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Landslide Incidence and Susceptibility
Earthquakes 1568 - 2009
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Landslide Incidence and Susceptibility
Earthquakes 1568 - 2009
Geomagnetism: The Magnetic Field of the Earth
Engineering Aspects of Karst
Dynamic Maps
Potentially Active Volcanoes


  Geologic Hazards

downGeomagnetic Storms

downGlossary of Terms

  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) provides real-time hazard information on earthquakes, landslides, geomagnetics, and volcanoes, as well as background information on all the types of hazards described below.
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A fault scarp from the 1983 Borah Peak, Idaho, earthquake
More than 6 feet was added to this fault scarp by vertical movement in the 1983 Borah Peak, Idaho, earthquake (magnitude 7.3).
Credit: U.S. Geological Survey
The 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
Subsidence at Government Hill School in Anchorage, AK, after the magnitude 8.5 earthquake of March 28, 1964, Prince William Sound, Alaska.
Credit: U.S. Geological Survey

The Earth 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.

  Illustration of types of plate boundaries
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.

Earthquake damage in Charleston SC
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. The New 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 humans, Freeway collapse in Northridge, CA
Freeway interchange that collapsed in the 1994 M6.9 Northridge, Calif. earthquake.
Credit: U.S. Geological Survey
structures, and the land surface. The USGS usually uses the Modified 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.

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  Geomagnetic Storms
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.

Power line towers
Electric power line towers.
Credit: U.S. Geological Survey
Why 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,Radio tower
Radio tower.
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.

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Landslide in La Conchita, CA
La Conchita, CA landslide, January 2005.
Credit: U.S. Geological Survey
A 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 which has demolished a portion of a house
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 forward.

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 a volcano Debris flow crossing a road
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 showing creep on the hillside
Earthflow on Mission Pass in the California coastal ranges. The lateral lines on the hillside show creep.
Credit: National Oceanic and Atmospheric Administration
Earthflows 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 flow rapidly.

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 induced.

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.

  A huge landslide from a 7,000-foot-high peak in the Alaska Range
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
Landslide at Thistle, Utah
The 1983 Thistle landslide at Thistle, Utah.
Photo by R.L. Schuster, U.S. Geological Survey
  One of 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.
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Aligned sinkholes
Aligned sinkholes.
Credit: U.S. Geological Survey
Sinkholes, 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.

  A sinkhole at Winter Park Florida
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
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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 tsunamis.

Tsunami damage in Hilo, HI, 1960
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. Navy
The 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
Tsunami damage to boats.
Credit: National Oceanic and Atmospheric Administration
The 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.

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A volcano 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.

  Aerial view of a steaming Mount Saint Helens, Washington, February 2005
Mount Saint Helens, Washington, February 2005.
Credit: U.S. Geological Survey, Cascades Volcano Observatory
  Volcanic 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.
  Illustration of volcano effects
Volcano effects.
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
A lava flow moves through an intersection.
Photo by J.D. Griggs, U.S. Geological Survey
A 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 people.

Trees being killed by carbon dioxide
Trees being killed by high carbon dioxide concentrations near Mammoth Mountain, CA.
Credit: U.S. Geological Survey
Magma 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 engulfed by lava flow and flames
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.

  Aerial view of Redoubt Volcano, Alaska during a low-level eruption of steam and ash
Redoubt Volcano, Alaska, during a continuous, low-level eruption of steam and ash, December 18, 1989.
Photo by W. White, U.S. Geological Survey
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  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.

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 surface.

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.

Modified 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 lava.

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.

Richter 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).

Spreading zone—Also called a divergent plate boundary. An area where two plates are moving away from each other and new crust is being formed.

Subduction 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 by eruptions.

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