Volcanoes are openings, or vents where lava, tephra (small rocks), and steam erupt onto the Earth's surface. Volcanic eruptions can last days, months, or even years.

What is a volcano?

Many mountains form by folding, faulting, uplift, and erosion of the Earth's crust. Volcanic terrain, however, is built by the slow accumulation of erupted lava. The vent may be visible as a small bowl shaped depression at the summit of a cone or shield-shaped mountain. Through a series of cracks within and beneath the volcano, the vent connects to one or more linked storage areas of molten or partially molten rock (magma). This connection to fresh magma allows the volcano to erupt over and over again in the same location. In this way, the volcano grows ever larger, until it is no longer stable. Pieces of the volcano collapse as rock falls or as landslides.

How do volcanoes erupt?

Molten rock below the surface of the Earth that rises in volcanic vents is known as magma, but after it erupts from a volcano it is called lava. Magma is made of molten rock, crystals, and dissolved gas—imagine an unopened bottle of soda with grains of sand inside. The molten rock is made of the chemicals oxygen, silicon, aluminum, iron, magnesium, calcium, sodium, potassium, titanium, and manganese. After cooling, liquid magma may form crystals of various minerals until it becomes completely solid and forms an igneous or magmatic rock.

Originating many tens of miles beneath the ground, magma is lighter than surrounding solid rock. It is driven towards Earth's surface by buoyancy, it is lighter than the surrounding rock, and by pressure from gas within it. Magma forces its way upward and may ultimately break though weak areas in the Earth's crust. If so, an eruption begins.

Magma can be erupted in a variety of ways. Sometimes molten rock simply pours from the vent as fluid lava flows. It can also shoot violently into the air as dense clouds of rock shards (tephra) and gas. Larger fragments fall back around the vent, and clouds of tephra may move down the slope of the volcano under the force of gravity. Ash, tiny pieces of tephra the thickness of a strand of hair, may be carried by the wind only to fall to the ground many miles away. The smallest ash particles may be erupted miles into the sky and carried many times around the world by winds high in the atmosphere before they fall to the ground.

How many volcanoes are there?

USGS scientists monitor over 160 active and potentially active volcanoes in the United States. Some of these are erupting now and others could erupt at some point in the future. Most of these volcanoes are located in Alaska, a state where eruptions occur almost every year. The rest of the volcanoes are located throughout the American West, and in Hawaii (see our volcano activity map for their locations). Kīlauea volcano on the Island of Hawai‘i is one of the most active volcanoes on Earth. It has been erupting almost nonstop since 1983!

There are about 1,350 potentially active volcanoes worldwide, not counting the volcanoes under the oceans. About 500 of these have erupted in the past 100 years. Many of these are located around the Pacific Ocean in what is known as the "Ring of Fire." In the U.S., volcanoes along the west coast and in Alaska (Aleutian volcanic chain) are part of the Ring of Fire, while Yellowstone and Hawaiian volcanoes form over a "hot spot."

What are the main types of volcanoes?

Cinder Cone

Cinder cones are the simplest type of volcano. They are made of small pieces of solid lava, called cinder, that are erupted from a vent. The ground shakes as magma rises from within the Earth. Then, a powerful blast throws molten rocks, ash, and gas into the air. The rocks cool quickly in the air and fall to the earth to break into small pieces of bubbly cinder that pile up around the vent. They accumulate as a small cinder cone that can be as high as a thousand feet above the surrounding ground. If the wind is blowing during the eruption, cinder is carried downwind before it's deposited into an oval shape. Eruptions that form cinder cones also feed lava flows that spread outward from the eruptive vent. When you climb a cinder cone you can usually find the bowl-shaped crater marking the location of the vent. If eruptions of cinder and lava flows happen repeatedly from the same vent, the overlapping layers can form a composite volcano (stratovolcano). When looking at a map, you will find that thousands of cinder cones exist in western North America and in other volcanic areas of the world.

