|| A geologic
hazard is a natural geologic event that can endanger human lives
and threaten human property. Earthquakes, geomagnetic storms, landslides,
sinkholes, tsunamis, and volcanoes are all types of geologic hazards.
The U.S. Geological Survey (USGS)
hazard information on earthquakes, landslides, geomagnetics,
and volcanoes, as well as background information on all the types
of hazards described below.
More than 6 feet was added to this fault
scarp by vertical movement in the 1983 Borah Peak, Idaho, earthquake
Credit: U.S. Geological SurveyThe
term "earthquake" refers
to the vibration of the Earth's surface caused by movement along
a fault, by a volcanic eruption, or even by manmade explosions.
The vibration can be violent and cause widespread damage and injury,
or may be barely felt. Most destructive earthquakes are caused
by movements along faults. Earthquakes can occur at the surface
of the Earth or as deep as 400 miles below the surface. An earthquake
can trigger additional hazards such as landslides or tsunamis.
Earthquakes occur all over the world and often occur without significant
warning. These geohazards can have far-reaching affects on humans
and on the surface of the Earth. Small, localized earthquakes may
cause no noticeable damage and may not even be felt by people living
in the affected area. In contrast, a large earthquake may cause
destruction over a wide area and be felt thousands of miles away.
Subsidence at Government Hill School in Anchorage, AK, after the
magnitude 8.5 earthquake of March 28, 1964, Prince William Sound,
Credit: U.S. Geological Survey
is formed of several layers that have very different physical and
chemical properties. The outer layer, which averages about 43 miles
(70 kilometers) in thickness, consists of about a dozen large,
irregularly shaped plates that slide over, under, and past each
other on top of the partly molten inner layer. The plate boundaries
are fault zones, and are where most earthquakes occur. In fact,
the locations of earthquakes and the kinds of ruptures they produce
help scientists define the plate boundaries.
There are three types of plate boundaries: spreading zones, transform
faults, and subduction zones. At spreading
zones, molten rock
rises, pushing two plates apart and adding new material at their
edges. Most spreading zones are found in oceans; for example, the
North American and Eurasian plates are spreading apart along the
mid-Atlantic ridge. Spreading zones usually have earthquakes at
shallow depths (within 19 miles (30 kilometers) of the surface).
Transform faults are found where plates slide past one another.
An example of a transform-fault plate boundary is the San Andreas
fault, along the coast of California and northwestern Mexico.
Earthquakes at transform faults tend to occur at shallow depths
and form fairly straight linear patterns.
A cross section illustrating the main types of plate boundaries.
Illustration by Jose F. Vigil from This Dynamic Planet -- a wall
map produced jointly by the U.S. Geological Survey, the Smithsonian
Institution, and the U.S. Naval Research Laboratory.
Subduction zones are found where one plate overrides, or subducts, another,
pushing it downward into the mantle where it melts. An example
of a subduction-zone plate boundary is found along the northwest
coast of the United States, western Canada, and southern Alaska
and the Aleutian Islands. Subduction zones are characterized by
deep-ocean trenches, shallow to deep earthquakes, and mountain
ranges containing active volcanoes.
Damage in Charleston, SC after the August
31, 1886 earthquake.
Credit: U.S. Geological Survey Earthquakes can also occur within plates, although plate-boundary
earthquakes are much more common. Less than 10 percent of all earthquakes
occur within plate interiors. As plates continue to move and plate
boundaries change over geologic time, weakened boundary regions
become part of the interiors of the plates. These zones of weakness
within the continents can cause earthquakes in response to stresses
that originate at the edges of the plate or in the deeper crust.
Madrid earthquakes of 1811-1812 and the 1886
Charleston earthquake occurred within the North American plate.
Earthquake strength is measured as both magnitude and intensity.
Magnitude measures the relative strength of an earthquake and is
recorded with the Richter
scale. Each earthquake only has one magnitude. People usually
cannot feel earthquakes with magnitudes of 3.0 or less. Intensity
measures the severity of an earthquake in terms of its effect on
Freeway interchange that collapsed in the
1994 M6.9 Northridge, Calif. earthquake.
Credit: U.S. Geological Surveystructures, and the land surface. The USGS usually uses
Mercalli intensity scale to describe earthquake intensity.
