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Volcanoes
Chapter 6
© 2011 Pearson Education, Inc.
You will learn
• What volcanoes are, where they occur, and why
they form
• The different types of volcanoes and the hazards
associated with each type
• How scientists study volcanoes and come to
understand and predict their eruptions
• How people can lower the risks posed by
volcanoes
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Over the Volcano
Figure 6-2 Over the Volcano
If you fly over the Pacific rim, you will probably find yourself looking down
on volcanoes. This erupting volcano, in the Aleutian Islands of Alaska, is on
the main flight path between Asia and North America.
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What Volcanoes Are
• Magma: (molten rock underground) rises from great depths
to the upper level of the crust and erupts (as lava) onto the
surface.
• hot and less dense (more buoyant) than surrounding solid
rock
• quickly cools and solidifies on the surface—or at relatively
shallow depths in the crust
• magma contains dissolved gases (e.g. H2O vapor, CO2, SO2)
• As magma rises:
• confining pressure of the overlying rocks decreases
• dissolved gases bubble out of solution and release the
pressure
• the more dissolved gas, the more explosive the magma
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Lava Flow
Figure 6-3 Volcanoes Are Rock Factories
Magma quickly cools and solidifies into igneous rock. The surface of this lava
flow in Hawaii is changing to a gray color as it solidifies.
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Numbers of Volcanic Eruptions
•
How many volcanoes have been active in the last 10,000 years?
• 1300–1500 volcanoes on land with probable eruptions
• only about 550 volcanic eruptions eyewitness accounts
• 50–70 volcanoes erupt on land in an average year
• many more volcanic eruptions occur in the deep oceans
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Potentially Active Volcanoes of
the Western U.S.
Figure 6-4
Many people do not realize that even
in the continental United States there
are several potentially active
volcanoes dangerously close to large
population centers.
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Types of Magmas
• Eruptions range from spectacular explosions to
oozing red-hot lava flows.
• This difference reflects the fact that not all magmas
are the same.
• different temperatures
• compositions
• dissolved-gas contents
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Types of Magmas (cont.)
• Viscosity = resistance to flow, depends on
temperature, composition
• Composition (e.g, SiO2 content) has greatest influence on
magma viscosity
• high SiO2 = more viscous
• magmas with high SiO2 solidify at lower temperatures
• their viscosity increases even more
• Amount of SiO2 depends on:
• composition of source rocks
• % partial melting of source—low % partial melt = higher
SiO2
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Gas Content in Lavas
• H2O—most abundant gas dissolved in magma, along
with CO2, SO2, and others
• lowers viscosity (by reducing the tendency of SiO4 ions to
combine)
• Gases may separate from magma during ascent due to:
•
•
•
•
•
the magma may partially crystallize (and thus hold less liquid)
temperature may go down
being trapped, accumulating in more viscous magmas
decrease in the confining pressure
gas (vapor) pressure increases
• Explosive eruptions!
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Shield Volcanoes of the Big
Island of Hawaii
• Kilauea is the currently
active volcano on the island.
Loihi is just beginning to
form underwater to the
South.
Figure 6-6
The Big Island of Hawaii consists
of five overlapping shield
volcanoes formed at different
times over the last few hundred
thousand years.
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Types of Volcanoes
• Shield volcanoes
• wide and gently sloping (like an upturned warrior’s shield)
• almost entirely composed of layers of solidified mafic
(basaltic) lava
• mafic magma is hot and less viscous, so it flows long
distances
• largest on Earth and build over hundreds of thousands
of years
• Example: island of Hawaii
• 5 successively younger, overlapping shield volcanoes
• the island Hawaii is the largest mountain on Earth
• high point ≈10,200 meters (33,500 ft) above
seafloor
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Types of Volcanoes (cont.)
• Flood basalts
• mafic magmas that erupt from fissures—cover extensive areas with
basalt
• form stacks (over thousands of years) of basaltic flows
• the amount of magma erupted can be tremendous
• Columbia River flood basalt—covers 130,000 km2 (50,000 mi2)
• 100,000 km3 (24,000 mi3) of basalt was erupted ≈ 5 million years
ago.