Composite Volcano (Stratovolcano)

Some of the Earth's grandest mountains are composite volcanoes—sometimes called stratovolcanoes. They are usually tall with steep even sides and are made out of repeating layers of lava flows, volcanic ash, cinders, blocks, and volcanic bombs. Some composite volcanoes rise over 8,000 feet above their surroundings, but they reach much higher elevations when compared to the level of the sea (called above sea level). Ojos del Salado in Chile is the tallest composite volcano on Earth with a summit elevation (height above sea level) of 22,615 feet; the tallest in the U.S. is Mount Rainier in Washington State with a summit elevation of 14,410 feet. Some of the most famous and beautiful mountains in the world are composite volcanoes, including Mount Fuji in Japan, Mount Cotopaxi in Ecuador, Mount Shasta in California, Mount Hood in Oregon, and Mount St. Helens in Washington.

Shield Volcano

Shield volcanoes are built almost entirely of fluid lava flows. Lava pours out of vents in all directions, either from the summit (top) or along two to three rift zones (fractures) that radiate out from the summit like spokes on a bicycle wheel. As lava flows overlap one another, they construct a broad, gently sloping dome shape that from far away appears similar to a warrior's shield. Shield volcanoes build up slowly by the growth of thousands of lava flows that spread widely over great distances, and then cool as thin sheets. On Earth, some of the most massive volcanoes are shield volcanoes. In northern California and Oregon, many shield volcanoes are up to 3 or 4 miles wide and as tall as 1,500 to 2,000 feet. The Hawaiian Islands are made of a chain of shield volcanoes including Kīlauea and the world's largest active volcano, Mauna Loa. Looking at pictures of volcanoes, you can usually identify them by shape as being a shield volcano or stratovolcano.

Lava Dome

A volcano can host many lava domes over a long period of time, so these are technically not a "volcano type" but rather an eruption phenomenon. Lava domes are technically lava flows made up of lava that is too thick to flow away from the vent. Lava squeezes out of the vent and accumulates as a giant pile over and around the vent. Some domes form pointy spines, while others appear as a giant muffin, as opening flower petals, or as steep-sided stubby flows or tongues. Lava domes often grow within craters or upon the flanks of large steep-sided composite volcanoes. Lava domes can be dangerous. They grow largely by expansion from within. As fresh magma fills the inside, the cooler and harder outer surface shatters and spills hot rock and gases down the mountainside. The circle-shaped Novarupta Dome that formed during the 1912 eruption of Katmai Volcano, Alaska, measures 800 feet across and 200 feet high. This dome was one of the last squirts of lava to emerge during a much larger and long-term eruption. The eruption at Katmai was the largest and most violent eruption ever to occur within the United States.

You can do many things to protect yourself and your family from the dangers a volcanic eruption can cause. The best way to protect yourself and your family is to follow the advice of local officials.

Major health threats from a volcanic eruption

Volcanoes spew hot, dangerous gases, ash, lava, and rock that are powerfully destructive. People have died from volcanic blasts. Volcanic eruptions can result in additional threats to health, such as floods, mudslides, power outages, drinking water contamination, and wildfires. Health concerns after a volcanic eruption include infectious disease, respiratory illness, burns, injuries from falls, and vehicle accidents related to the slippery, hazy conditions caused by ash. When warnings are heeded, the chances of adverse health effects from a volcanic eruption are very low.

Volcanic ash

Exposure to ash can be harmful. Infants, elderly people, and people with respiratory conditions such as asthma, emphysema, and other chronic lung diseases may have problems if they breathe in volcanic ash. Ash is gritty, abrasive, sometimes corrosive, and always unpleasant. Small ash particles can abrade (scratch) the front of the eye. Ash particles may contain crystalline silica, a material that causes a respiratory disease called silicosis.

Gases

Most gases from a volcano quickly blow away. However, heavy gases such as carbon dioxide and hydrogen sulfide can collect in low-lying areas. The most common volcanic gas is water vapor, followed by carbon dioxide and sulfur dioxide. Sulfur dioxide can cause breathing problems in both healthy people and people with asthma and other respiratory problems. Other volcanic gases include hydrogen chloride, carbon monoxide, and hydrogen fluoride. Amounts of these gases vary widely from one volcanic eruption to the next.