The intensity of a given earthquake will vary from place to place.
We tend to picture most earthquake damage as resulting directly
from ground shaking, but there are many other related impacts from
an earthquake. For example, ground shaking can result in soil liquefaction,
damage to dams or levees with resultant flooding, landslides, and
fires caused by ruptured fuel and power lines. In addition, earthquakes
may trigger tsunamis or seiches. Structural damage or collapse
may be caused by any of these effects, which may be local or may
occur hundreds or even thousands of miles from the epicenter of
the earthquake. A Federal Emergency
Management Agency study considered just capital (damages to
buildings and their contents) and income-related costs, and provided
an estimate of $4.4 billion as the minimum average annualized loss
due to earthquakes in the United States.
Most earthquakes in the United States occur in Alaska and California,
although Hawaii, Nevada, Washington, and Idaho also experience
many earthquakes. Four of the five largest
earthquakes in the United States occurred in Alaska. The USGS Earthquake
Hazards Program maintains detailed records on historical earthquakes
and continuously monitors earthquake activity around the world.
While we can't prevent earthquakes or even accurately predict
when they will occur, we can take steps to lessen their human impact.
In the United States and many other countries, building codes take
into account the local earthquake risk so that buildings and other
structures can be designed to withstand all but the most severe
earthquakes. In addition, those who live in earthquake-prone areas
should know how
to be prepared for an earthquake and what
to do if one occurs.
Credit: National Park Service
The Earth has an associated magnetic field, referred to as the
geomagnetic field, which is caused by electric currents both within
the Earth and in the area surrounding the Earth. The geomagnetic
field is what causes a compass to point north. The interaction
of the geomagnetic field with the solar wind is what gives us aurora - the
Northern or Southern Lights.
Electric power line towers.
Credit: U.S. Geological SurveyWhy is geomagnetism considered a geologic hazard? Occasionally
the earth experiences a "magnetic storm", which is a rapid variation
in the geomagnetic field, caused either by a gust in the solar
wind or by a temporary linking of the Sun's magnetic field with
the geomagnetic field. Magnetic storms can disturb long-range radio
communication, degrade global positioning systems, damage satellites,
affect long-distance pipelines, and produce surges on electric
power grids resulting in blackouts. In addition,
Credit: USDA Forest Service magnetic storms
can expose astronauts and high-altitude pilots to increased levels
of radiation. Large magnetic storms can produce dramatic auroral
displays. Variations in the magnetic field don't have a direct
impact on human health, but do affect the technology that is important
to our modern society.
Because the geomagnetic field covers the entire Earth, problems
caused by geomagnetic storms can occur almost anywhere. The National
Oceanic and Atmospheric Administration (NOAA), National
Weather Service, Space
Environment Center (SEC) monitors geomagnetic storm activity
and provides real-time information on their Space
Weather Now site. The SEC has defined five types of solar
radiation storms, ranging from mild to extreme; their definitions
include a description of the possible damaging effects of each
class of magnetic storm.
The USGS National Geomagnetism
Program provides additional information to the public on
the geomagnetic field and geomagnetic hazards.
La Conchita, CA landslide, January 2005.
Credit: U.S. Geological SurveyA
landslide is the movement of soil, rock, or other earth materials,
downhill in response to gravity. Landslides include rock
falls and topples, debris
flows and debris avalanches, earthflows,
mudflows, creep, and lateral
spread of rock or soil.
Frequently landslides occur in areas where the soil is saturated
from heavy rains or snowmelt. They can also be started by earthquakes,
volcanic activity, changes in groundwater, disturbance or change
of a slope by man-made construction activities, or any combination
of these factors. A variety of other natural causes may also result
in landslides, and they may trigger additional hazards, such as
tsunamis caused by submarine landslides. A landslide occurs when
the force that is pulling the slope downward (gravity) exceeds
the strength of the earth materials that compose the slope.
A large boulder demolished a portion of
park housing at Zion National Park. Fortunately, no one was injured.
Credit: National Park Service Rock falls or topples are usually sudden and occur on steep slopes.
In a rock fall, rocks may fall, bounce, or roll down the slope.
