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Volcanoes and
Volcanic Hazards
Lecture by Dr. Ken Galli, Boston College
EESC116301 Environmental Issues and Resources
July 28, 2015
Please do not distribute beyond the EESC116301 Class.
1
Volcanoes are mountains
• very different from other mountains; they are not formed by folding and crumpling
or by uplift and erosion.
• Instead, volcanoes are built by the accumulation of their own eruptive products
-- lava, bombs (crusted over lava blobs), ashflows, and tephra (airborne ash and
dust).
• A volcano is most commonly a conical hill or mountain built around a vent that
connects with reservoirs of molten rock below the surface of the Earth.
• The term volcano also refers to the opening or vent through which the molten rock
and associated gases are expelled.
Driven by buoyancy and gas pressure the molten rock, which is lighter than the
surrounding solid rock, forces its way upward and my ultimately break through zones
of weaknesses in the Earth's crust.
• If so, an eruption begins, and the molten rock may pour from the vent as nonexplosive
lava flows, or it may shoot violently into the air as dense clouds of lava fragments.
• Larger fragments fall back around the vent, and accumulations of fallback fragments may
move downslope as ash flows under the force of gravity.
• Some of the finer ejected materials may be carried by the wind only to fall to the ground
many miles away.
• The finest ash particles may be injected miles into the atmosphere and carried many times
around the world by stratospheric winds before settling out.
Volcano Terms:
• volcanic dust: finest (dust-sized) particles blown into air by a volcanic eruption
• ash: unconsolidated volcanic debris, less than 4 mm in diameter, physically blown out of
a volcano during an eruption
• cinder: small volcanic bomb the size of a golf ball
• TEPHRA: any material ejected and physically blown out of a volcano; mostly ash.
•
IGNEOUS PROCESSES & VOLCANIC HAZARDS
UNDER WHAT CIRCUMSTANCES CAN MAGMA FORM?
OBVIOUS ANSWER is one way: ROCK BECAME UNUSUALLY HOT & MELTED.
—WORKS FOR 1/3 - 1/2 OF VOLCANOES (ISLAND ARCS, CONTINENTAL RHYOLITES.
—NEED ANOTHER EXPLANATION ALSO FOR REST (HAWAII, MID-OCEAN RIDGES, ETC.
ROCK ———————> MELT
LESS VOLUME
Surface
Depth
GREATER VOLUME (expands)
Temp. Increases ——>
rock
Phase diagram: shows
P, T relations between
solid and melted rock.
melt
LIQUID
rock
Pressure
Increases
melt
SOLID
rock
melt
Geothermal Gradient: Temp. rises 22°C for every kilometer in depth
Ice Analogy: squeeze ice with skate blade—> melts.
Ice’s behavior is opposite to rock behavior.
If take pressure off rock, it melts!
Ice
Squeeze—————>
Less dense stuff
Water
More dense stuff
Suppose have plastic rock—can flow. (~ 1”/year is enough)
Less dense HOT rock below; denser rock above
Are rocks good insulators or poor?
So, rising package stays HOT.
Gravity
T ——>
This is
the
way
most
magma
forms!
Constant T> arrow stays vertical
As body rises, P << so arrow is >>
Depth
P
magma
solid
1. Starts off hot enough
2. Stays hot as it rises
3. Rises far enough that it melts!
Index maps show locations of active and potentially active volcanoes and nearby population
centers (not all labeled). (From Wright, T. L., and Pierson, T. C. 1992. U.S. Geological Survey
Circular 1073)
The word "volcano" comes from the little island of Vulcano in the Mediterranean Sea off Sicily. Centuries ago, the
people living in this area believed that Vulcano was the chimney of the forge of Vulcan -- the blacksmith of the Roman
gods. They thought that the hot lava fragments and clouds of dust erupting form Vulcano came from Vulcan's forge as he
beat out thunderbolts for Jupiter, king of the gods, and weapons for Mars, the god of war. In Polynesia the people attributed
eruptive activity to the beautiful but wrathful Pele, Goddess of Volcanoes, whenever she was angry or spiteful. Today we
know that volcanic eruptions are not super-natural but can be studied and interpreted by scientists.
Volcanoes are mountains, but they are very different from other mountains; they are
not formed by folding and crumpling or by uplift and erosion. Instead, volcanoes are
built by the accumulation of their own eruptive products -- lava, bombs
(crusted over lava blobs), ashflows, and tephra (airborne ash and dust). A
volcano is most commonly a conical hill or mountain built around a vent that
connects with reservoirs of molten rock below the surface of the Earth. The term
volcano also refers to the opening or vent through which the molten rock and
associated gases are expelled.
Driven by buoyancy and gas pressure the molten rock, which is lighter than
the surrounding solid rock, forces its way upward and my ultimately break
through zones of weaknesses in the Earth's crust. If so, an eruption begins, and
the molten rock may pour from the vent as nonexplosive lava flows, or it may
shoot violently into the air as dense clouds of lava fragments. Larger fragments
fall back around the vent, and accumulations of fallback fragments may move
downslope as ash flows under the force of gravity.
