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Chapter 5 Lecture
Natural Hazards
Fourth Edition
Volcanoes
Tim Frazier
The University of Idaho
© 2015 Pearson Education, Inc.
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.
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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.
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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
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Volcanic Ash Shuts Down Air Travel
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Fire and Ice
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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
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The “Ring of Fire”
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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
1. Decompression melting
• Pressure exerted on hot rock is decreased
• Divergent plate boundaries, continental rifts, and hot spots
2. Addition of volatiles
• Chemical compounds that lower the melting temperature
of the rock
• Subduction zones – responsible for “Ring of Fire”
3. Addition of heat
• Induces melting if temperature exceeds melting temperature
• Continental hot spots
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Magma Formation
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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
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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
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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
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Volcano Characteristics
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5.2 Volcano Types, Formation, and Eruptive
Behavior
• Volcanoes vary greatly
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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
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Volcano Characteristics
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Volcanoes and Eruption Types
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Volcanic Explosivity Index (VEI)
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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
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Stratovolcano
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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
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Lava Dome
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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
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Shield Volcano and Volcano Dimensions
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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
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Cinder Cone
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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
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Large Calderas Form by Explosion and
Collapse
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Crater Lake Caldera
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Ash Fall Hazard
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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
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United States Volcanic Hazards
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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
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Selected Historic Volcanic Events
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Locations of Volcanoes in the United States
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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
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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
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Lava Flows
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Lava Tubes
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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
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Pyroclastic Activity
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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”
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Volcanic Tephra on Buildings
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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
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Pyroclastic Flow
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Plaster Casts of Volcano Victims
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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)
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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
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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
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Volcanic Mudflow Catastrophe
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Mount Rainier Volcanic Hazard Map
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Case Study 5.3 Eyjafjallajokull 2010 Eruption
- Applying the 5 Fundamental Concepts
• Eruption on May 18, 1980
• Exemplifies many types of events expected from a
Cascade volcano
• Unique and complex eruption
– Have learned a great deal from the eruption
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Mount St. Helens, Before and After
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Mount St. Helens, Before and After
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Case Study 5.3 Eyjafjallajokull 2010 Eruption
- Applying the 5 Fundamental Concepts,
cont.
• 120 years of dormancy
• March 1980 – seismic activity and small explosions
• May 1 – bulge begins to grow on northern flank at
rate of 1.5 m (5 ft.) per day
• May 18, 8:32 A.M. – 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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Case Study 5.3 Eyjafjallajokull 2010 Eruption
- Applying the 5 Fundamental Concepts,
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 to 55 km/hr
(18 to 34 mph)
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Mount St. Helens Erupts
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Debris Avalanche and Ash Cloud
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Barren Landscape Produced by Eruption
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Case Study 5.3 Eyjafjallajokull 2010 Eruption
- Applying the 5 Fundamental Concepts,
cont.
• Summary
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57 people were killed
Flooding destroyed > 100 homes
800 feet of timber flattened
Damage > $1 billion
September 23, 2004, Mount St. Helens reawakens
• Lava dome begins to form on crater floor
• Continues to form today
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Continued Growth of Dome in Crater
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5.5 Linkages between Volcanoes and Other
Natural Hazards
• Fire
– Hot lava may ignite plants and structures
• Earthquakes
– Usually accompany or precede volcanic activity
• Landslides
– Mudflows, ashflows, and landslides are common
secondary effects
• Climate change
– Debris can reflect sunlight causing global cooling
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5.6 Natural Service Functions 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 non-metallic rocks
– Used for soap, building stone, aggregate for roads,
railroads, etc.
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5.6 Natural Service Functions 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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5.7 Human Interactions with Volcanoes
• Humans do not affect the frequency or severity of
eruptions
• Minimization of loss of life and property damage is
best action
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5.8 Minimizing the Volcanic Hazard
• Forecasting is major component to reduce volcanic
hazards
• “Forecast” is a probabilistic statement
– Describes time, place, and character
– Analogous to forecasting weather
– Not as precise as a prediction
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Forecasting
• Unlikely to accurately forecast the majority of
volcanic activity in the near future
– Need experience with actual eruptions
– Better able to predict eruptions in Hawaiian Islands
• Forecasting uses information gained by
– Monitoring seismic activity
– Monitoring thermal, magnetic, and hydrologic conditions
– Monitoring the land surface to detect tilting or swelling of
the volcano
– Monitoring volcanic gas emissions
– Studying the geologic history of a particular volcano or
volcanic center
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Forecasting, cont.
• Seismic activity monitoring
– Shallow earthquakes and swarms can precede eruption
– May not provide enough time for evacuation
• Thermal, magnetic, and hydrologic monitoring
– Hot magma changes temperatures, magnetic properties,
and groundwater level
– Detected by satellite remote-sensing or infrared aerial
photography
– May also melt snow, showing new “hot” activity
– Ground or aerial magnetic surveys show new magnetic
properties
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A Volcano Reawakens
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Forecasting, cont.