• Cinder cones
• from mafic magma, rich in gas, spew globs of lava into air
• erupt at shield volcanoes, flood basalt areas, and stratovolcanoes
• pieces solidify and rain down as cinders and larger blocks (lava
bombs)
• this loose debris falls down around the vent to form cone-shaped
piles
• typically short-lived and low volume
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Types of Volcanoes (cont.)
• Stratovolcanoes
• towering, steep-sloped, and
frequently symmetrical
mountains
• complexly alternating layers of
lava and other volcanic debris
• intermediate to felsic in
composition (some mafic
magma)
• many stratovolcanoes
commonly caped with glaciers
• magmas are viscous and gasrich, and can erupt explosively
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Pyroclastics (Tephra)
Figure 6-13
Pyroclastic debris ranges from (a)
fine ash (small crystals, rock
fragments, and bits of glassy frozen
magma), (b) pumice (glassy
solidified magma with many holes
created by gas bubbles), (c) to large
blocks of rock; (d) glowing lava
bombs and cinders
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Large Calderas
• Calderas are large, circular-tooblong depressions that form
when magma chambers erupt
their contents and the volcanic
mountain above collapses into
the empty magma chamber.
• Caldera-forming eruptions can
expel tens to thousands of
cubic kilometers (cubic miles)
of ash that completely blankets
whole regions. (Volcanic ash
that becomes lithified is tuff.)
• Crater Lake National Park
formed from the collapse of
Mount Mazama, forming the
caldera—which subsequently
filled with water.
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Huge Caldera Eruptions
• La Garita Caldera (New Mexico)
erupted 5000 km3 (1200 mi3) of
tuff —28 million years ago
• Toba Caldera (Sumatra in
Indonesia) erupted 2800 km3
(670 mi3) of tuff 75,000 years ago
= largest volcanic eruption during
the last 2 million years
• Yellowstone Caldera—
eruptions at:
• 2.0 million years ago
• 1.2 million years ago
• .64 million years ago—Lava
Creek Tuff 1000 km3 (240 mi3)
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Types of Volcanoes (cont.)
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The Volcanic Explosivity Index (VEI)
• The VEI assigns a
volcano a number from
0 to 8 based on:
• volume of material
ejected by past
eruptions
• height of eruption
column
• style of the eruption
(such as lava flows vs.
explosive eruptions)
• duration of the eruption
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Benefits of Volcanoes
• Ultimate source of valuable natural resources
• metal deposits are found in the roots of extinct volcanoes
• solid debris (e.g., pumice and ash) mined for construction
materials and abrasives
• Mountain scenery that draws tourists (Mount Fuji in
Japan)
• Ski resorts (Mount Hood in Oregon)
• Geothermal springs/spas (Yellowstone National Park)
• Geothermal power (ancient and current)
• Weathering transforms ash and lava into fertile soil
that supports agriculture in many lands (Mount
Vesuvius in Italy)
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Volcanoes: Divergent Plate Boundaries
• About 62% of the magma erupted on Earth is at mid-ocean
ridges.
• Mantle material decompresses as it rises below divergent plate
boundaries.
• As the mantle material rises—pressure and melting point
decrease
• At 100 km (62 mi), temperature of the mantle is 1200°–1400°C
• Melting doesn’t occur—pressures keep the melting point high
• Material rises to lower-pressure regions, where the melting point
is lower than the temperature of the mantle material
• Melting produces a mafic magma, derived from partially melted
ultramafic mantle rock
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Volcanoes: Convergent Plate Boundaries
• About 26% of the magma erupted on Earth is generated at
subduction zones—where oceanic lithosphere sinks into the
mantle
• descending plate carries water
• trapped in oceanic crust and sediments
• in the mantle, the oceanic plate recrystallizes, releases water
and other volatiles, such as carbon dioxide
• these rise into the overlying mantle and lower its melting point
• overlying mantle begins to melt producing mafic (basaltic)
magma
• BUT the magma that erupts at subduction-zone volcanoes is
primarily intermediate in composition (60% silica), making it an
andesite. HOW?
