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Natural Disasters Volcano Presentation

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  • 1. 1 Volcanoes CHAPTER 6: MATERIALS, HAZARDS, AND ERUPTIVE MECHANISMS CHAPTER 7: TYPES, BEHAVIOR, AND RISKS
  • 2. 2 Volcanoes • Volcanism. Refers to the rise of magma onto the earth’s surface. • Magma - a mixture of liquid rock, mineral crystals, and dissolved gases. • Volcanoes are conical or dome shaped landforms built by the emission of magma and its contained gasses onto the earth’s surface.
  • 3. Volcanic activity is controlled by plate tectonics, because plate movements relate to where sources of magma originate inside the earth. Nearly all active volcanoes are located in one of three plate tectonic settings: •Subduction zones at convergent plate boundaries Example: Volcanoes lining the trenches of the Pacific Ocean, forming the Pacific “Ring of Fire”. •Rifting and sea floor spreading at divergent plate boundaries, Example: Volcanic eruptions at mid-ocean ridges, and in some rift zones on the continents, like the East African Rift Valley. •Hot spots Example: The Hawaiian Islands and the Galapagos Islands. 3 Locations of Volcanic Activity and Volcanic Hazards
  • 4. Map of the World’s Active Volcanoes Note that the majority of above sea active volcanoes (about 66%) occur in the Pacific Ring of Fire. 4
  • 5. 5 Magma and the Driving Force Behind Eruptions • Magma may be ejected onto the earth’s surface as: – Lava – Pyroclastics or tephra- flour sized to boulder sized particles which are thrown in the air due to the built up pressure of gasses. . • The violence of a volcanic eruption depends on the magma’s viscosity and gas content. The more viscous (thick) and more gaseous the magma, the more explosive the eruption. Above: Gigantic eruption cloud of tephra Above: aa lava Right: pahoehoe lava
  • 6. Viscosity Viscosity is a measure of a fluid’s resistance to flow. The main factor that determines the viscosity of magma is its silica (SiO2) content. The more silica in the magma, the more viscous it is. The more viscous (silica-rich) the magma, the more violent the eruption. We are concerned, then, with three types determined by the chemical composition of the magma. 1.Mafic or Basaltic magma – low silicon and oxygen, high iron and magnesium. Simple silicate minerals. Dark magmas. Low Viscosity. 2.Intermediate or andesitic magma - intermediate silicon and oxygen, intermediate iron and magnesium. High Viscosity. 3.Felsic or rhyolitic magma - high silicon and oxygen, low iron and magnesium. Complex silicate minerals. Pale magmas. High Viscosity. 6 Left: The basic building block of all silicate minerals - the silica tetrahedron. Four oxygen atoms surrounding a single atom of silicon
  • 7. Gas Content The gas content of a magma also relates to its behavior. A magma with low gas content will tend to flow out of a volcano as relatively quiet lava. A magma with high gas content will tend to blow apart violently upon erupting. The higher the gas content, the more violent the eruption. The composition of the gases in magma are: Water vapor Carbon Dioxide Minor amounts of sulfur dioxide, hydrogen sulfide, chlorine, and flourine gases. But it is the amount of dissolved water that typically inspires a volcano to violence. 7
  • 8. 8 Steps to a Volcanic Eruption • Volcanoes form wherever rock melts at depth and the magma can rise to erupt at the surface. Rock deep in the earth may melt by – Increasing its temperature – Decreasing its pressure – Adding water (to lower the melting temperature). • Magmas that are generated deep within the Earth begin to rise because they are less dense than the surrounding solid rock. • As they rise they may encounter a depth (or pressure) where the dissolved gas no longer can be held in solution in the magma, and the gas begins to form a separate phase (makes bubbles) and will continue to grow in size as pressure is reduced.
  • 9. 9 Steps to a Volcanic Eruption: Two Possibilities • If the magma has a low viscosity, the gas will easily expand to atmospheric pressure at the earth’s surface and simply burst, and a non-violent eruption will occur, usually as a lava flow. • If the magma has a high viscosity the gas will not be able to expand very easily creating a high pressure inside which will cause them to burst explosively on reaching atmospheric pressure. This will cause and explosive volcanic eruption.