• 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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Inflation and Tilting Before Eruption
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Volcanic Alert or Warning
• USGS alert notification system
– Ground-based volcano alert levels
– Aviation-based color code levels
• Four levels
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Normal - Green
Advisory - Yellow
Watch - Orange
Warning - Red
– For some eruptions, hazard on the ground or in the air will
differ and higher alerts or codes will be issued
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U.S. Geological Survey Volcanic Alert
Levels and Aviation Color Codesa
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5.9 Perception of and Adjustment to the
Volcanic Hazard
• Perception of volcanic hazards
• Adjustments to volcanic hazards
• Attempts to control lava flows
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Perception of Volcanic Hazards
• Reasons for people to live near a volcanic hazard
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Place of birth
Fertile land for farming
People believe eruption is unlikely
Economic limitations
• Good education can help people understand
volcanic hazard
• Volcanic crisis can develop when scientists predict a
volcanic hazard for near future
– Improved communication among scientists, emergency
managers, educators, media, and private citizens is key
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Adjustments to Volcanic Hazard
• Primary adjustment is evacuation
– Relocating people out of the hazard area prior to eruption
• Also psychological adjustment to losses
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Attempts to Control Lava Flows
• Hydraulic chilling
– Water is used to chill and control the lava flow
– Used in Iceland
• Edges and surface of the flow were cooled with fire hoses
• Bulldozers were moved up on the slow flow for a large water
pipe
• The plastic pipe did not melt as long as water was flowing in
it
• Small holes allowed the cooling of hot spots along parts of
the flow
• Wall construction
– Walls used to redirect lava flow
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Fighting Lava Flows
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Eyjafjallajökull 2010 Eruption – Applying the
5 Fundamental Concepts
• Iceland sits atop a hot spot
– Volcanic eruption once every three to four years
– Fortunately, most Icelandic eruptions are generally
effusive and have a VEI of no more than 1
– Most volcanoes are localized
– Changed with Eyjafjallajökull volcano in 2010
• Several signs
– Seismic activity became more shallow
– Uplift and tilting detected
– Rate of deformation was as much as 5 mm per day
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Icelandic Volcano Monitoring
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GPS and Seismic Data
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Eyjafjallajökull 2010 Eruption – Applying the
5 Fundamental Concepts, cont.
• Eruption in phases
– Effusive, Icelandic-type eruption
• Lasted about 3 weeks
• Generated a spectacular fire curtain display and lava falls
– Explosive eruption
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Forced evacuations
Caused flooding
Damage from debris and icebergs
Towering eruption column of ash
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Geotourists Witness Fissure Eruption
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Eyjafjallajökull 2010 Eruption – Applying the
5 Fundamental Concepts, cont.
• Volcanic ash
– Winds blew ash column towards Europe
• “No fly zones”
• By April 18, most air travel was suspended
• Airspace closed for more than a week
– Winds spared most of Iceland
• Southeastern corner covered by 1-2 inches of ash
• Total darkness from April 17 to early April 18
• Toxic levels of water-soluble fluoride in ash could harm
people and animals
• Eruptions not declared over until 6 months later
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Ash Shuts Down Air Travel in Europe
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Chapter 5 Summary
• Volcanic activity is directly related to plate tectonics.
Most volcanoes are located at plate boundaries,
where magma is produced in the spreading or
sinking of lithospheric plates.
• Lava is magma that has been extruded from a
volcano.
• Magma comes from melting of the asthenosphere.
• Features of volcanoes include vents, craters, and
calderas.
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Chapter 5 Summary, cont.
• Most explosive volcanic eruptions are from the
classic, cone-shaped stratovolcanoes that occur
above subduction zones, particularly around the
Pacific Rim.
• Volcanic domes are smaller then stratovolcanoes
but develop similarly above subduction zones and
are composed of viscous magma.
• The largest volcanoes, shield volcanoes, are
common at mid-ocean ridges such as Iceland, and
over mid-plate hot spots such as the Hawaiian
Islands.
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Chapter 5 Summary, cont.
• Large calderas are created by infrequent, huge,
violent eruptions.
• Specific geographic regions of North America at risk
from volcanoes include the northwestern coast of
California; the western coasts of Oregon and
Washington and parts of British Columbia and
Alaska, Long Valley, and the Yellowstone National
Park area.
• Primary effects of volcanic activity include lava
flows, pyroclastic hazards, and, occasionally, the
emission of poisonous gases.
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Chapter 5 Summary, cont.
• Lava flows, which can move as fast as 35 mph, but
often move much slower, do not represent a serious
hazard for loss of life because they can often be
avoided.
• Pyroclastic hazards include volcanic ash falls, which
may cover large areas with carpets of cool ash that
can destroy some structures and ruin agricultural
land, but loss of life is less common.
• Secondary effects of volcanic activity include debris
flows and mudflows, generated when melting snow
and ice or precipitation mix with volcanic ash.
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Chapter 5 Summary, cont.
• Volcanoes are linked to other natural hazards such
as fire, earthquakes, landslides, and climate
change.
• Volcanoes provide fertile soils, a source of power,
mineral resources, recreational opportunities, as
well as newly created land.
• Sufficient monitoring of seismic activity; thermal,
magnetic, and hydrologic properties; and change in
the land surface, combined with knowledge of the
recent geologic history of volcanoes, may eventually
results in reliable forecasting of volcanic activity.
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Chapter 5 Summary, cont.
• Forecasts of eruptions have been successful,
particularly for Hawaiian volcanoes and Mount
Pinatubo in the Philippines.
• The U.S. Geological Survey has developed an alert
notification system for volcanic activity that has four
levels for hazards on land and four color codes for
aviation hazards.
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Chapter 5 Summary, cont.
• Efforts to meet the goal of reducing volcanic
hazards are focusing on human and societal issues
of communication; the objective is to prevent a
volcanic crises from becoming a disaster or
catastrophe.
• Perception of the volcanic hazard is apparently a
function of a person’s age and length of residency
near the hazard. Community-based education plays
an important role in informing people about the
hazards of volcanoes.
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