• mafic magma may differentiate
• heat may partially melt crust
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Volcanoes: Within Plates
Volcanoes that are far from plate boundaries (Hawaii; Yellowstone)
Hypothesis:
• caused by local anomalies in the mantle called hot spots
• voluminous mantle material rises and melts to form mafic magma
• crust above hot spots melts to produce intermediate and felsic magma
Example: Hawaiian Islands—or Emperor-Hawaiian Seamount Chain
>80 volcanoes—progressively younger from N to S (80 mya to now),
change in age along the chain has been attributed to progressive
movement of the Pacific Plate over a fixed mantle hot spot.
However, melting and magma movement in the mantle can be
influenced by:
• mantle’s thermal structure
• mantle composition (larger % of lower-melting-point material)
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The Hawaiian Islands
• Recent research is suggesting
that hot spots may not all be
stationary.
Figure 6-21
(a) The progressive change in age of the
Emperor-Hawaiian seamount chain—getting
younger from north to south—has been
interpreted to reflect the movement of the
Pacific plate over a “fixed” mantle hot spot,
as shown in part (b).
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The Long Valley Caldera Erupted the
Bishop Ash 760,000 Years Ago
FIGURE 6-22
(a) This volcanic eruption covered
most of the southwestern United
States, including Los Angeles, with
ash. The continued volcanic activity,
which produced Mammoth Mountain
(b) with its popular ski slopes, is still
active and is being closely monitored
by the U.S. Geological Survey.
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Mount St. Helens Erupts
Figure 6-24 Ash, steam, water, and debris were blasted to a height of 18,300
meters (60,000 ft) during the May 18, 1980, eruption of Mt. St. Helens.
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Pyroclastic Flows and Surges
Hot ash/solid debris—>800oC (1500oF), up to160 km/hr (100 mph)
Causes:
1) Gravitational collapse of ash column above a stratovolcano
- hot ash, gas in the column rise—reach cooler altitudes, lose buoyancy
- column falls back in on itself, spreads out on ground as a pyroclastic flow
2) Lateral eruption
- side of a volcano may collapse, releasing pressure on magma, initiating
a lateral (sideways) pyroclastic eruption (1980, Mount St. Helens, WA)
3) Collapse of a dome plugging a vent
Base flow = dense, ground-hugging mass of ash, cinders, blocks of rock
Pyroclastic surge = envelope of hot gas and ash around the base flow,
lighter; behaves like a fluid—can detach from the main flow and flow up
and over ridges
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Volcanic Ash
Figure 6-27 Unhealthy to Breathe
Volcanic ash contains tiny particles of
glassy, solidified magma (shown here in a
scanning electron micrograph) and small
rock and crystal fragments. The size of this
particle is about 0.01 cm. Ash is abrasive
and can cause respiratory problems.
Figure 6-28
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Lahars
• Gravity-driven, wet debris flow
• heat from eruption melts snow and ice, which begins to
flow
• slurry of ash, lava debris, and water
• very dense, massive, fast-flowing
• lahars can kill long after a volcanic eruption
• Example: Nevado del Ruiz, Colombia, November 13, 1985
• relatively small eruption melted snow and ice
• water-saturated loose flank debris triggered lahars
• destroyed >5000 homes and killed >23,000 people
• Example: Mount Pinatubo, Phillipines, 1991
• lahars can kill long after a volcanic eruption
• loose flank debris mixed with water (e.g., from heavy rains)
• after the 1991 eruption, lahars continued to flow for years, burying whole
villages
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A Lahar
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Stratovolcano Hazards
FIGURE 6-32 Stratovolcano Eruptions Can Create Many Hazards
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After the Eruption of Mount St.
Helens in 1980
FIGURE 6-34 (a) Surrounding forests
were devastated by the 1980 eruption of
Mount St. Helens. Over four billion board
feet of usable timber was damaged or
destroyed.
(b) Lahars severely damaged property and
stream drainages. The lahar that
devastated the Muddy River drainage
carried this huge boulder along with it.
