Frequently Asked Questions About Volcanoes
and Volcanologists
Q: Can scientists really anticipate volcanic eruptions?
- A: Yes, in many cases, but most reliably only for volcanoes
that have been studied geologically. By studying deposits of rock
and ash formed by past events, volcanologists can reconstruct the
history of a volcano in considerable detail. This allows them to
make general forecasts about future activity, because the past is
often, though not always, a good guide to the future. For example,
USGS scientists who studied Mount St. Helens during the 1970's
recognized that, for thousands of years, it had been the most
active and explosive volcano in the entire Cascade Range. On that
basis, they forecast that Mount St. Helens might be the next
Cascade volcano to erupt, possibly before the year 2000. In 1980,
their detective work paid off when the volcano erupted for the
first time in 123 years.
Q: Are specific predictions of an eruption's time, place, and
character possible?
- A: In some cases the answer is yes, but specific predictions
require more and different kinds of information. Using
seismometers and other sensitive monitoring instruments, USGS
scientists are keeping an eye on more than three dozen dangerous
volcanoes in the western United States. At the first sign of
trouble they'll intensify their monitoring efforts. Taking the
pulse of a restless volcano in this way allows scientists to
refine their assessment of hazards and make increasingly specific
statements about future activity, including the time, location,
and type of activity expected.
For example, USGS scientists correctly predicted days in
advance more than a dozen dome-building eruptions at Mount St.
Helens during 1980-1986. In 1991, an accurate prediction of the
largest eruption on Earth in almost 80 years saved thousands of
lives and millions of dollars worth of property near Mount
Pinatubo in the Philippines.
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Q: How does the USGS provide eruption warnings?
- A: The USGS warning system varies depending on the nature and
proximity of volcanic hazards to surrounding communities or
aircraft. Before a crisis starts, we provide hazards-zone maps and
other information about the frequency of eruptions and extent of
specific hazards to public officials, land-use planners, and
emergency-management agencies. The USGS works with the Federal
Aviation Administration and National Weather Service to provide
airline pilots with timely information about hazardous volcanic
ash clouds.
When communities are at risk, scientists give hazards
information directly to public officials to help them make
decisions about land-use or evacuations. Unlike the movie,
warnings are delivered only after a thorough analysis of all
existing information and careful consultation among members of the
USGS response team. Our goal is always to keep natural processes
from becoming natural disasters.
Q: How many active volcanoes are there in the United States?
- A: There are about 65 volcanoes in the United States that
scientists consider active. Most of these are located in Alaska,
where eruptions occur virtually every year. The others are located
in the Cascade Range (Washington, Oregon, northern California), or
in Hawaii on the islands of Hawaii and Maui. Kilauea volcano in
Hawaii is one of the most active volcanoes on Earth. It has been
erupting almost continuously since 1983!
Q: How many active volcanoes are there on Earth?
- A: There are about 1500 potentially active volcanoes
worldwide, aside from the continuous belt of volcanoes on the
ocean floor. About 500 of these have erupted in historical time.
Many of these are located along the Pacific Rim in what is known
as the "Ring of Fire." In the U.S., volcanoes in the Cascade Range
and Alaska (Aleutian volcanic chain) are part of the Ring, while
Hawaiian volcanoes form over a "hot spot" near the center of the
Ring.
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QUESTIONS ABOUT ERUPTION PRECURSORS
Before a volcano erupts, magma must force its way upward through
solid rock beneath a restless volcano. This process causes the ground
above to heave and shake as rock is shoved aside or broken. At the
same time, gases are released from the magma as it rises to shallower
levels where the pressure is lower. These phenomena--ground
movements, earthquakes, and changes in volcanic gases--provide the
clues that scientists use to recognize a restless volcano and
anticipate what might happen next.
Q: What kinds of unusual activity might be noticed before an
eruption?
