited gassesIn 1986 Lake Nyros released immense volumes of CO2 killing 1,700 people and 3000 cattleGeologists made degassing fountains to get rid of CO2SO2 makes acid rainFog + Volcanic material makes VOG, bad for healthSecondary Effects- Debris flows and mud flows, land slides, floods, and firesDebris flows and mud flows AKA Lahar- large volume of loose volcanic ash becomes saturated with water and then moves downslopeIn Debris flow over 50% of particles are smaller than sandMud flow are with bigger particlesMt. Pinatubo and Mt. St. Helens were big volcanoes in 1991 and 1981 respectivelyForcasting Volcanic activity- Goal- to reduce volcanic hazardsSeismic ActivityEarthquakes preceed Volcanoes!Thermal, magnetic, and hydrologic monitoringHot materials change the local magnetic, thermal, hydrologic, and geochemical conditionsTopographic graph monitoringHawaiian volcanoes, especially Kilauea, tilts and swells before eruptionMonitoring volcanic gas emissionsChanges in gas composition and emission (steam, CO2, SO2, and gas emission) are correlated with surface volcanic pressuresGeologic HistoryUnderstanding geologic history is good predictor of types of eruption that will occurVolcanic Alert or warningGreen- Everything’s coolYellow-WatchOccurs once per decadeAt least one earthquake with magnitude >5 orEvidence of magma movementOrange-WarningEruption likely within hoursOccurs every few hundred yearsStrong evidence of magma movementRed- You’re fucked, eruption underwayOccurs every few hundred yearsAdjustment to the perception of volcanic hazardThe longer one has been living in an area with volcanoes and the older they are= more knowledge about volcanoes= better adjustment and chances of survivalChapter 7- Volcanic ActivityOverview50-60 volcanoes erupt each year worldwideVolcanic activity and volcanoes are directly related to plate tectonicsSpreading or sinking lithospheric plates interact with earth materials and create MAGMAOutside of volcano magma is called LAVAVolcano TypesShield volcanoesShape- Gentle slope (like a shield)Silica content of magma- Low (around 50%)Viscocity (Resistance to flow)- lowRock Type formed- BasaltEruption Type- lave flows, tephra ejection aka pyroclastic debris (volcanic debris) Example- Mauna Loa, HawaiiComposite volcano/ stratocvolcano Shape- Steep, cone shapedSilica content of magma- medium (around 60%)Viscosity- mediumRock type formed- AndesiteEruption type- combination of lava flows and explosive activityExample- Mt. Fuji, JapanVolcanic Dome-Shape- Dome ShapedSilica content of magma- High (around 70%)Viscosity- HighRock type formed- rhyoliteEruption type- Highly explosiveExample- Mt. Lassen, CACinder coneShape- cone shaped, steep sides, often with summit craterSilica content of magma- LowViscosity- lowRock type