  • 10. Volcanic Explosivity Index The violence and size of a volcano’s eruption is expressed by the Volcanic Explosivity Index (VEI). Values for the VEI range from 0 to 8, and are based on: 1. the volume of material (lava and particles) erupted 2. the height of the eruption column 3. how long the eruption lasts The larger the VEI value, the larger the eruption. 10 This figure plots VEI values versus the eruption cloud height.
  • 11. Volcanic Explosivity Index Two main factors determine the size and explosivity of a volcano’s eruption: 1. The amount of time that has passed since the last eruption. 11 This figure shows that individual eruptions become more violent as the time gap between eruptions increases.
  • 12. Volcanic Explosivity Index 2. The viscosity (thickness) and gas content of the magma. Both relate to plate tectonic setting: • Divergent boundaries (MOR) and hot spots both draw their magmas from the upper mantle. This magma is called MAFIC (basaltic) MAGMA, and is characterized by relatively low silica and low gas content. (the eruptions have low VEI). • The magma at convergent boundaries comes from melting of subducted oceanic plates. Subduction forms FELSIC (rhyolitic) to INTERMEDIATE (andesitic) MAGMA, and is characterized by relatively high silica and high gas content. (the eruptions have high VEI). 12 (Refer to pg. 147)
  • 13. Bottom line: The world’s most dangerous volcanoes are those at convergent plate boundaries! 13
  • 14. 14 Types of Volcanic Hazards and Products The figure at right shows a composite of the potential hazards associated with a volcano. The main hazards are: •lava flows •ash falls •pyroclastic flows •lahars •gas emissions
  • 15. 15 Lava Flows • Lava is molten magma that flows out and onto the Earth’s surface. • Lava flows are typically formed from low viscosity mafic magma that erupts at divergent boundaries and hot spots. Fluid basalt lava flows are called Hawaiian type lava and can come in two varieties. Smooth, runny pahoehoe (top) and chunky aa (bottom).
  • 16. Lava Flows • The lower silica content (and therefore low viscosity) of mafic magma allows the lava to run down slopes easily. • Lava flow eruptions are fairly gentle and quiet. They may cause property damage, but rarely fatalities. 16(Refer to pg. 134-135)
  • 17. Lava flows destroy whatever they overrun. Everything in the path of an advancing lava flow will be knocked over, surrounded, buried, or ignited by the extremely hot temperature of lava. 17 Hawaii visitor’s center before (above)…. …..and after (right)
  • 18. 18 Left: An example of diversion of a lava flow. The bulldozer is making a barrier to force a moving lava flow away from its path toward a village in Hawaii. Right: Barrier constructed to protect the main tourist complex on Etna, Sicily. Note the thirty foot (10 m) thick aa flow approaching the barrier. The barrier remained intact until the eruption ended. Left: Aerial view of aa flow against the barrier on Etna, Sicily. Diverting Lava Flows
  • 19. 19 Ash Falls • Ash falls form when an eruption column of tephra and gas is blown into the air by an explosive eruption. The eruption column can rise up more than 20 km into the atmosphere. • Tephra is a general term for any size of fragmental material blown out of a volcano.
  • 20. Large-sized tephra typically falls back to the ground on or close to the volcano. 20 Shown left: Pele is the goddess of the volcano. Teeny tiny volcanic bombs are known as Pele’s tears. Shown right: Volcanic Bombs form from the rapid cooling of lava thrown in the air. When the lava cools in mid-air, it forms the characteristic ellipsoidal shape. Tephra
  • 21. Volcanic Ash Volcanic ash, the smallest tephra fragments, can travel hundreds to thousands of kilometers downwind from a volcano. 21 1980 Eruption of Mt. St. Helen and map of ash fall distribution and thickness.
  • 22. Volcanic Ash Ash spreads in upper atmosphere around the globe. The suspended ash can decrease the insolation from the sun and lower global temperatures! 22
  • 23. Volcanic ash can be hazardous for a variety of reasons... Daylight turns into darkness… Roofs may collapse from added weight… Machinery and vehicles will be abraded and engines may seize… 23
  • 24. Farmland will be covered… Roads will be slippery or blocked… Waste-water systems may clog… 24
  • 25. 25 Pyroclastic Flows Pyroclastic flows (Ash Flows) are avalanches of a very hot (1300- 1800F) mixture of hot rock particles and hot gas that are blown out of the vent of the volcano as an eruption column which subsequently collapses and moves very rapidly down the flanks of the volcano at speeds from 50 to over 200 km per hour and can travel for 10’s of kms burning, burying and suffocating everything in their path. Above: 1997 eruption on island of Montserrat in the West Indies. Right: 1986 eruption of Augustine Volcano, Alaska.