- A: Common symptoms of volcanic unrest include an increase in
the frequency or intensity of earthquakes beneath a volcano; the
occurrence of volcanic tremor; swelling, subsidence, or cracking
of the ground; increased steam emission or small steam explosions;
melting snow or ice; changes in existing fumaroles or hot springs,
or the appearance of new ones; and increased discharge of magmatic
gases. Volcanologists assess the significance of volcanic unrest
partly by monitoring the pace and intensity of such activity.
Q: What is volcanic tremor, and how does it differ from
earthquakes?
- A: Tremor is a seismic vibration, similar to a volcanic
earthquake, but of longer duration and more continuous than
earthquakes of the same amplitude. Volcanic tremor can last from
minutes to days. It may be caused by magma moving through narrow
cracks, boiling and pulsation of pressurized fluids within the
volcano, or escape of pressurized steam and gases from fumaroles.
Q: Do volcanoes produce different kinds of earthquakes?
- A: Yes. A variety of earthquake types can occur at a volcano
that is getting ready to erupt. These include earthquakes caused
by rocks breaking along faults or fractures, termed tectonic-type
earthquakes. Another common type a long-period or volcanic
earthquake. These can occur when bubble-filled magma is on the
move beneath a volcano. The differences between tectonic- type and
volcanic-type earthquakes are so subtle that they can be
distinguished only by using seismometers.
Q: What kind of gases escape from volcanoes?
- A: The fumes escaping from a volcano consist mostly of water
vapor (steam). Steam may be emitted from the hot interiors of
volcanoes even when they are dormant. But steaming usually
increases dramatically as magma intrudes and heats groundwater
beneath a volcano. Magma gives off carbon dioxide (CO2)
and hydrogen sulfide (H2S, rotten egg gas) that do not
totally dissolve in groundwater and can therefore show up at the
surface. As water inside the volcano boils away, other more
water-soluble volcanic gases can reach the surface, signaling an
increasingly grave situation. These gases include sulfur dioxide
(SO2) and common halogen gases such as hydrogen
chloride (HCl), and hydrogen fluoride (HF).
Q: Are there other restless volcanoes in the United States
today besides Long Valley Caldera?
- A: Several. An increase in earthquake activity has been
observed at Iliamna Volcano in the Cook Inlet region of Alaska
since May, 1996. Airborne surveys in August and October 1996
showed increased CO2 emissions from the volcano. Modest
increases in sulfur dioxide were also observed. The USGS is
continuing to monitor Iliamna's activity with the University of
Alaska Fairbanks Geophysical Institute and the Alaska Division of
Geological and Geophysical Surveys in a cooperative program at the
Alaska Volcano Observatory.
Also in Alaska, the Katmai region, which produced the Valley of
Ten Thousand Smokes during a colossal eruption in 1912, has
experienced swarms of small earthquakes in recent years. And
Pavlof volcano has been erupting sporadically since September 1996
and remains restless.
In Hawaii, the largest volcanic edifice on Earth, Mauna Loa,
has been slowly swelling and producing earthquakes since its last
eruption in 1984.
At Yellowstone in northwest Wyoming, one of the largest active
volcanic systems on Earth, spectacular hydrothermal activity
(geysers, hot springs, mud volcanoes), frequent earthquakes, and
large ground movements remind us constantly that future eruptions
are likely. The likelihood of an eruption at Yellowstone anytime
soon, though, is much lower than at many other volcanoes in the
western United States.
QUESTIONS ABOUT VOLCANO MONITORING
To anticipate the awakening or reawakening of a volcano,
volcanologists watch for changes caused by moving or pressurizing
magma and associated changes in the hydrothermal system surrounding
the magma. Magma moving toward the surface can cause swarms of
earthquakes; swelling, subsidence, or cracking of the volcano's
flanks; and changes in the amount or types of gases that are emitted
from a volcano. The USGS continuously monitors many volcanoes in the
states of Washington, Oregon, California, Hawaii, Alaska, and Wyoming
(Yellowstone) to detect unusual activity.
Q: Does the USGS have a team of volcanologists that can respond
to volcanic unrest on short notice?