A topple occurs when part of a steep slope breaks loose and rotates
A debris flow is a combination of water-saturated loose soil,
rock, organic matter, and air, with material varying in size from
grains of clay to large boulders. Such flows are formed when loose
masses of unconsolidated wet debris become unstable. A lahar is
a special type of debris flow that originates from the slopes of
Debris flow crossing a road.
Credit: U.S. Geological Survey(see
the Volcano section for further information.) Water for a debris
flow may be supplied by rainfall, by melting of snow or ice, or
by overflow of a lake, and the flow may be either hot or cold,
depending on how it starts and the temperature of the constituent
debris. When moving, a debris flow resembles a mass of wet concrete
and tends to flow along channels or stream valleys. It can travel
down a hillside at speeds up to 200 miles per hour (more commonly,
30 to 50 miles per hour), depending on the slope angle, the water
content, and the type of earth and debris in the flow. Burned areas
are particularly susceptible to debris flows. Very rapidly moving
debris flows are known as debris avalanches.
Earthflow on Mission Pass in the California
coastal ranges. The lateral lines on the hillside show creep.
Credit: National Oceanic and Atmospheric
AdministrationEarthflows usually occur on moderate
slopes, and consist of saturated soil or fine-grained rock deposits
that flow downhill. Dry earthflows are also possible. A mudflow
is an earthflow consisting of material that is wet enough to
Creep is the imperceptibly slow, steady, downward movement of
slope-forming soil or rock. Creep can occur seasonally, where movement
is within the depth of soil affected by seasonal changes in soil
moisture and soil temperature, or can be continuous or progressive.
Creep is indicated by curved tree trunks, bent fences or retaining
walls, tilted poles or fences, and small soil ripples or ridges.
Most landslides happen on steep or moderate slopes, but lateral
spreads usually occur on very gentle slopes or in flat terrain.
These spreads are caused by liquefaction, the process whereby saturated,
loose sediments that will not stick together (usually sands and
silts) are transformed from a solid into a liquefied state. Lateral
spread is usually triggered by rapid ground motion, such as that
experienced during an earthquake, but can also be artificially
The combination of two or more types of landslides is known as
a complex landslide.
Landslides constitute a major geologic hazard because they are
widespread, occurring in all 50 States, and because they cause
more than $2 billion in damages and more than 25 fatalities on
average each year. Casualties in the United States are primarily
caused by rockfalls, rock slides, and debris flows. Worldwide,
landslides cause thousands of casualties and billions in monetary
losses annually. The USGS Landslide
Hazards Program collects and distributes information on landslides
to the public, scientists, and civil authorities, and works to
reduce losses and deaths from landslides.
This huge landslide from an unnamed 7,000-foot-high
peak in the Alaska Range, less than 10 miles west of the Trans-Alaska
Oil Pipeline, was triggered by the 2002 Denali Fault earthquake.
Credit: U.S. Geological Survey
The 1983 Thistle landslide at Thistle,
Photo by R.L. Schuster, U.S. Geological Survey
the largest landslides in the world the 20th century occurred at
Mount St. Helens, Washington, in 1980. A moderate earthquake caused
roughly 1.7 cubic miles of rocks and mud to break free and slide
down the side of the volcano, releasing pent-up pressure to produce
the major eruption of May 18. Although this was the largest landslide
recorded in historic time, fewer than 60 people were killed because
most residents and visitors had been evacuated. The most costly
landslide in U.S. history was a relatively slow-moving event in
Thistle, Utah, in the spring of 1983. The landslide, caused by
the wet El Nino winter of 1982-83, dammed the Spanish Fork River
and buried U.S. Highway 6 and the main line of the Denver and Rio
Grande Western Railroad. The town of Thistle was inundated under
the floodwaters rising behind the landslide dam. Total losses were
estimated at more than $400 million in 1983 dollars.
Credit: U.S. Geological SurveySinkholes, like landslides, are a form of ground movement that
can happen suddenly and with little warning, and that can cause
major damage. Sinkholes
are common where the rock below the land surface is limestone,
carbonate rock, salt beds, or rocks that can naturally be dissolved
by ground water circulating through them. As the rock dissolves,
spaces and caverns develop underground. Sinkholes are dramatic
because the land usually stays intact for a while until the underground
spaces just get too big, then a sudden collapse occurs. These collapses
can be small and have little impact on people, or they can be huge
and can occur where a house, road, or other structure is on top.
