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Learning Objectives. Explain the relationship of volcanoes to plate tectonics. Identify the different types of volcanoes and their associated features. Locate on a map the geographic regions at risk from volcanoes. Learning Objectives, cont.
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Learning Objectives • Explain the relationship of volcanoes to plate tectonics. • Identify the different types of volcanoes and their associated features. • Locate on a map the geographic regions at risk from volcanoes.
Learning Objectives, cont. • Describe the effects of volcanoes and how they are linked to other natural hazards. • List the potential benefits of volcanic eruptions. • Discuss how humans can minimize the volcanic hazard. • Recommend adjustments we can make to avoid death and damage from volcanoes.
Eyjafjallajökull 2010 Eruption • Iceland home to more than 30 active volcanoes • Volcanic eruption in April 2010 • Ash plume 9.5 km (~6 mi) high • Caused shutdown of airspace throughout Europe • Largest aerial closure since WWII • Increased seismic activity in December 2009 • Future eruptions • 2010 eruption was small taste of what could happen if the more explosive Katla erupts
5.1 Introduction to Volcanism • Volcanic activity is directly related to plate tectonics • Most volcanoes are near plate boundaries • Approximately 2/3 of active volcanoes within “Ring of Fire” • At plate boundaries, magma is created • Magma is molten rock • Lava is magma on the Earth’s surface • Volcanoes form around a vent • Not all volcanoes are the same • Different processes in formation • Depends on tectonic settings
How and Where Magma Forms • Most magma come from the asthenosphere • Weak layer of rock that is close to melting temperature • Three principal magma generating processes • Decompression melting • Pressure exerted on hot rock is decreased • Divergent plate boundaries, continental rifts, and hot spots • Addition of volatiles • Chemical compounds that lower the melting temperature of the rock • Subduction zones – responsible for “Ring of Fire” • Addition of heat • Induces melting if temperature exceeds melting temperature • Continental hot spots
Magma Properties • Described by silica content and amount of dissolved gasses • Three types of magma based on silica content (low to high) • Basaltic, andesitic, and rhyolitic • Magma less dense than surrounding rock • Rises toward the surface • Accumulates in magma chambers • Composition changes • Viscosity • Volatile content
Magma Properties, cont. • Viscosity • Resistance to flow • Affected by temperature and composition • As magma cools, viscosity increases • As silica content increases, viscosity increases • Affects • The flow of lava • Shape of resulting volcano • Correlated to the volatile content
Magma Properties, cont. • Volatile content • Determines how explosive the eruption will be • High concentration of dissolved volatiles will explode violently • Volatile-poor magma results in effusive eruptions • Volatile content increases with increasing silica content • Pyroclastic debris • Volcanic materials (like ash) that are explosively ejected
5.2 Volcano Types, Formation, and Eruptive Behavior • Volcanoes vary greatly • Size, shape, composition • Number of eruptions in formation • How and where magma is formed • Amount of magma evolution • Volatile content • Viscosity and volatile content still primary control of eruption explosiveness • Volcanic explosivity index (VEI) • Relative scale to compare exlopsions
5.2 Volcano Types, Formation, and Eruptive Behavior, cont. • Stratovolcanoes • Known for beautiful conical shapes • Result of high viscosity magma • Lava does not flow far resulting in steep sides • Mixture of explosive activity and lava flows • Produce combination of lava flows and pyroclastic deposits • Also called composite cones • Can be extremely explosive • Responsible for over 80% of eruptions • Responsible for most of the death and destruction • Common in the “Ring of Fire” • Examples: Mount St. Helens and Mount Rainer in United States, Mount Fuji in Japan
5.2 Volcano Types, Formation, and Eruptive Behavior, cont. • Lava Domes • Small dome with steep sides • Often form in the vent of a stratovolcano after an explosive eruption • Can grow in single event or over decades • Made from highly viscous magma • Exhibit highly explosive eruptions • Common in the “Ring of Fire” • Examples: Mount Lassen in CA, Mt. Unzen dome in Japan
5.2 Volcano Types, Formation, and Eruptive Behavior, cont. • Shield Volcanoes • Largest volcanoes in the world • Thin lava flows build up volcano with gentle slopes • Wider than they are tall • Still among tallest mountains on Earth (measured from bases) • Associated with basaltic magma • Low viscosity, low volatile content • Gently flowing lava with non-explosive eruptions • Develop a cladera • Common at hot spots in the oceanic lithosphere and divergent plate boundaries, continental rifts • Hawaiian Islands, Iceland, and in the East African Rift • Examples: Mauna Loa and Kilauea in Hawaii