- A: Yes. The USGS Volcano Hazards Team includes experts in all
aspects of volcano hazard assessment, monitoring, information
dissemination, and volcano-emergency response. As depicted in the
movie, a group of USGS scientists will respond to any potentially
hazardous volcanic activity in the United States.
Q: Does the USGS have a team for rapid response to volcano
emergencies abroad?
- A: Yes. Such a team is operated by the U.S. Geological Survey
as part of the Volcano Disaster Assistance Program (VDAP). The
team was formed in cooperation with the U.S. Office of Foreign
Disaster Assistance (OFDA) of the U.S. State Department following
the 1985 eruption of Nevado del Ruiz Volcano, Colombia, in which
over 23,000 people lost their lives. At the request of host
countries and working through OFDA, VDAP scientists quickly
determine the nature of volcanic unrest and assess its possible
consequences. VDAP has responded to volcano emergencies in more
than a dozen countries during the past decade.
In addition to helping people in other countries to get out of
harm's way, VDAP's international work directly benefits
volcano-hazard mitigation in the United States. Through VDAP, we
gain experience at active volcanoes that will help during future
crises in the western United States, and we collect important
scientific data on eruption precursors that are used to better
understand how U.S. volcanoes work.
Q: How does the USGS monitor volcanoes in the United States?
- A: One of the earliest signs of an impending eruption is often
a subtle change in seismic activity beneath a volcano. In
cooperation with universities and state agencies, the USGS
monitors seismic activity near volcanoes using networks of
seismometers. When unusual activity is detected, more seismometers
and other instruments may be installed by a response team to
better determine if an eruption is likely.
Q: How are earthquakes monitored?
- A: By installing seismometers that send information
continuously via radio to a central recording site (observatory),
scientists can determine the sizes and locations of earthquakes
near a volcano. They look for specific types of earthquakes that
are often associated with volcanic activity, including long-
period volcanic earthquakes and volcanic tremor. An increase in
the number or size of earthquakes beneath a volcano usually means
that an eruption is more likely.
Q: How are ground movements measured?
- A: Ground deformation (swelling, subsidence, or cracking) is
measured with a variety of techniques, including Electronic
Distance Meters (EDM), the Global Positioning System (GPS),
precise leveling surveys, strainmeters, and tiltmeters. EDMs use
lasers to accurately measure changes in distance between
benchmarks (fixed points) with repeated measurements. GPS makes
use of satellites orbiting the Earth to determine and track the
locations of points. Strainmeters and tiltmeters are used to
monitor subtle changes in shape of the ground surface.
Q: How are volcanic gases measured?
- A: Instruments to measure sulfur dioxide and carbon dioxide
can be mounted in aircraft to determine the quantity of gas being
emitted on a daily basis. Such instruments can also be used in a
ground-based mode. An instrument that detects carbon dioxide can
be installed on a volcano and configured to send data continuously
via radio to an observatory. Sulfur dioxide in volcanic clouds can
also be measured from space with instruments aboard satellites.
Q: What else do scientists measure at volcanoes?
- A: Field observations by experienced volcanologists go hand in
hand with more sophisticated equipment and techniques to form a
complete system for monitoring volcanoes. Field observations may
include water temperature and pH (acidity) measurements, or
observations of ground cracking and new areas of avalanching
rocks. An experienced observer can integrate many different types
of data on the spot and design simple measurements to further
assess the significance of volcanic unrest. There is no substitute
for well-trained, experienced observers when trying to figure out
how a volcano will behave.
QUESTIONS ABOUT VOLCANO HAZARDS
A restless volcano endangers any nearby residents with clouds of
ash, falling blocks of rock, pyroclastic flows or ash hurricanes,
lava flows, and floods of debris or lahars. These hazards are typical
of snow- and ice-covered stratovolcanoes like those in the Pacific
Northwest and Alaska. Since 1980, volcanic activity has killed more
than 29,000 people worldwide. Most of the deaths were caused by
lahars and pyroclastic flows; a few hundred people were killed by ash
falls, which collapsed the roofs of buildings.
Q: What kinds of hazards are associated with volcanic
eruptions?
