Natural Hazards, 2e

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Natural Hazards, 2e

• Volcanoes
• Chapter 4

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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

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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

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Introduction

• Most volcanoes are near plate boundaries.
• Plate boundaries are where the magma is.
• Magma is molten rock.
• Lava is magma on the earths surface.
• Some active plate boundaries
• Subduction zones
• Mid-ocean ridges
• Continental rift zones

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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

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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.

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Magma Types

• Basaltic
• Low silica content, low viscosity
• Andesitic
• Intermediate silica content, intermediate
  viscosity
• Rhyolitic
• High silica content, high viscosity

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Volcano Types
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Shield Volcanoes

• Largest volcanoes in the world
• Built almost entirely of lava flows
• Resemble a warriors shield
• Associated with basaltic magma
• Low viscosity, low gas content
• Gentle flowing lava with nonexplosive eruptions
• Can form lava tubes underground

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Shield Volcanoes, cont.

• Found in Hawaiian Islands, Iceland, and around
  Indian Ocean

Figure 4.4
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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

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Composite Volcanoes

• Ex. Mt. St. Helens, Mt. Rainer, Mt. Fuji

Figure 4.6
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Volcanic Domes

• Made from highly viscous rhyolite magma
• Exhibit highly explosive eruptions
• Ex. Lassen Peak and Mono Craters

Figure 4.7
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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

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Cinder Cone Volcanoes, cont.
Figure 4.8
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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

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Volcanic Features, cont.

• Hot springs
• Hot rocks heat groundwater discharged at surface
• Geysers
• Groundwater boils, erupting steam at surface

Figure 4.11b
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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

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Caldera Eruptions
Figure 4.14
Figure 4.15
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Volcanic Activity and Plate Tectonics
Figure 4.16
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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

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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

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Geographic Regions

• Ring of fire
• Pacific Ocean subduction zones
• Hot spots
• Hawaii and Yellowstone Park
• Mid-ocean ridges
• Iceland
• Rift valleys
• East Africa

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Figure 4.19
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Effects of Volcanoes

• 5060 volcanoes erupt each year.
• In U.S. 23 volcanoes
• 500 million people live close to volcanoes.
• Japan, Mexico, Philippines, and Indonesia
• Several U.S. cities vulnerable

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Figure 4.20
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Lava Flows

• Rhyolitic, andesitic, and basaltic lavas
• Basaltic lavas flow most abundantly
• Pahoehoe 1 m/hr
• AA-1-3 m/day

Figure 4.21
Figure 4.22
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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.

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Ash Fall

• Vegetation destroyed
• Contaminates surface water
• Damage to buildings
• Health hazards
• Aircraft engine failure

Figure 4.24
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Pyroclastic Flow

• Responsible for more deaths than any other hazard
• Flow at 160 km/hr (100 mph)
• Temperatures gt1000C

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Poisonous Gases

• 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

Figure 4.26b
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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.

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Figure 4.28
Figure 4.29
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Landslides

• Secondary effects of volcanoes
• Can cause tsunamis

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Mt. St. Helens

• Scene of volcanic explosion in recent history
• Well-studied example of Cascade volcanic eruption

Figure 4.30a
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Mt. St. Helens Before
Figure 4.30b
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Mt. St. Helens After
Figure 4.30c
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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, 832 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)

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Bulge Avalanche
Figure 4.31a
Figure 4.31b
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Lateral Blast Vertical Eruption
Figure 4.31c
Figure 4.31d
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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).

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Debris Avalanche and Ash Cloud
Figure 4.32a
Figure 4.32b
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Mt. St. Helens Summary

• 57 people were killed
• Flooding destroyed gt100 homes
• 800 feet of timber flattened
• Damage gt1 billion
• September 23, 2004, Mt. St. Helens reawakens
• Lava dome begins to form on crater floor
• Continues to form today

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Figure 4.34
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Links to other Hazards

• Earthquakes
• Landslides
• Fire
• Hot lava ignites plants and structures.
• Climate Change
• CO2 (and other gasses) from eruption alters
  climate.

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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.

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Benefits of Volcanoes cont.

• Recreation
• Health spas and hot springs
• Hiking, snow sports, and education
• Kilauea National Park
• Creation of New Land
• Hawaiian Islands

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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.

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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.

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Volcanic Alert or Warning
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Attempts to Control Lava Flows

• Hydraulic chilling
• Water used to chill and control the lava flow
• Iceland
• Wall construction
• Walls used to redirect lava flow

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End

• Volcanoes
• Chapter 4