Although gases usually blow away rapidly, it is possible that people who are close to the volcano or who are in the low-lying areas downwind may be exposed to levels that may affect health. At low levels, gases can irritate the eyes, nose, and throat. At higher levels, gases can cause rapid breathing, headache, dizziness, swelling and spasm of the throat, and suffocation.

Volcanic eruptions are one of Earth's most dramatic and violent agents of change. Not only can powerful explosive eruptions drastically alter land and water for tens of kilometers around a volcano, but tiny liquid droplets of sulfuric acid erupting into the stratosphere can change our planet's climate temporarily.

Volcanic eruptions often force people living near volcanoes to abandon their land and homes, sometimes forever. Those living farther away are likely to avoid complete destruction, but their cities and towns, crops, industrial plants, transportation systems, and electrical grids can still be damaged by tephra, ash, lahars, and flooding.

Fortunately, volcanoes exhibit precursory unrest that, when detected and analyzed in time, allows eruptions to be anticipated and communities at risk to be forewarned. The warning time preceding volcanic events typically allows sufficient time for affected communities to implement response plans and mitigation measures.

Explosive eruptions produce ash

All explosive volcanic eruptions generate tephra, fragments of rock that are produced when magma or or rock is explosively ejected. The largest fragments, blocks and bombs (>64 mm, 2.5 inches diameter), can be expelled with great force but are deposited near the eruptive vent. Lapilli-sized material (6-64 mm, 0.24-2.5 inches diameter) can be carried upward within in a volcanic plume and downwind in a volcanic cloud, but fall to the ground as the eruption cloud cools. The smallest material, volcanic ash (2 mm diameter) is both easily convected upward within the plume and carried downwind for very long distances; as it falls out of suspension it can potentially affect communities and farmland across hundreds, or even thousands, of square kilometers (miles).

Ash endangers aviation and infrastructure

Ashfall rarely endangers human lives, but it can have devastating effects on the things that we rely upon from day to day. As a result of its fine-grained abrasive character and widespread distribution by wind, ashfall and volcanic ash clouds are a major hazard to aviation. The primary hazard from Alaska volcanoes is ashclouds impacting aviation and ashfall reaching areas downwind, owing to widespread dispersal by wind.

Ash fallout to the ground can pose significant disruption and damage to buildings, transportation, water and wastewater, power supply, communications equipment, agriculture, and primary production leading to potentially substantial societal impacts and costs, even at thicknesses of only a few millimetres or inches. Additionally, fine grained ash, when ingested can cause health impacts to humans and animals.

Impacts from ashfall are more complex and multi-faceted than for any of the other volcanic hazards. Variabilities include the distance from the eruption source, orientation and dispersion of the eruption cloud, the amount of ashfall received, physical and chemical properties of the ash, characteristics of the receiving environment (such as climate and land use), and the ability of affected community to adapt to ashfall.

Lava flows typically move slowly enough to outrun them, but they will destroy everything in their path.

Lava type dictates flow shape and travel distance

Lava flows are streams of molten rock that pour or ooze from an erupting vent. Lava is erupted during either nonexplosive activity or explosive lava fountains. The speed at which lava moves across the ground depends on several factors, including (1) type of lava erupted and its viscosity; (2) steepness of the ground over which it travels; (3) whether the lava flows as a broad sheet, through a confined channel, or down a lava tube; and (4) rate of lava production at the vent.

Fluid basalt flows can extend tens of kilometers from an erupting vent. The leading edges of basalt flows can travel as fast as 10 km/h (6 mph) on steep slopes but they typically advance less than 1 km/h (0.27 m/s or about 1 ft/s) on gentle slopes. But when basalt lava flows are confined within a channel or lava tube on a steep slope, the main body of the flow can reach velocities >30 km/h (19 mph).

Viscous andesite flows move only a few kilometers per hour (couple feet per second) and rarely extend more than 8 km (5 mi) from their vents. Viscous dacite and rhyolite flows often form steep-sided mounds called lava domes over an erupting vent. Lava domes often grow by the extrusion of many individual flows >30 m (100 ft) thick over a period of several months or years. Such flows will overlap one another and typically move less than a few meters per hour.