  • 26. Pyroclastic Flows….. ...destroy by direct impact. bury sites with hot rock debris…. ...burn forests, crops, and buildings 26
  • 27. 27 The eruption of Mount Saint Helens in 1980 produced a devastating pyroclastic flow, seen here blasting out to the right as a lateral blast. The eruption was so violent that the entire top of the mountain was blown off. The Mount Saint Helens pyroclastic flow knocked down vast acres of forest, stripping the bark entirely off trees. The Eruption of Mount Saint Helens
  • 28. 28 One of the greatest volcanic disasters in history was the destruction of the city of Saint Pierre, Martinique, by a pyroclastic flow from Mount Pelee in 1902. Before the eruption right…... ….and after the eruption left. An estimated 30,000 people died gruesomely by burning, burial, and suffocation. The Eruption of Mount Pelee Refer to page 179-181.
  • 29. 29 Pyroclastic flows have buried entire cities and their inhabitants. Perhaps the most famous examples are Pompeii and Herculaneum, which were buried in A.D. 79 by the eruption of Mount Vesuvius. Cast of victims. The cast is obtained by the filling the cavity in the ashes with liquid chalk. Left: Ruins of Pompeii, with Vesuvius in the background The Eruption of Mount Vesuvius
  • 30. 30 Mudflows or Lahars Lahars have caused more fatalities than any other volcanic hazard because they are more common, and they can occur at any time. Volcanic Mudflows or Lahars form by mixing water with loose volcanic ash and debris on the flanks of a volcano. As the mud moves downslope, it gathers rocks of all sizes accelerating as it goes. The water can come in several ways including: A major rainstorm. An eruption melts large amounts of snow and ice on the flanks of the volcano.
  • 31. When moving, a lahar looks like a mass of wet concrete that carries rock debris ranging in size from clay to boulders. Most lahars travel much too fast for people to outrun. 31
  • 32. 32 Landscape before and after the 1991 eruption of Mt. Pinatubo in the Philippines. Mudflows from Mount Pinatubo after the 1991 eruption destroyed roads and bridges, and buried farmland and towns with sediment. The Eruption of Mount Pinatubo
  • 33. 33 Right: This valley (north fork of the Toutle River) was filled by a muddy lahar several 10’s of feet thick during the 1980 eruption of Mt. St. Helens. Left: Mudlines high on tree trunks show the depth of the Toutle River mudflow. Note the person on the right for scale. Right: A lahar carries away a bridge spanning the Toutle River about 55 km downstream from Mount St. Helens. Before arriving at the bridge, the lahar swept through a logging camp and picked up thousands logs. The Eruption of Mount Saint Helens
  • 34. 34 Located in Washington State, Mount Rainier is the largest and highest volcano of the Cascade Range. Due to its location near heavily populated areas including Seattle and Tacoma, Lahar hazard makes this stratovolcano the most dangerous volcano in the U.S. The Risk of Lahars of Mount Rainier
  • 35. 35 Start of landslide Landslide enters valley Landslides Landslides are large masses of rock and soil that fall, slide, or flow very rapidly under the force of gravity. A number of factors can trigger a landslide: * intrusion of magma into a volcano * explosive eruptions * large earthquake * intense rainfall that saturates a volcano or adjacent tephra- covered hillslopes with water
  • 36. 36 Explosions (red) begin to rip through the landslide (green) Exploded rock debris (red) forms a pyroclastic surge that quickly overtakes the landslide (green) These illustrations show the landslide (green) and directed blast (red) that occurred during the first few minutes of the eruption of Mount St. Helens in 1980. Volcanic landslides can trigger volcanic explosions.
  • 37. 37 Above and Below: Stands of dead trees killed by excess CO2 seeping from the ground from magma close to the surface. Poisonous Gases Gases emitted during volcanic eruptions may be toxic and/or corrosive. The most common hazardous gas is CO2. Carbon Dioxide is deadly to people, animals, and trees in high concentrations.