Some of the finer ejected materials may be carried by the wind only to fall to the
ground many miles away. The finest ash particles may be injected miles into the
atmosphere and carried many times around the world by stratospheric winds
before settling out.
Volcano Types Island-Arc, Oceanic, and Continental Volcanoes
Some volcanoes crown island areas lying near the continents, and
others form chains of islands in the deep ocean basins.
Volcanoes tend to cluster along narrow mountainous belts where
folding and fracturing of the rocks provide channel-ways to the
surface for the escape of the magma.
Significantly, major earthquakes also occur along these belts,
indicating that volcanism and seismic activity are often closely related,
responding to the same dynamic Earth forces. --
Excerpt from: Tilling, 1985,
Volcanoes: USGS General Interest Publication
lie along the crest of an arcuate, crustal
ridge bounded on its convex side by a deep
oceanic trench.
The granitelike layer/ continental crust goes
beneath the ridge to the vicinity of the
trench.
Basaltic magmas, generated in the mantle
beneath the ridge, rise along fractures
through the granitic layer.
Commonly modified in composition during
passage through the granitic layer and erupt
on the surface to form volcanoes built
largely of non-basaltic rocks.
Augustine Island Volcano, Alaska
-- USGS Photo by Harry Glicken, 1986
Augustine volcano, a postglacial island volcano in lower Cook
Inlet, is part of the eastern Aleutian arc.
In a typical "oceanic" environment,
volcanoes are aligned along the crest of a
broad ridge that marks an active fracture
system in the oceanic crust.
Basaltic magmas, generated in the upper
mantle beneath the ridge, rise along fractures
through the basaltic layer.
Because the granitic crustal layer is absent, the
magmas are not appreciably modified or
changed in composition and they erupt on the
surface to form basaltic volcanoes
Mauna Loa Shield Volcano, Hawaii
Mauna Loa is considered the world's largest active volcano, with an estimated
volume of around 40,000 cubic kilometers, based on the assumption that the
volcano extends from the surface to the top of the isostatically depressed
oceanic crust.
In the typical "continental"
environment, volcanoes are located
in unstable, mountainous belts that
have thick roots of granite or
granite-like rock. Magmas,
generated near the base of the
mountain root, rise slowly or
intermittently along fractures in the
crust.
During passage through the granite
layer, magmas are commonly
modified or changed in
composition and erupt on the
surface to form volcanoes
constructed of nonbasaltic rocks.
Mount Rainier, Washington, from Paradise Area
-- USGS Photo by Lyn Topinka, 1975
In the Pacific Northwest, the Juan de Fuca Plate plunges beneath
the North American Plate. As the denser plate of oceanic crust is
forced deep into the Earth's interior beneath the continental plate, a
process known as subduction, it encounters high temperatures and
pressures that partially melt solid rock. Some of this newly formed
magma rises toward the Earth's surface to erupt, forming a chain of
volcanoes (the Cascade Range) above the subduction zone
Caldera: Large depression commonly formed by collapse of the ground following
explosive eruption of a large body of stored magma. Calderas at Yellowstone and
Long Valley are associated with eruption of silicic magma as pyroclastic flows that
covered large areas around and within the caldera. Kilauea caldera, by contrast, is
thought to be associated with draining of basaltic magma from beneath Kilauea's
summit. The caldera now filled by Oregon's Crater Lake was produced by an
eruption that destroyed a volcano the size of Mount St. Helens and sent volcanic
ash as far east as Nebraska.
From: Brantley, 1994, Volcanoes of the United States: USGS General Interest Publication
The largest and most explosive volcanic eruptions eject tens to hundreds of cubic kilometers of
magma onto the Earth's surface. When such a large volume of magma is removed from beneath a
volcano, the ground subsides or collapses into the emptied space, to form a huge depression called a
caldera. Some calderas are more than 25 kilometers in diameter and several kilometers deep.
Calderas are among the most spectacular and active volcanic features on Earth. Earthquakes,
ground cracks, uplift or subsidence of the ground, and thermal activity such as hot springs, geysers,
and boiling mud pots are common at many calderas. Such activity is caused by complex interactions
among magma stored beneath a caldera, ground water, and the regional buildup of stress in the large
plates of the Earth's crust. Significant changes in the level of activity at some calderas are common;
these new activity levels can be intermittent, lasting for months to years, or persistent over decades
to centuries. Although most caldera unrest does not lead to an eruption, the possibility of violent
explosive eruptions warrants detailed scientific study and monitoring of some active calderas.
Recently, scientists have recognized volcanic unrest at two calderas in the United States, Long
Valley Caldera in eastern California and Yellowstone National Park, Wyoming. Whether unrest at
these calderas simply punctuates long periods of quiet or is the early warning sign of future eruptions
is an important but still unanswered question.