- A: Debris flows, or lahars, are slurries of muddy debris and
water caused by mixing of solid debris with water, melted snow, or
ice. Lahars destroyed houses, bridges, and logging trucks during
the May 1980 eruption of Mount St. Helens, and have inundated
other valleys around Cascade volcanoes during prehistoric
eruptions. Lahars at Nevado del Ruiz volcano, Colombia, in 1985,
killed more than 23,000 people. At Mount Rainier, lahars have also
been produced by major landslides that apparently were neither
triggered nor accompanied by eruptive activity. Lahars can travel
many tens of miles in a period of hours, destroying everything in
their paths.
Tephra (ash and coarser debris) is composed of fragments of
magma or rock blown apart by gas expansion. Tephra can cause roofs
to collapse, endanger people with respiratory problems, and damage
machinery. Tephra can clog machinery, severely damage aircraft,
cause respiratory problems, and short out power lines up to
hundreds of miles downwind of eruptions. Explosions may also throw
large rocks up to a few miles. Falling blocks killed people at
Galeras Volcano in Colombia in 1992, and at Mount Etna, Italy, in
1979.
Pyroclastic surges and flows are hot, turbulent clouds of
tephra (known as surges), or dense, turbulent mixtures of tephra
and gas (known as flows). Pyroclastic flows and surges can travel
more than a hundred miles per hour and incinerate or crush most
objects in their path. Though most extend only a few miles, a
pyroclastic surge at Mount St. Helens in 1980 extended 18 miles
(28 km) and killed 57 people. Pyroclastic surges at El
Chichón volcano in Mexico in 1982 killed 2000 people, and
pyroclastic flows at Mount Unzen, Japan, in June, 1991, killed 43
people. Speeding vehicles cannot outrun a pyroclastic flow or
surge.
Lava flows erupted at explosive stratovolcanoes like those in
the Pacific Northwest and Alaska are typically slow-moving, thick,
viscous flows. Kilauea volcano on the Island of Hawaii has
produced thin, fluid lava flows throughout its history, and almost
continuously since 1983. Lava flows destroyed a visitor center at
Kilauea in 1989 and overran the village of Kalapana on the
volcano's southeast flank in 1991.
Q: Can volcanoes be dangerous even when they don't erupt?
- A: Definitely. Many stratovolcanoes have a plumbing system of
hot acid water that progressively breaks down hard rock to soft,
clay-rich material. The volcano is gradually weakened, and large
parts may suddenly fail. Resulting water-rich landslides are
especially dangerous because they can occur without any volcanic
or seismic warning.
The risk of mudflows formed this way is especially high along
rivers downstream from Mount Rainier, because of the large
population on floodplains, the huge weakened edifice of the
volcano, and a long history of large flows that occurred when the
volcano was otherwise dormant.
Q: How can residents who live near volcanoes prepare for future
eruptions?
- A: Residents can obtain copies of USGS volcano-hazard reports
to determine whether they live or work in areas at risk from
volcanic activity. Everyone should plan how they and their family
will respond to a natural disaster, including unrest or eruptive
activity at nearby volcanoes. Preparation might include knowing
where to go when family members are separated, where to go for
emergency housing, what emergency supplies to keep on hand, and
how to be self sufficient for several days, as recommended by
local emergency management agencies. Residents who live within 100
miles of a volcano should also find out what their local officials
are doing to prepare their community for the possibility of
renewed volcanic activity. Lastly, enjoy the scenic, recreational,
and inspirational benefits of living near an active volcano!
MOVIE FACT OR FICTION?
Dante's Peak, a volcano-disaster thriller from Universal Studios,
available on videocassette, dramatizes some real-world concerns faced
by communities located near active volcanoes in the United States.
Set in the northern Cascade Range of Washington State, the movie
portrays the roles of U.S. Geological Survey (USGS) scientists and
local public officials during the reawakening and eruption of a
fictional volcano - one that resembles dozens of real volcanoes in
Alaska, British Columbia, Washington, Oregon, and northern
California. To separate fact from fiction, here are answers to some
frequently asked questions about the movie and the USGS mission to
reduce the risk from dangerous volcanoes.