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Hazards of Shield Volcanoes
Lava flows—velocity is controlled by gravity and viscosity
Figure 6-35 Pahoehoe flows are fast, hot mafic fluid and develop smooth, hummocky,
or ropy surfaces when they solidify. Aa flows, which are lower in temperature and more
viscous than pahoehoe, develop a rough surface of broken lava blocks. Here an aa
flow is advancing over a previously solidified pahoehoe flow.
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Volcanic Gases
• Hazards to people
• CO2 build-ups in volcanic lakes
• geological disturbances churn up clouds of colorless,
odorless CO2 gas from the depths of volcanic lakes
• gas bubbles out of solution—flows out of lake
(heavier than air)
• Lake Nyos (Cameroon, 1986)—1700 persons and
numerous wildlife and livestock; clouds spread over
300 km2 (116 mi2)
• VOG (volcanic fog)—sulfur dioxide (SO2) and other
gases react chemically with moisture and oxygen to
form a type of air pollution
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Volcanic Gases (cont.)
• Hazards to plant life
• Sulfur gases combine with moisture in air,
generating sulfuric acid (acid rain)
• Climate changes
• SO2 gas erupts and combines with water to
form aerosols of sulfuric acid
• aerosols reduce sunlight reaching surface,
lowering global
temperatures
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Living with Volcanoes
Worldwide, about 500 million people live in volcanic hazard zones.
Strategies for protection:
• Dropping bombs to divert lava flows (in 1935, 1942, 1975–1976
in Hawaii)
• Pump water onto flow to cool and solidify lava flows (Heimaey,
Iceland—36,000 tons of seawater per hour pumped onto the
encroaching lava flows—for 5 months!)
• Mount Etna, Sicily
• Blasting through walls of channels feeding flows (1983)
• Build earth barriers to slow flows
• Dig channels to divert flows into old lava tubes (1992) or into
artificial channels (2001)
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Hazard Assessments
• Geologic mapping and dating of lava and ash from past
eruptions can reveal approximately how often a volcano has
large eruptions.
• Hazard mapping involves locating and describing geological
deposits near the volcano to identify them as lava, ash,
pyroclastic flows, lahars, etc.
• Laboratory work can date these deposits, sometimes
precisely, down to individual lava flows and the calendar
year they erupted.
• Volume and thickness of individual deposits provides clues
to the magnitude of past eruptions, which in turn puts some
constraints on how large a future eruption could be.
• Long-range forecasts—state the probability that an
eruption of a certain size will occur within a certain time
period.
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Monitoring Volcanic Activity
Figure 6-40 Volcanologists and other scientists frequently monitor volcanoes to
determine when an eruption may be imminent. They may visit active volcanoes to
gather data collected by sensors, take samples of magma or volcanic gases, and
make close-up observations of ongoing activity.
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Monitoring Eruption Precursors
Earthquakes
• magma pushing upwards shatters surrounding rocks
• increase in small- to medium-sized earthquakes at shallow depths
• specific patterns, associated with changes in volcanic “plumbing”
Monitoring ground deformation
• mechanical tilt meters—monitor inflation as magma pushes upwards
• Global Positioning System (GPS) receivers, or field surveying
• remote sensing techniques, such as satellite radar mapping
Gas composition and concentration
• monitoring done from a distance with a correlation spectrometer
(COSPEC)
Other precursors
• minute changes in the force of gravity on a volcano’s slopes
• fluctuations in electrical conductivity reflect changes in the magma
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SUMMARY
• Volcanic activity is commonplace on Earth
• Most volcanoes form along divergent and
convergent plate boundaries
• The most explosive magmas are rich in silica,
viscous, and gas-rich
• Mafic magmas are silica-poor, less viscous, and
erupt as lava flows
• tend to form shield volcanoes
• Lava flows from shield volcanoes destroy
property, but are associated with less loss of life
than eruptions at stratovolcanoes
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SUMMARY (cont.)
• Stratovolcanoes are the most explosive and the
most dangerous
• create pyroclastic flows, lahars, and ash plumes in
the atmosphere
• Volcanic gases, released by all types of
volcanoes, can be extremely hazardous, and in
large eruptions can influence the global climate
• Scientists study eruption history, map distribution
and character of erupted material, identify
hazards, and predict future eruptions
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