Lava flows may instigate other types of hazards

Everything in the path of an advancing lava flow will be knocked over, surrounded, buried, or ignited by the extremely hot temperature of lava. When lava erupts beneath a glacier or flows over snow and ice, meltwater from the ice and snow can result in far-reaching lahars. If it enters a body of water or water enters a lava tube, the water may boil violently and cause an explosive shower of molten spatter over a wide area. Methane gas, produced as lavaburies vegetation, can migrate in subsurface voids and explode when heated. Thick viscous lava flows, especially those that build a dome, can collapse to form fast-moving pyroclastic flows.

Deaths caused directly by lava flows are uncommon because most move slowly enough that people can move out the way easily. Death and injury can result when onlookers approach an advancing lava flowtoo closely or their retreat is cut off by other flows. Deaths attributed to lava flows are often due to related causes, such as explosions when lava interacts with water, the collapse of an active lava delta that forms where lava enters a body of water, asphyxiation due to accompanying toxic gases, pyroclastic flows from a collapsing dome, and lahars from meltwater.

Other natural phenomena such as hurricanes, tornadoes, tsunami, fires, and earthquakes often destroy buildings, agricultural crops, and homes, but the owner(s) can usually rebuild or repair structures and their businesses in the same location. Lava flows, however, can bury homes and agricultural land under tens of meters of hardened black rock; landmarks and property lines become obscured by a vast, new hummocky landscape. People are rarely able to use land buried by lava flows or sell it for more than a small fraction of its previous worth.

If you are on the floor of a valley that leads from a volcano and become aware of an oncoming lahar, seek high ground.

Lahar is an Indonesian term that describes a hot or cold mixture of water and rock fragments that flows down the slopes of a volcano and typically enters a river valley. Small seasonal events are sometimes referred to as "debris flows", especially in the Cascades. Lahars generally occur on or near stratovolcanoes, such as those of the Aleutian volcanic arc in Alaska and the Cascade Range in the Western U.S.

A moving lahar looks like a roiling slurry of wet concrete, and as it rushes downstream, the size, speed, and amount of material carried can constantly change. The initial flow may be relatively small, but a lahar may grow in volume as it entrains and incorporates anything in its path – rocks, soil, vegetation, and even buildings and bridges. The flowing slurry may consume additional water through melting of snow and ice or by engulfing river or lake water. Voluminous lahars commonly grow to more than 10 times their initial size as they move downslope. In steep areas, lahars can exceed speeds of 200 km/hr (120 mi/hr), but as they move farther away from a volcano and decelerate in lowland areas, they eventually begin to deposit some of the load and decrease in size.

Lahars can occur with or without a volcanic eruption

Eruptions may trigger lahars by melting snow and ice or by ejecting water from a crater lake. Pyroclastic flows can generate lahars when extremely hot, flowing rock debris erodes, mixes with, and melts snow and ice as it travel rapidly down steep slopes.

Lahars can also be formed when high-volume or long-duration rainfall occurs during or after an eruption. On steep slopes, rainwater can easily erode and transport fine-grained, loose volcanic sediment and form a slurry, especially if vegetation has not had time to grow back on recent volcanic deposits.

Some of the largest lahars begin as landslides of wet, hydrothermally altered rock on the steep flanks of volcanoes. These types of collapse and resultant laharsare natural events during a stratovolcano's life history and can occur long after it stops erupting.

Lake breakout floods that occur without an eruption can also lead to lahars. They commonly occur after a stream becomes blocked by a volcanic landslide or pyroclastic flow that forms a natural dam. The most frequent cause of a lake breakout is the overflow of water across a newly formed natural dam, followed by rapid erosion of the loose rock debris. By further erosion and entrainment of sediment and water, the initial flood can transform into a slurry and increase in volume as it races downvalley.

Lahars and excess sediment cause serious economic and environmental damage to river valleys and flood plains

Large lahars can crush, abrade, bury, or carry away almost anything in their paths. Buildings and valuable land may be partially or completely buried. By destroying bridges and roads, lahars can also trap people in areas vulnerable to other hazardous volcanic activity, especially if the lahars leave fresh deposits that are too deep, too soft, or too hot to cross.