From: Brantley, 1994, Volcanoes of the United States: USGS General Interest Publication
Yellowstone Caldera is one of the largest and most active calderas in the world. The spectacular
geysers, boiling hot springs, and mud pots that have made Yellowstone famous -- and even the
strikingly beautiful Grand Canyon of Yellowstone through which the Yellowstone River plunges -owe their existence to the tremendous volcanic forces that have affected the region during the past
2 million years. Cataclysmic eruptions 2.0, 1.3, and 0.6 million years ago ejected huge volumes
of rhyolite magma; each eruption formed a caldera and extensive layers of thick pyroclastic-flow
deposits. The youngest caldera is an elliptical depression, nearly 80 kilometers long and 50
kilometers wide, that occupies much of Yellowstone National Park. The caldera is buried by several
extensive rhyolite lava flows erupted between 75,000 and 150,000 years ago.
The Earth's crust beneath Yellowstone National Park is still restless. Precise surveys have
detected an area in the center of the caldera that rose by as much as 86 centimeters between 1923
and 1984 and then subsided slightly between 1985 and 1989. Scientists do not know the cause of
these ups and downs but hypothesize that they are related to the addition or withdrawal of magma
beneath the caldera, or to the changing pressure of the hot groundwater system above Yellowstone's
large magma reservoir. Also, Yellowstone National Park and the area immediately west of the
Park are historically among the most seismically active areas in the Rocky Mountains. Smallmagnitude earthquakes are common beneath the entire caldera, but most are located along the
Hebgen
Lake fault zone that extends into the northwest part of the caldera. A magnitude 7.5 earthquake
occurred along this zone in 1959 [Map,20K,InlineGIF].
Compiled From: 1 Smithsonian Institution - Global Volcanism Program Website,
1998, and 2 Wright and Pierson, 1992, Living With Volcanoes, The U. S.
Geological Survey's Volcano Hazards
Program: USGS Circular 1073
Yellowstone Caldera
Location: Wyoming, Montana, Idaho
Latitude: 44.43 N
Longitude: 110.67 W
Height: 2,805 Meters
Type: Calderas
Number of eruptions in past 200 years: 0
Latest Eruptions: 70,000 years ago
Present thermal activity: Numerous hydrothermal activity
Remarks: Numerous hydrothermal explosions, geysers, geothermal activity;
currently restless, shown by seismicity and ground deformation 2
Ash plume from June 12, 1991 Eruption of Mount Pinatubo, Philippines
Among the more prominent theories of events that have triggered global climatic changes and
lead to repeated glaciation are: (1) known astronomical variations in the orbital elements of
the Earth (the so-called Milankovitch theory); (2) changes in energy output from the Sun; and
(3) increases in volcanism that could have thrown more airborne volcanic material into the
stratosphere, thereby creating a dust veil and lowered temperatures.
The June 1991 eruption of Mount Pinatubo was global. Slightly cooler than usual
temperatures recorded worldwide and the brilliant sunsets and sunrises have been
attributed to this eruption that sent fine ash and gases high into the stratosphere,
forming a large volcanic cloud that drifted around the world.
The sulfur dioxide (SO2) in this cloud -- about 22 million tons -- combined with
water to form droplets of sulfuric acid, blocking some of the sunlight from
reaching the Earth and thereby cooling temperatures in some regions by as much
as 0.5 degrees C. An eruption the size of Mount Pinatubo could affect the weather
for a few years.
The renowned Krakatau volcano lies
in the Sunda Strait between Java and
Sumatra. Caldera collapse, perhaps in
416 AD, destroyed the ancestral
Krakatau edifice, forming a 7kilometer-wide caldera. Remnants of
this volcano formed Verlaten and
Lang Islands; subsequently Rakata,
Danan and Perbuwatan volcanoes
were formed, coalescing to create the
pre-1883 Krakatau Island.
Caldera collapse during the
catastrophic 1883 eruption destroyed
Danan and Perbuwatan volcanoes, and
left only a remnant of Rakata volcano.
The post-collapse cone of Anak
Krakatau (Child of Krakatau),
constructed within the 1883 caldera at
a point between the former cones of
Danan and Perbuwatan, has been the
site of frequent eruptions since 1927.
The August 1883 eruption of Krakatau is often cited as a classic example of
caldera formation by collapse following eruption of large volumes of pumice.
The Krakatau edifice grew as one or more stratovolcanoes of dominantly hyperstheneaugite andesite composition. The geology of Krakatau has been described by Effendi
and others (1985,1986), who identified five main evolutionary periods. Period 1 was
an early growth phase that included accumulation of lavas and pyroclastics. Period 2
was marked by caldera formation, accompanied by pyroclastic flows and partly
welded tuffs (ignimbrites). The cones of Rakata, Danan, and Perbuwatan grew during
the third period and were largely destroyed during the fourth period, which included
the paroxysmal eruption of August 1883.
Tsunami, or giant sea wave, generated by the historic 1883 eruption of Krakatau,
Indonesia. The largest wave, which reached heights of 40 meters (140 feet) above
sea level and killed over 34,000 people, stranded this ship 2 1/2 kilometers (1 1/2
miles) inland. example of an eruption-caused tsunami.
A series of tsunamis washed away 165 coastal villages on Java and Sumatra, killing 36,000
people.
... tsunamis are seismic sea waves caused by earthquakes, submarine landslides, and,
infrequently, by eruptions of island volcanoes.