In the United States, the most damage from sinkholes tends to
occur in Florida, Texas, Alabama, Missouri, Kentucky, Tennessee,
and Pennsylvania, in what is known as karst topography. Sinkholes
are also a problem in many other places around the world.
Sinkhole at Winter Park Florida. This sinkhole occurred in 1981,
in the time span of one day. The city of Winter Park stabilized
and sealed the sinkhole, converting it into an urban lake.
Credit: U.S. Geological Survey
Tsunamis are large, destructive waves that are caused by the sudden
movement of a large area of the sea floor. Tsunamis are often incorrectly
called tidal waves, but unlike regular ocean tides they are not
caused by the gravitational pull of the moon and sun. Most tsunamis
are caused by earthquakes, some are caused by submarine landslides,
a few are caused by submarine volcanic eruptions and on rare occasions
they are caused by a large meteorite impact in the ocean. The December
26, 2004 magnitude 9.0 earthquake near Sumatra produced the
largest trans-oceanic tsunami in over 40 years, and killed more
people than any tsunami in recorded history. The Krakatau volcanic
eruption of 1883 generated giant waves reaching heights of 125
feet above sea level, killing thousands of people and wiping out
numerous coastal villages.
While tsunami means "harbor wave" in Japanese, a tsunami
is actually a series of large waves created by the sudden movement
of the seafloor. The energy generated by the earthquake or other
event is transmitted through the water as a large train of waves,
but the movement of these waves is very different from the movement
of waves generated by wind. NASA's Physics
Behind the Wave explains the structure of tsunamis. Tsunamis
can travel rapidly across oceans, causing destruction far from
the location where they were generated.
All oceanic regions of the world experience tsunamis, although
tsunamis in the Atlantic, Mediterranean, and Caribbean tend to
be smaller and less destructive than those in the Pacific and Indian
Oceans. About 90 percent of recorded tsunamis occur in the Pacific
Ocean. The reasons for this lie in the geologic structure of the
Pacific basin - the ocean is surrounded by a geologically active
series of mountain chains, deep ocean, trenches, and island arcs,
sometimes called "the ring of fire." The earthquakes and volcanic
eruptions that occur in the ring of fire are the source of many
Tsunami damage in Hilo, HI, 1960. The
tsunami was generated by a magnitude 8.6 earthquake near Chile.
Property damage in Hawaii was estimated at 24 million dollars.
Credit: U.S. NavyThe height of a tsunami in the deep ocean is small - usually
about 1 foot - and they cannot be seen or felt by ships at sea.
The distance between wave crests can be more than 100 miles. The
speed at which the tsunami travels decreases as water depth decreases.
In the deep waters of the mid-Pacific, a tsunami can reach a speed
of more than 500 miles per hour, but in the shallow waters near
land the speed drops to 100 miles per hour or less. As tsunamis
reach shallow water the height of the waves increases dramatically,
and can reach 100 feet or more. These huge waves can wash far inland,
carrying large amounts of debris, destroying buildings and other
structures, causing widespread flooding, and dramatically altering
shorelines. Most tsunamis consist of a series of waves, and the
first wave to reach shore may not be the largest.
Locally generated tsunamis may reach a shoreline with only a few
minutes warning, while distant events may allow several hours warning.
Warning signs of an approaching tsunami include a strong earthquake
felt near the shore or a rapid fall in the water level - like a
sudden and extremely low tide. Either of these signs should be
taken as a warning that a tsunami is imminent and that coastal
areas should be immediately evacuated. In addition, many coastal
areas have tsunami alert systems that sound sirens or provide information
through local media. The United States has a tsunami warning system
in place for the west coast, Hawaii, and Alaska. The West
Coast and Alaska Tsunami Warning Center (WCATWC) in Palmer,
Alaska, provides information for Alaska, Washington, Oregon, California,
and British Columbia. WCATWC also provides online tsunami
safety advice. Information for the remaining portions of the
Pacific basin is supplied by the Pacific
Tsunami Warning Center in Hawaii.
Tsunami damage to boats.
