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VOLCANIC Magma Types

Chapter 18. VOLCANIC Magma Types. Introduction (1)‏. ~ 1,500 active volcanoes on Earth 400 erupted in the last century ~ 50 eruptions per year Most activity concentrated along major plate boundaries Impact risks depend on the types of volcanoes. Introduction (2)‏.

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VOLCANIC Magma Types

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  1. Chapter 18 VOLCANIC Magma Types

  2. Introduction (1)‏ • ~ 1,500 active volcanoes on Earth • 400erupted in the last century • ~ 50 eruptions per year • Most activity concentrated along major plate boundaries • Impact risks depend on the types of volcanoes

  3. Introduction (2)‏ • ~ 500 million people living near volcanoes • 100,000 deaths during the last 125 years • 23,000 in the last 20 years • Densely populated countries in the volcanic zones (e.g., Philippines, Indonesia, Japan, Mexico) • Some major cities are located near volcanoes

  4. Volcanism in Space (1)‏ • Highly related to plate tectonic movement • Concentrated along the Pacific ring of fire • In the United States: Alaska, Cascades, and Hawaii

  5. Animation • Types of Volcanic Activity

  6. Animation • Crater Lake

  7. Three types of Magma • The composition of magmas and lavas is controlled by the most abundant elements in the Earth Si, Al, Fe, Ca, Mg, Na, K, H, and O • Three distinct types of magma are more common than others: • Basaltic, containing about 50 percent SiO2 • Andesitic, about 60 percent SiO2 • Rhyolitic, about 70 percent SiO2

  8. Types of Volcanoes (1)‏ • Volcanic eruption style • Depending on • lava’s viscosity and • amount of dissolved gas content • Viscosity: Liquid’s resistance to flow • Determined by: • silica content (lava composition) and • lava temperature • Quiet flow (low viscous basalt flow)‏ • Violent explosion (high viscous lava eruption)‏

  9. Magma Viscosity • The internal property of a substance that offers resistance to flow is called viscosity • The more viscous a magma, the less easily it flows • Viscosity of a magma depends on temperature and composition (especially the silica and dissolved-gas contents)‏ • The higher the temperature, the lower the viscosity, and the more readily magma flows

  10. Viscosity • The greater the silica content, the larger is the polymerized group • For this reason, rhyolitic magma (70% silica) is always more viscous than basaltic magma (50% silica)‏ • Andesitic magma has a viscosity that is intermediate between the two (60% silica)‏

  11. Nature of Volcanic Eruption • Factors affecting viscosity continued • Lower silica content = lower viscosity or more fluid-like behavior (e.g., mafic lava such as basalt)‏ • Dissolved Gases • Gas content affects magma mobility • Gases expand within a magma as it nears the Earth’s surface due to decreasing pressure • The violence of an eruption is related to how easily gases escape from magma

  12. Relative size of Volcanoes 9 km 150 km Shield volcano (e.g. Hawaii)‏ 3 km 15 km Composite volcano (e.g. Vesuvius)‏ 0.3 km 1.5 km Cinder cone (e.g. Sunset crater)‏

  13. Volcanic Features • Craters and vent • Volcanic cones • Caldera: Collapsed craters typically from explosive eruptions • Hot springs and geysers • Fissure line: Basaltic lava flow

  14. Volcanic Hazard • Eruptions present five kinds of hazards: • Hot, rapidly moving pyroclastic flows and laterally directed blasts can overwhelm people before they can evacuate; e.g., • Mont Pelee in 1902 • Mount St. Helens in 1980 • Tephra and hot poisonous gases can bury or suffocate people • 79 Mount Vesuvius in A.D. 79

  15. Volcanic Hazard • Mudflows, called lahars, can be devastating • In 1985, the Colombian volcano Nevado del Ruiz experienced a small, nonthreatening eruption. But, when glaciers at the summit melted, massive mudflows of volcanic debris moved swiftly down the mountain , killing 20,000 • Violent undersea eruptions can cause powerful sea waves called tsunamis • Krakatau, in 1883, killed more than 36,000 on Java and nearby Indonesia islands • A tephra eruption can disrupt agriculture, creating a famine

  16. Volcanic Impact Risks (1)‏ • Lava flows: From the vent of a crater or along a line of fissure • Most common and abundant type: Basaltic lava low • Pahoehoe lava:Less viscous, higher temp, with a smooth ropy surface texture • Aa lava: More viscous, lower temp, with a blocky surface texture

  17. Volcanic Impact Risks (2)‏ • Pyroclastic flow • Enormous amount of rock fragments, volcanic glass fragments, and volcanic bombs • Associated with explosive volcanic eruptions • More deadly if lateral blast • Pyroclastic avalanches • Hot temperature and fire hazards

  18. Volcanic Impact Risks (3)‏ • Ash flow • Covering large area, 100s or 1,000s of km2 • Wider impact if ash flows reach upper atmosphere • Hot temp (nueé ardentes) ash and moving at rapid speed (100 km/h)‏ • Harm to human health and structures • Blocking away solar radiation • Hazardous for air traffic

  19. Nuée ardente • Pyroclastic flows are also known as nuée ardente (glowing cloud)‏ • Historic observations indicate that pyroclastic flows can reach velocities of more than 700 km/h • In 1902, a pyroclastic flow rushed down the flanks of Mont Pelee Volcano at an estimated speed of 200 KM/h, instantly killing 29,000 people

  20. Volcanic Impact Risks (4)‏ • Poisonous Gases • Volcanic gases: H2O, CO2, CO, SO2, H2S • Floating in air • Dissolved in water • Dangerous for health, plants, and animals • Producing smog air, acid rain, and toxic soil

  21. Volcanic Impact Risks (5)‏ • Debris and Mudflows • Collapse of volcano slopes • Sudden melting of snow caps and glaciers at the top of a volcano • Rapid downslope flow at the speed of 50 km/h • Long flowing distance: Tens of miles from volcano

  22. Case Study (1)‏ • Mount Pinatubo • June 15–16, 1991 • Killed 350 people and destroyed a U.S. military base • Nearly 1-ft depth of ash covered buildings over a 40-km radius • Huge cloud of ash 400 km wide into nearly 40 km elevation • Affected global climate (cooler summer the next year; global temp differences −0.5°C, ~1°F)‏

  23. Case Study (2)‏ • Mount St. Helens • May 18, 1980, erupted after a 120-year dormancy • Earthquake (4–5 magnitude) precursor, triggered massive landslide displacing water in Spirit Lake and traveling an 18-km distance down the Toutle River • Lateral blast impacted 19 miles at 1000 km/h • Mudflows reached nearly 100 km (60 miles) away Cowlitz and Columbia Rivers

  24. Case Study (2)‏ • Mount St. Helens (continued)‏ • Ash/tephra materials spread over WA, ID, and west MT • Its maximum altitude (peak) reduced 450 meters (over 1476 ft)‏ • Killed 54 people, damaged 100 homes, 800 million feet of timber: Total cost $3 billion

  25. Forecasting Volcanic Activity • Seismic Activities: Earthquakes as precursors • Thermal, magnetic and hydrologic conditions • Amount of volcanic gas emission • Topographic monitoring: Tilting and special bulging • Remote sensing: Radar 3-D interferometry • Geologic history of a volcano

  26. Public Perception and Adjustment • Perception of the volcanic hazards • Age and residence near a volcano affects one’s knowledge of volcanic activity and possible adjustment • Adjustment • Public awareness and education • Improvement in education • Better scientific info dissemination • Timely and orderly evacuation

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