Natural Hazards, 2e Volcanoes Chapter 4
Learning Objectives • Know the different types of volcanoes and their associated features • Understand the relationship of volcanoes to plate tectonics • Know what geographic regions are at risk from volcanoes • Know the effects of volcanoes and how they are linked to other natural disasters
Learning Objectives, cont. • Recognize the potential benefits of volcanic eruptions • Understand how we can minimize the volcanic hazard • Know what adjustments we can make to avoid death and damage from volcanoes
Introduction • Most volcanoes are near plate boundaries. • Plate boundaries are where the magma is. • Magma is molten rock. • Lava is magma on the earth’s surface. • Some active plate boundaries: • Subduction zones • Mid-ocean ridges • Continental rift zones
Magma • Described by silica content and amount of dissolved gasses. • Silica content affects viscosity. • Energy needed to make a liquid flow • High silica content, high viscosity • Gas content determines how explosive the eruption will be. • High gas content, greater the explosion
Tephra • Gasses will cause lava and other debris to be expelled from the volcano. • Also called pyroclastic materials. • Range in size from dust-sized materials, gravel-sized lapilli, to large block-sized bombs.
Magma Types • Basaltic • Low silica content, low viscosity • Andesitic • Intermediate silica content, intermediate viscosity • Rhyolitic • High silica content, high viscosity
Shield Volcanoes • Largest volcanoes in the world • Built almost entirely of lava flows • Resemble a warrior’s shield • Associated with basaltic magma • Low viscosity, low gas content • Gentle flowing lava with nonexplosive eruptions • Can form lava tubes underground
Shield Volcanoes, cont. • Found in Hawaiian Islands, Iceland, and around Indian Ocean Figure 4.4
Composite Volcanoes • Associated with variety of magmas, basaltic to lavas between andesitic and rhyolitic • Higher viscosity and gas content • Built from a combination of lava flows and pyroclastic deposits • Have a cone shape, also called stratovolcanoes • Explosions more violent and dangerous
Composite Volcanoes • Ex.: Mt. St. Helens, Mt. Rainer, Mt. Fuji Figure 4.6
Volcanic Domes • Made from highly viscous rhyolite magma • Exhibit highly explosive eruptions • Ex.: Lassen Peak and Mono Craters Figure 4.7
Cinder Cone Volcanoes • Small volcanoes • Built entirely from tephra • Small pieces of black or red lava • Common on larger volcanoes, normal faults, or along cracks and fissures • Ex.: Paricutin, Mexico
Cinder Cone Volcanoes, cont. Figure 4.8
Volcanic Features • Craters • Depressions formed by explosion or collapse of volcano top • Calderas • Very large craters formed from violent explosions • Vents • Any opening for lava and debris • Can produce flood basalts
Volcanic Features, cont. • Hot springs • Hot rocks heat groundwater discharged at surface • Geysers • Groundwater boils, erupting steam at surface Figure 4.11b
Volcanic Features, cont. • Caldera eruptions • Very large, very violent eruptions • Produce calderas • Very rare • Most recent North American caldera eruptions 640 mya at Yellowstone National Park and 700 mya at Long Valley, California
Caldera Eruptions Figure 4.14 Figure 4.15
Volcanic Activity and Plate Tectonics Figure 4.16
Volcano Origins • Mid-ocean ridges • Basaltic magma from asthenosphere • Shield volcanoes • Ex.: Iceland at Mid-Atlantic Ridge • Subduction zones • Andesitic magma from melting tectonic plate • Composite volcanoes • Ex.: Cascade Mountains
Volcano Origins • Hot spots beneath oceans • Basaltic magma • Shield volcanoes • Ex.: Big Island of Hawaii • Hot spots beneath continents • Rhyolitic magma from mixes of rising magma and continental crust • Caldera eruptions • Ex.: Yellowstone National Park
Geographic Regions • Ring of fire • Pacific Ocean subduction zones • Hot spots • Hawaii and Yellowstone Park • Mid-ocean ridges • Iceland • Rift valleys • East Africa
Effects of Volcanoes • 50–60 volcanoes erupt each year. • In U.S. 2–3 volcanoes • 500 million people live close to volcanoes. • Japan, Mexico, Philippines, and Indonesia • Several U.S. cities vulnerable
Rhyolitic, andesitic, and basaltic lavas Basaltic lavas flow most abundantly: Pahoehoe – 1 m/hr A’A’-1-3 m/day Lava Flows Figure 4.21 Figure 4.22
Pyroclastic Activity • Tephra is blown into atmosphere. • Ash fall • Ash is blown high into air and falls onto areas. • Lateral blast • Rock fragments are blown horizontally from volcano. • Pyroclastic flow • Avalanches of hot rock, ash, glass fragments.