Credit: National Oceanic and Atmospheric
AdministrationThe tsunami warning centers issue two types of bulletins to advise
of a possible approaching tsunami. A Tsunami Watch Bulletin is
released when an earthquake occurs with a magnitude of 6.75 or
greater on the Richter scale. A Tsunami Warning Bulletin is released
when information from tidal stations indicates that a potentially
destructive tsunami exists. Tidal stations record information about
the water around them and issue a warning when characteristics
of the sea begin to match those of a potential tsunami.
While we can't prevent tsunamis, we can take steps to lessen
their impact. Those who live in or visit tsunami-prone areas should
know the warning signs of an approaching tsunami, and what to do
when a tsunami is imminent.
is a vent at the Earth's surface through which magma and
associated gases erupt, and also the cone
built by eruptions. A volcano that is currently erupting or showing
signs of unrest (earthquakes, gas emissions) is considered active.
A volcano that is not currently active but which could become active
again is considered dormant. Extinct volcanoes are those considered
unlikely to erupt again.
Mount Saint Helens, Washington, February 2005.
Credit: U.S. Geological Survey, Cascades Volcano Observatory
eruptions are one of Earth's most dramatic and violent agents of
change. They pose significant geologic hazards because their eruptions
and associated activities can affect large areas and go on for
extended periods of time. Many kinds of volcanic activity can endanger
the lives of people and property, and the affects of these activities
are felt both close to and far away from the volcano. Explosive
eruptions can spread lava, gas and other materials over a wide
area, and may drastically alter the landscape. Slow eruptions or
flows can also alter landscapes, while associated earthquakes,
atmospheric effects, landslides, and floods all may damage or destroy
property and threaten human lives.
Credit: U.S. Geological Survey
Some volcanic eruptions are mild and slow, while others are powerful
and dramatic. An eruption happens when magma, gases, or steam break
through vents in the Earth's surface. A mild eruption may simply
discharge steam and other gases, or quietly extrude lava. A strong
eruption can consist of violent explosions that send great clouds
of gas-laden debris into the atmosphere, or may consist of explosions
that blast sideways from a collapsed portion of the volcano, as
happened in the 1980
eruption of Mount St. Helens.
Eruptions can alter the land and water locally through lava flows,
lahars, pyroclastic flows, and landslides. An eruption cloud of
ash and gas may spread the impact of a volcano over many miles
or even around the Earth.
A lava flow moves through an intersection.
Photo by J.D. Griggs, U.S. Geological SurveyA lava
flow is molten rock that has reached the surface of the
Earth. It may flow quickly or slowly, but destroys everything in
its path, including vegetation and manmade structures, and may
bury homes and agricultural land under tens of feet of hardened
black rock. People are rarely able to use land buried by lava flows
or to sell it for more than a small fraction of its previous worth.
Lava flows usually do not travel far from their volcanic source.
Lava entering the sea poses special risks. With temperatures higher
than 2,000 degrees Fahrenheit (1,100 degrees Celsius), lava can
instantly transform seawater to steam, causing explosions that
blast hot rocks, water, and molten lava fragments into the air.
A lava delta, created as lava enters the sea, looks like a stable
platform that extends tens to hundreds of feet into the ocean.
However, the lava delta may not be well supported and can collapse
into the ocean with little or no warning.
A lahar is a mixture of volcanic ash, rock, debris, and water
that can travel quickly down the slopes of a volcano. They are
generated when a high volume of hot or cold water mixes with ash
and rock and starts down slope. The water may come from melting
snow or ice, heavy rainfall during an eruption, or the breakout
of a lake. When moving, a lahar looks like a mass of wet concrete.
As a lahar rushes downstream from a volcano, its size, speed, and
the amount of water and rock debris it carries constantly change.
The beginning surge of water and rock debris often erodes rocks
and vegetation from the side of a volcano and along the river valley
it enters. This initial flow can also incorporate water from melting
snow and ice or from the river it overruns. By eroding rock debris
and incorporating additional water, lahars can easily grow to more
than 10 times their initial size. But as a lahar moves farther
away from a volcano, it will eventually begin to lose its heavy
load of sediment and decrease in size.
A pyroclastic flow is a rapidly-moving
mixture of hot, dry rock fragments, ash, and hot gases which knocks
down, buries, or burns everything in its path. Pyroclastic flows
are caused by explosive eruptions or by the collapse of a lava
flow, can reach temperatures as high as 1,300 degrees Fahrenheit
(700 degrees Celsius), and may melt snow and ice to cause lahars.