5.2 Volcano Types, Formation, and Eruptive Behavior, cont. • Cinder Cones • Cone shaped with summit crater • Built from an accumulation of tephra • Small pieces of black or red lava • Formed when lava meets groundwater • Associated with basaltic eruptions • Low to intermediate explosivity • Also called scoria cones • Common on larger volcanoes, normal faults, or along cracks and fissures • Examples: SP Crater in AZ, Parícutin in Mexico, Eldfell in Iceland
5.2 Volcano Types, Formation, and Eruptive Behavior, cont. • Continental Caldera • Large summit depression • Collapse of the land surface or volcanic edifice • Associated with rhyolite eruptions • Violent explosions • ultra-Plinian extrude a great deal of pyroclastic debris on mainly ash • Largest known as the supervolcanic type • Very rare • Examples: Mount Mazama (Crater Lake), Yellowstone caldera
5.3 Geographic Regions at Risk from Volcanoes • Direct volcanic risk • Ring of Fire: surrounds Pacific Ocean basin • Hot spots: Hawai’i and Yellowstone Park • Mid-ocean ridges: Iceland • Rift valleys: East Africa • Indirect volcanic risk • Ash fall and ash clouds: all locations in path
5.4 Effects of Volcanoes • 50 to 60 volcanoes erupt each year worldwide • In the United States, 2 to 3 per year, mostly in Alaska • Most eruptions are in sparsely populated regions • 500 million people live close to volcanoes • Japan, Mexico, Philippines, and Indonesia and several U.S. cities are vulnerable • Primary Effects • Lava flows, ash fall, pyroclastic flows, lateral blasts, and release of volcanic gases • Secondary Effects • Debris flows, mudflows, landslides or debris avalanches, floods, fires, and tsunamis • Global cooling of the atmosphere in a large eruption
Lava Flows • One of most familiar products of volcanic activity • Results when magma reaches the surface through crater or from a vent • Three types: basaltic, andesitic, rhyolitic • Basaltic is the most abundant • Can form lava tubes • Can move slowly or more rapidly • Basaltic lavas are the most rapid at 15–35 km/h (10–30 mph) • Pahoehoe lavas are smooth and ropey • AA are blocky flows • Move slow enough for people to get out of the way
Pyroclastic Activity • Explosive volcanism that blasts magma and rocks from a vent • Known as tephra • Fine dust to sand-sized ash (less than 2mm) • Small gravel-sized lapilli (2 to 64 mm) • Large angular blocks and smooth-surfaced bombs (greater than 64mm) • Falls cool and lightly like snow or hat, fast, and heavy like a freight train • Accumulation forms a pyroclastic deposit
Pyroclastic Activity, cont. • Ash Fall • Explosive fragmentation of magma during an eruption • Can cover hundreds or thousands of square kilometers • Direct hazards • Vegetation may be destroyed • Surface water may be contaminated by sediment • Fine particles clog the gills of fish and kill other aquatic life • Ash accumulation on roofs may cause structural damage • Irritation of the respiratory system and eyes • Engines of jet aircraft may “flame out”
Pyroclastic Activity, cont. • Pyroclastic Flows • One of the most lethal aspects of volcanic eruptions • Hot and race down side of volcano at speeds exceeding 400 km/hr (~250 mph) • Hot expanding gases carry low-density ash upward • Base of flow contains larger debris • Also known as ash flows, hot avalanches, or “glowing clouds” • Catastrophic if populated area in path • Responsible for more deaths than any other hazard • Formation • Large ash-generating eruptions (VEI>3) • Lateral blasts: explosion destroys part of the volcano • Lava dome collapse: most common
Poisonous Gases • Emitted gases • Carbon dioxide and water vapor account for 90 percent of emissions • Carbon dioxide gas is odorless and heavy • It can accumulate suffocating people • Sulfur dioxide • Can produce acid rain • Others: carbon monoxide and hydrogen sulfide • Toxic concentrations rarely reach populated areas • Chemicals can contaminate soil and plants • Can cause air pollution known as vog (volcanic smog)
Debris Flows, Mudflows, and Volcanic Landslides • Also known as lahars • Loose volcanic ash becomes saturated with water, becomes unstable, and moves down volcano • Can occur in the absence of an eruption • Debris flows • Glaciers and ice are melted by volcano and mix with sediment and rock • Similar to wet concrete
Debris Flows, Mudflows, and Volcanic Landslides, cont. • Mud flows • Finer than debris flows • Populous areas of Pacific Northwest are built on old mudflows • Not unlikely for new flows to occur • Landslides • May be triggered outside of an eruption • May affect areas far from their source • Can cause tsunamis