Q: Is the eruption depicted in Dante's Peak realistic?
- A: In many but not all respects, the movie's depiction of
eruptive hazards hits close to the mark, especially as regards the
enormous power unleashed during an eruption. Stratovolcanoes in
the Cascade Range and Alaska erupt explosively and produce
pyroclastic flows, clouds of volcanic ash, and debris flows
(lahars) that behave much as shown in the movie. Lava flows at
these volcanoes, though, are usually thick and slow moving, unlike
the fluid flows in the movie. Fast-flowing flows of basalt lava
are common in Hawaii, though. Real eruptions may be considerably
larger or smaller, and affect larger or smaller areas, than those
shown in the film.
Q: Can eruptions really threaten helicopters, as in the movie,
and other aircraft?
- A: Yes. Encounters between aircraft and clouds of volcanic ash
are a serious concern. Jet engines and other aircraft components
are vulnerable to damage by fine, abrasive volcanic ash, which can
drift in dangerous concentrations hundreds of miles downwind from
an erupting volcano.
During the past 15 years, at least 80 aircraft have
accidentally encountered volcanic ash clouds, and in 6 cases jet
engines temporarily lost power. An international consortium of
government agencies, including the U.S. Geological Survey, Federal
Aviation Administration, and National Weather Service, is now
monitoring ash-producing volcanoes and tracking volcanic ash
clouds to reduce the likelihood of future encounters.
Q: Can the temperature of hot springs near a restless volcano
change quickly enough to injure bathers?
- A: Temperature changes can and do occur, but usually more
slowly than shown in the movie. In fact, the temperature of hot
springs may increase, decrease, or stay the same during volcanic
unrest. Increases in water temperature, when they do occur,
usually take days or weeks to develop, rather than a few seconds
as shown in the movie.
In rare cases, earthquakes can suddenly disrupt a volcano's hot
groundwater system, changing its temperature. And earthquakes have
been known to temporarily increase the flow of water from hot
springs, sometimes causing geyser-like activity that could
threaten bathers.
Q: Do earthquakes large enough to collapse buildings and roads
accompany volcanic eruptions?
- A: Not usually. Earthquakes associated with eruptions rarely
exceed magnitude 5, and these moderate earthquakes are not big
enough to destroy the kinds of buildings, houses, and roads that
were demolished in the movie. The largest earthquakes at Mount St.
Helens in 1980 were magnitude 5, large enough to sway trees and
damage buildings, but not destroy them. During the huge eruption
of Mount Pinatubo in the Philippines in 1991, dozens of light to
moderate earthquakes (magnitude 3 to 5) were felt by several
hundred thousand people. Many houses collapsed, but not primarily
because of the shaking. Heavy, wet ash from the eruption and a
hurricane accumulated on roofs and crushed them.
Stronger earthquakes sometimes DO occur near volcanoes as a
result of tectonic faulting. For example, four magnitude 6
earthquakes struck Long Valley caldera, California, in 1980, and a
magnitude 7.2 earthquake struck Kilauea Volcano, Hawaii, in 1975.
Both volcanoes were quiet at the time. The Hawaii earthquake
triggered a small eruption at the summit of Kilauea. No eruption
has yet occurred at Long Valley, but the area has been restless
since the 1980 earthquakes.
Q: Can a town's water supply become contaminated when a volcano
is restless?
- A: Yes, but probably not as quickly as shown in the movie. If
a town's water supply originates directly from a volcano's
groundwater system or from a stream that has been covered with
volcanic ash, the water could become contaminated with
foul-smelling gases or fine ash and other sediment. Some volcanic
gases such as sulfur dioxide dissolve in groundwater, making the
water acidic. Sulfurous odors, however, are caused by hydrogen
sulfide gas, which smells like rotten eggs.
Q: Do scientists drive across moving lava flows?