These flows vary considerably in size and speed, but even relatively
small flows can destroy buildings, forests, and farmland. Even
on the margins of pyroclastic flows, death and serious injury to
people and animals may result from burns and inhalation of hot
ash and gases.
Volcanic landslides are common and can be caused by an eruption
or associated heavy rainfall, by an earthquake under the volcano,
or by the collapse of a slope weakened by underlying volcanic activity.
A landslide caused by collapse of part of the volcano's cone may
also trigger an eruption as pressure on the underlying volcanic
systems is decreased. Historically, landslides have caused explosive
eruptions, buried river valleys with tens to hundreds of feet of
rock debris, generated lahars, triggered waves and tsunami, and
created deep horseshoe-shaped craters. Moving rapidly and with
great momentum, a large volcanic landslide may flow up and over
ridges, and may cause damage far from the volcano.
Tephra is fragments of volcanic rock and
lava that are blasted into the air by explosions or carried upward
by hot gases in eruption columns or lava fountains. These fragments
may be as small as ash or as large as several feet in diameter.
Tephra includes combinations of pumice, glass
shards, crystals from different types of minerals, and shattered
rocks. Large tephra typically falls back to the ground near the
volcano while smaller fragments are carried away by wind. Volcanic
ash, the smallest tephra fragments, can travel hundreds to thousands
of miles downwind from a volcano. Ash usually covers a much larger
area and disrupts the lives of far more people than the other more
lethal types of volcano hazards. Ash fall may injure livestock
and crops, collapse buildings, damage communications and power-supply
facilities, cause driving and visibility problems, damage or disable
aircraft, and cause respiratory and eye irritation problems in
Trees being killed by high carbon dioxide
concentrations near Mammoth Mountain, CA.
Credit: U.S. Geological SurveyMagma contains dissolved gases that are released into the atmosphere
during eruptions, primarily as acid aerosols (tiny acid droplets),
compounds attached to tephra particles, and microscopic salt particles.
Volcanic gases may also escape continuously into the atmosphere
from the soil, volcanic vents, fumaroles,
and hydrothermal systems. The volcanic gases that pose the greatest
potential hazard to people, animals, agriculture, and property
are sulfur dioxide, carbon dioxide, and hydrogen fluoride. Sulfur
dioxide gas can lead to acid rain and air pollution downwind from
a volcano, and large amounts may lead to lower surface temperatures
and promote depletion of the Earth's ozone layer. Concentrations
of carbon dioxide gas can be lethal to people, animals, and vegetation,
while hydrogen fluoride can contribute to acid rain and is a powerful
irritant that can deform or kill animals.
Scientists monitor active volcanoes and try to anticipate when
an eruption will occur. Volcano monitoring methods detect and measure
changes in the state of a volcano caused by magma movement beneath
the volcano. Rising magma typically will trigger numerous earthquakes,
cause swelling or subsidence of a volcano's summit or flanks, and
lead to the release of volcanic gases from the ground and vents.
In the United States, the USGS Volcano Hazards Program has established
a series of volcano
warning schemes that are used to notify the public and civil
authorities of impending volcanic activity or eruptions.
Volcanic activity since 1700 has killed more than 260,000 people,
destroyed entire cities and forests, and severely disrupted local
economies for months to years. Even with our improved ability to
identify hazardous areas and warn of impending eruptions, increasing
numbers of people face certain danger. Scientists face a formidable
challenge in providing reliable and timely warnings of eruptions
to so many people at risk.
The Wahaula Visitor Center in Hawaii Volcanoes National Park was
overrun by lava flows in June 1989.
Photo by J.D. Griggs, U.S. Geological Survey
The USGS Volcano Hazards
Program provides current
updates of worldwide volcanic activity and additional information
on various types of volcanic hazards.
Redoubt Volcano, Alaska, during a continuous, low-level eruption
of steam and ash, December 18, 1989.
Photo by W. White, U.S. Geological Survey
||Glossary of Terms
Creep—The slow, steady, downward movement
of slope-forming soil or rock.