  • 38. Poisonous Gases Being heavier than oxygen, it can pour downslope and displace oxygen at the surface. Because it is colorless and odorless, it can suffocate without warning. 38 These cattle, and about 1700 people, were victims of asphyxiation from a massive CO2 discharge from Lake Nyos (Cameroon, West Africa) in 1986. Lake Nyos lies within a volcanic crater.
  • 39. 39 Types of Volcanoes Shield Volcano Composite or Stratovolcano Cinder Cone They differ in igneous rock chemistry, eruption style, physical features and geographic location. Note the size differences!
  • 40. 40 Shield Volcanoes Shield Volcano Stats • Found along hotspots and divergent boundaries. • Mafic (basaltic) magma • Low viscosity • Low gas content (no water because no subduction). • GENTLE eruptions of lava flows. • Landform composed almost entirely of relatively thin lava flows. • Gently sloping due to the low viscosity of the magma which allows lava to flow great distances before it cools. • The largest structures on earth! 9 km from seafloor to summit! • Basalt is the most common rock type.
  • 41. Most of the lava of a shield volcano flows down the flanks of ridges that radiate outward from the volcano summit. 41 Left: Fissure eruption on Kilauea's East Rift Zone. Shield Volcano
  • 42. The big island of Hawaii has three major volcanoes: Mauna Loa, Mauna Kea, and Kilauea. Mauna Loa is Hawaii’s largest volcano but its activity is infrequent. Kilauea is in the main growth stage having erupted almost continuously since 1983. Hawaii
  • 43. Because they form from relatively quiet eruptions of low viscosity, fluid magma, shield volcanoes are very large, with gently sloping sides and a convex shape. 43 Left: a shield volcano in the Galapagos Islands created over a hot spot. Right: Iceland created where a mid- ocean ridge sticks up above sea level. Left: Mt. Etna, Sicily. The largest continental volcano on earth and the most active volcano in Europe, Etna erupts almost continuously, with an occasional violent episode. Shield Volcano
  • 44. 44 Stratovolcano Stats • Found along convergent boundaries paralleling subduction zones. • Andesitic (intermediate) to rhyolitic (felsic) magma. • High viscosity • High gas content (subduction drags down water) • VIOLENT eruptions. Dangerous and Explosive. • Landform composed of alternating layers of tephra and lava. • Steeply sloping due to the piling up of tephra around the central vent and the high viscosity of the lava that glues it together. • Can reach heights of roughly 3500 meters. • Andesite and rhyolite are the most common rock type. Composite or Stratovolcanoes
  • 45. Stratovolcano Hazards This figure shows a composite of the main hazards associated with a typical stratovolcano. The main hazards are: pyroclastic flows lahars landslides ash falls Poisonous gas emissions 45
  • 46. 46 Map of the world’s active volcanoes, showing that the majority of above sea active volcanoes (about 66%) are stratovolcanoes produced by subduction in the Pacific Ring of Fire.
  • 47. 47 Because the western U.S.A. borders the Pacific Ring of Fire volcanic hazards exist primarily in the western states. On the map, regions with the greatest risk of volcanic hazards are shown in red. *Yellowstone Caldera The volcanoes of the Cascade Range, which extends from northern California north into Oregon and Washington, are related to the presence of the Cascadia Subduction Zone offshore. The Volcanoes of the Cascade Range
  • 48. 48 Stratovolcano eruptions are intermittent and violent. They can remain dormant for tens to hundreds of years. Stratovolcano Eruptions
  • 49. 49 Mt. St. Helens Above: Mount St. Helens eruptive sequence on May 18, 1980 Below: Events leading up to Mount St. Helens eruption on May 18. 1980.
  • 50. Lava Domes Lava domes are steep sided structures that form when viscous lava is erupted slowly and piles up in and around a volcanic vent. As it grows its outer surface cools and hardens, then shatters, spilling loose fragments down its sides. 51 Below: Dome extrusion often follows explosive eruptions. This lava dome in the crater of Mount St. Helens built by periodic lava extrusion between 1980- 1986. Above: Dome collapse can produce pyroclastic flows. Shown is a pyroclastic flow sweeping down the eastern flank of the lava dome of Soufrière Hills volcano on Montserrat on January 16, 1997.