- A: No. Any attempt to drive across an active lava flow, even
one that has partly solidified to form a thin crust, is likely to
lead to disaster. With a temperature of 1,700 degrees Fahrenheit
or higher, fresh lava will quickly melt rubber tires and ignite
gas tanks. And if a vehicle gets stuck in moving lava, well, you
know the rest of the story.
Q: Can carbon dioxide gas from volcanoes kill trees and
wildlife?
- A: Yes. At several volcanoes around the world, carbon dioxide
gas released from magma has accumulated in the soil in sufficient
concentrations to kill vegetation or has collected in low areas
and suffocated animals. At Mammoth Mountain in California, carbon
dioxide has killed about 100 acres of trees since 1989, and
visitors to this area have occasionally suffered symptoms of
asphyxiation when entering cabins or below-ground excavations.
USGS scientists have concluded that the gas is escaping from a
magma body beneath Mammoth Mountain. The magma itself is not
currently moving toward the surface, but the USGS is monitoring
the situation carefully.
Q: Can volcanoes suddenly become restless and erupt within one
week of the first signs of activity?
- A: Yes. The first steam eruption at Mount St. Helens on March
27, 1980, was preceded by only 7 days of intense earthquake
activity. The climactic eruption, on May 18, followed seven weeks
later. An eruption of Redoubt Volcano in Alaska on December 13,
1989, was preceded by only 24 hours of intense earthquake
activity. But other volcanoes have been restless for months or
years before an eruption occurred, and sometimes a period of
unrest doesn't produce an eruption at all.
Q: Are robots used by the USGS to monitor volcanoes?
- A: No. We rely on observations and measurements made by
experienced scientists and on critical data sent by radio or
satellite relay from monitoring instruments installed around a
volcano. These instruments include seismometers, tiltmeters,
Global Positioning System (GPS) receivers, gas sensors, mudflow
(lahar or debris flow) sensors, and temperature probes.
NASA has tested a robot named Dante at Mount Erebus volcano in
Antarctica and Mount Spurr volcano in Alaska. The USGS believes
that, on Earth, experienced volcanologists are a better and more
cost-effective alternative for monitoring dangerous volcanoes.
Q: Can volcanoes produce large explosive eruptions and rivers
of fluid lava at the same time?
- A: Not usually. During a single eruption, a volcano CAN
produce both lava flows and ash, sometimes simultaneously. The
red, glowing lava fountains and lava flows in Dante's Peak
(including the active flow across which Harry Dalton drives) are
characteristic of a fluid magma, called basalt. In contrast,
explosive gray ash columns and pyroclastic flows shown in other
scenes are characteristic of more viscous magmas, called andesite,
dacite, or rhyolite. It's uncommon for a volcano to erupt magmas
of widely different composition at the same time.
Q: Can lakes near volcanoes become acidic enough to be
dangerous to people?
- A: Yes. Crater lakes atop volcanoes are typically the most
acid, with pH values as low as 0.1 (very strong acid). Normal lake
waters, in contrast, have relatively neutral pH values near 7.0.
The crater lake at El Chichon volcano in Mexico had a pH of 0.5 in
1983 and Mount Pinatubo's crater lake had a pH of 1.9 in 1992. The
acid waters of these lakes are capable of causing burns to human
skin but are unlikely to dissolve metal quickly. Gases from magma
that dissolve in lake water to form such acidic brews include
carbon dioxide, sulfur dioxide, hydrogen sulfide, hydrogen
chloride, and hydrogen fluoride. However, the movie's rapidly
formed acidic lake capable of dissolving an aluminum boat in a
matter of minutes is unrealistic.
If you want to learn more about volcanoes in other parts of the
United States, visit the other USGS volcano websites:
| Alaska Volcano Observatory
| Cascades Volcano
Observatory
| Hawaiian Volcano Observatory |
| USGS Volcano Hazards Program |
 
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U.S. Department of the Interior, U.S. Geological Survey, Menlo Park, California, USA
URL http://lvo.wr.usgs.gov/FAQs.html
Contact: Long Valley Web Team
Last modification: 14 October 1999 (SRB)