Crust—The thin, solid, outermost layer
of the Earth.
Debris avalanche—A very rapidly moving
Debris flow—A type of landslide made up of
a mixture of water-saturated loose soil, rock, organic matter,
and air, with a consistency similar to wet cement. Debris flows
move rapidly downslope under the influence of gravity. Sometimes
referred to as earthflows or mudflows.
Earthflow—See debris flow.
Earthquake—A sudden ground
motion or vibration of the Earth, produced by a rapid release of
stored-up energy. Includes sudden slip on a fault and the resulting
ground shaking and radiated seismic energy caused by the slip,
or motion caused by volcanic activity or by other sudden stress
changes in the earth.
Epicenter—The point on the Earth's surface
located directly above the focus of an earthquake.
Eruption—When solid, liquid, or gaseous
volcanic materials are ejected into the Earth's atmosphere or surface
by volcanic activity. Eruptions may occur as quiet lava flows or
violent explosive events.
Fault—A fracture in the Earth
along which one side has moved in relative to the other.
Focus—The location where an earthquake begins.
Fumarole—Vents from which volcanic gas
escapes into the atmosphere.
Karst—A distinctive landscape that develops
where the underlying bedrock is partially dissolved by surface
or ground water.
Lahar—A type of mudflow that originates on
the slopes of volcanoes when volcanic ash and debris becomes saturated
with water and flows rapidly downslope.
Landslide—The downslope movement of rock,
soil, or mud.
Lateral spread—A landslide on a gentle slope,
with rapid, fluid-like movement.
Lava—Molten rock that has reached the Earth's
Magma—Molten or partially molten rock beneath
the Earth's surface.
Mantle—The part of the Earth below the crust.
The uppermost layer of the mantle is solid, while the layers below
are partially molten.
Mercalli intensity scale—A measure of earthquake
i intensity based on the effect of the earthquake on buildings
and on the reactions of people. Intensity levels range from
not felt (I) to total destruction (XII). Generally the larger
the earthquake, the larger the area affected and the higher
the maximum intensity.
Molten—Liquefied by heat.
Mudflow—See debris flow.
Plates—Thick, moving slabs of rock composed
of crust and the uppermost layer of the under lying mantle.
Pumice—A light-colored, frothy, glassy volcanic
rock. The texture is formed by rapidly expanding gas in erupting
Pyroclastic flow—An extremely
hot mixture of gas, ash and pumice fragments, that travels down
the flanks of a volcano or along the surface of the ground at speeds
of 50 to 100 miles (80 to 160 kilometers) per hour.
magnitude scale—A measure of an earthquake's
size. It describes the total amount of energy released during
an earthquake. In the 1930's, C.F. Richter devised a way measure
the magnitude of an earthquake using an instrument called a
seismograph to measure the speed of ground motion during an
earthquake. Geologists discovered that the energy released
in an earthquake goes up with magnitude faster than the ground
speed by a factor of 32.
Rock fall—Falling, bouncing, or rolling of
debris down a steep slope
Seiche—The sloshing of a closed body of water
as a result of an earthquake.
Siesmic—Referring to earthquakes.
Soil liquefaction—A process by which water-saturated
soil temporarily loses strength and acts as a fluid.
Solar wind—The outward flux of solar particles
and magnetic fields from the sun. Typically, solar wind velocities
are near 215 miles/second (350 kilometers/second).
zone—Also called a divergent plate boundary.
An area where two plates are moving away from each other and
new crust is being formed.
zone—Also called a convergent plate boundary. An
area where two plates meet and one is pulled beneath the other.
Tephra—Material ejected into the air during
a volcanic eruption. The particles can be as small as volcanic
ash or as large as boulders and blocks, tens of feet in diameter
Topple—A landslide where part of a steep slope
breaks loose and falls forward.
Transform fault—Also called a transform plate
boundary. An area where two plates meet and are moving side to
side past each other.
Tsunami—A large wave series of waves that are
caused by a sudden disturbance that displaces water. The usual
cause is an earthquake, submarine landslide, volcanic eruption,
or meteor impact.
Vent—An opening in the Earth's crust through
which lava, gases, ash, or rock fragments are erupted.
Volcano—A vent at the Earth's surface through
which magma and associated gases erupt, and also the cone built