  • 51. 52 Cinder Cones • Cinder cones form when fluid basaltic magma rises along a fracture and encounters groundwater. The steam generated blows fragments of lava violently into the air which solidify and fall as cinders around the vent forming a circular or oval cone. • Cinder cones are commonly found on the flanks of shield volcanoes and stratovolcanoes. • Most cinder cones erupt for only a few months to a few years and rarely rise more than 300-500 m above their surroundings. Being unconsolidated they tend to erode rapidly.
  • 52. 53 Above: The photos the eruption of Paricutin Volcano, Mexico, a classic example of a cinder cone. Above: View of cinder cones near the summit of Mauna Kea Above: This cone is one of two cinder cones called the Red Cones located south of Long Valley Caldera in California. These basaltic cones were erupted about 5,000 years ago. Above: This cinder cone on the flank of Mount Etna is surrounded by a younger basaltic lava flow. Cinder Cones
  • 53. Caldera Some eruptions of stratovolcanoes are so violent that after the eruption ends the volcano collapses into the empty magma chamber beneath, leaving a large crater behind called a caldera. 54 The above photo shows Crater Lake in Oregon. The basin that holds Crater Lake formed when Mount Mazama destroyed itself in a gigantic eruption 7,700 years ago.
  • 54. Supervolcanoes – Giant Continental Calderas ‘Supervolcanoes’ are giant rhyolite volcanoes that form over continental hotspots. Resurgent caldera eruptions are by far the largest and presumably most destructive of all types of volcanic eruptions. 55 Bottom Image: The Yellowstone Volcano is the relic of three monstrous eruptions that occurred 2mya, 1.3mya, and 600,000ya. Each eruption left collapse calderas ~50km across. As magma continues to rise beneath the caldera, it raises a resurgent dome in its surface.
  • 55. Will Yellowstone Erupt Again? Geysers eruptions occur as a consequence of groundwater being heated to its boiling temperature in a confined space (ex. a fracture). The resulting steam forces water and steam up through the fractures and onto the ground. Two resurgent domes currently occupy the caldera-the Mallard Lake dome and the Sour Creek dome. According to a report from the U.S. Geological Survey. “Recurring earthquake swarms, swelling and falling ground, and changes in hydrothermal features are cited in the report as evidence of unrest at Yellowstone.” The Associated Press. May 9, 2005 A magma body beneath the caldera is about 60 km long NE-SW and up to 40 km wide. The two resurgent domes are where the magma body is closest to the surface, ~5-6 km. Hot springs are areas where heated groundwater reaches the surface of the earth.
  • 56. Mitigation and Prediction of Volcanic Hazards There is no effective way to mitigate most volcanic hazards after an eruption has occurred. One exception is lava flows, which have been mitigated by diverting the moving lava and/or by chilling the lava with sea water. Chilling with water was done successfully in Iceland in 1973 to save a town from being overrun by lava. 57
  • 57. Hazard Maps Mitigation before an eruption can be done by assessing a region for its potential volcanic hazards and creating a hazard-zone map, which indicates the type and degree of risk in particular areas. Involves studying the geology of the volcano to determine the types of hazards posed by the volcano and the frequency at which these types of hazards have occurred in the past (radiometric age dating). 58 Right: An example of a volcanic hazard zone map, showing lava flow hazard zones for the big island of Hawaii.
  • 58. Prediction of Volcanic Hazards To predict an eruption, geologists depend on various precursors: events that occur prior to an eruption. The main precursors that signal an impending volcanic eruption are: 1. Tilting and swelling of the volcano’s sides. As magma rises below a volcano, it forces the earth’s surface upward and outward, causing it to bulge. 59 Below: Tiltmeter site on crater floor of Mount St. Helens
  • 59. 2. Increased seismic activity. Many small earthquakes -- called earthquake swarms or harmonic tremors -- caused by fresh magma rising below the volcano. 3. Seismic exploration. Monitoring the movement of the s-wave shadow zone to determine the position and movement of magma. 60
  • 60. 4. Increased steam and gas emissions caused by rising magma emitting gas as it approaches the surface. 5. Changes in Groundwater System - As magma enters a volcano it may cause the water table to rise or fall and cause the temperature of the water to increase. 61 Direct gas sampling from fumeroles with laboratory analysis