Ash Fall • Vegetation destroyed • Contaminates surface water • Damage to buildings • Health hazards • Aircraft engine failure Figure 4.24
Pyroclastic Flow • Responsible for more deaths than any other hazard • Flow at 160 km/hr (100 mph) • Temperatures >1000C
Carbon dioxide (CO2) Odorless, heavy gas that can displace breathable air Sulfur dioxide Odorous gas that causes acid rain and can contaminate rock and soil Poisonous Gases Figure 4.26b
Debris & Mud Flows • Also known as lahars • Volcanic activity melts ice, snow, or glaciers on a volcano. • Water mixes with ash, other tephra • Mixture becomes unstable and flows down volcano • Populous areas of Pacific Northwest are built on old mudflows. • Not unlikely for new flows to occur.
Figure 4.28 Figure 4.29
Landslides • Secondary effects of volcanoes • Can cause tsunamis
Scene of volcanic explosion in recent history Well-studied example of Cascade volcanic eruption Mt. St. Helens Figure 4.30a
Mt. St. Helens – Before Figure 4.30b
Mt. St. Helens – After Figure 4.30c
Mt. St. Helens – Timeline • 120 years of dormancy • March 1980 – seismic activity & small explosions • May 1 – bulge begins to grow on northern flank at rate of 1.5m (5 ft) per day • May 18, 8:32 am – M 5.1 earthquake triggers landslide/debris avalanche of the bulge area • Seconds later, lateral blast from bulge area at rate of 480 km/hr (300 mph)
Bulge & Avalanche Figure 4.31a Figure 4.31b
Lateral Blast & Vertical Eruption Figure 4.31c Figure 4.31d
Mt. St. Helens – Timeline, cont. • One hour after blast: vertical cloud of ash extends to stratosphere. • 9 hours of ash falls to cover areas of Washington, northern Idaho, western and center Montana. • Pyroclastic flows begin at this time down the northern slope. • Mudflows begin at speeds of 29-55 km/hr (18-34 mph).
Debris Avalanche and Ash Cloud Figure 4.32a Figure 4.32b
Mt. St. Helens – Summary • 57 people were killed • Flooding destroyed >100 homes • 800 feet of timber flattened • Damage >$1 billion • September 23, 2004, Mt. St. Helens reawakens • Lava dome begins to form on crater floor • Continues to form today
Links to other Hazards • Earthquakes • Landslides • Fire • Hot lava ignites plants and structures. • Climate Change • CO2 (and other gasses) from eruption alters climate.
Benefits of Volcanoes • Volcanic Soils • Good for coffee, maize, pineapples, sugar cane, and grapes • Geothermal power • Can create energy for nearby urban areas • Mineral Resources • Gold, silver, etc. and nonmetallic rocks • Used for soap, building stone, aggregate for roads, railroads, etc.
Benefits of Volcanoes cont. • Recreation • Health spas and hot springs • Hiking, snow sports, and education • Kilauea National Park • Creation of New Land • Hawaiian Islands
Forecasting a Volcanic Eruption • Seismic activity • Shallow earthquakes and swarms can precede eruption. • May not provide enough time for evacuation. • Thermal, magnetic, and hydrologic monitoring • Accumulation of hot magma changes temperatures, magnetic properties, and temperature position of groundwater.
Forecasting a Volcanic Eruption • Land surface monitoring • Monitoring growth of bulges or domes. • Kilauea tilts and swells. • Monitoring volcanic gas emissions • Changes in CO2 amounts correlate with volcanic processes. • Geologic history • Mapping of volcanic rocks and deposits give idea of types of effects to be expected.