Over a period of weeks to years after a volcanic eruption, the erosion and transportation of loose volcanic deposits can lead to severe flooding in areas far downstream from a volcano. Intense rainfall easily erodes loose sediment on steep slopes to produce lahars that travel onto flood plains and bury entire towns and valuable agricultural land. These rainfall-induced lahars can wreak havoc on rivers and streams, sometimes depositing so much sediment that chronic flooding also becomes a problem.

Seasonal debris flows commonly occur on glacier-clad volcanoes

Ice-clad volcanoes are common locations for the initiation of small debris flows, owing to their steep slopes, abundant loose rocks and surface water, and diurnal, seasonal, and long-term variability of surface water flow. These events have some seasonality—one population of events occurs after the snowpack has diminished during mid-to late summer, and the other is triggered by early-season intense rains of winter before the snowpack has accumulated. The Cascades Volcano Observatory offers additional information about these events.

Research addresses lahar trigger mechanisms, flow dynamics, forecasting, and improved warning systems

To understand lahars, scientists observe and quantify what happens when these events occur naturally, derive equations to describe lahar behavior, and perform controlled experiments in a 310-foot (95-m) long debris flow flume. The results help us to understand flow dynamics and provide a foundation for a mathematical and computational model called D-Claw. D-Claw is a numerical software package that is used to simulate fluidly-flowing geologic events like landslides and lahars. The model predicts how a flow moves across terrain over time. The results are used to develop improved technologies for mitigating the destructive effects of lahars and other debris flows.

Because of the danger presented by lahars, scientists are ready to deploy lahar-detection systems when eruptions are imminent. Officials near Mount Rainier in Washington State maintain a permanent lahar-detection system and accompanying public notification system.

Magma contains dissolved gases, which provide the driving force that causes most volcanic eruptions. As magma rises towards the surface and pressure decreases, gases are released from the liquid portion of the magma (melt) and continue to travel upward and are eventually released into the atmosphere. Large eruptions can release enormous amounts of gas in a short time. The 1991 eruption of Mt. Pinatubo is thought to have injected more than 250 megatons of gas into the upper atmosphere on a single day. However, even if magma never reaches the surface, gases can often escape continuously into the atmosphere from the soil, volcanic vents, fumaroles, and hydrothermal systems.

By far the most abundant volcanic gas is water vapor, which is harmless. However, significant amounts of carbon dioxide, sulfur dioxide, hydrogen sulfide and hydrogen halides can also be emitted from volcanoes. Depending on their concentrations, these gases are all potentially hazardous to people, animals, agriculture, and property.

Carbon dioxide (CO₂) trapped in low-lying areas can be lethal to people and animals

Carbon dioxide constitutes approximately 0.04% of the air in the Earth's atmosphere. In an average year, volcanoes release between about 180 and 440 million tonnes of carbon dioxide. When this colorless, odorless gas is emitted from volcanoes, it typically becomes diluted to low concentrations very quickly and is not life threatening. However, because cold carbon dioxide gas is heavier than air it can flow into in low-lying areas where it can reach much higher concentrations in certain, very stable atmospheric conditions. This can pose serious risks to people and animals. Breathing air with more than 3% CO₂ can quickly lead to headaches, dizziness, increased heart rate and difficulty breathing. At mixing ratios exceeding about 15%, carbon dioxide quickly causes unconsciousness and death.

In volcanic or other areas where CO₂emissions occur, it is important to avoid small depressions and low areas that might be CO₂ traps. The boundary between healthy air and lethal gas can be extremely sharp; even a single step upslope may be adequate to escape death. In 2006, three ski patrol members were killed at Mammoth Mountain ski resort after falling into a snow depression surrounding a volcanic fumarole and filled with cool CO₂ gas. High concentrations of CO₂ gas in soils can also damage or destroy vegetation, as is visible in several areas on Mammoth Mountain.

In addition to their direct hazard, volcanic CO₂ emissions also have the capacity to affect the global climate, but scientific studies indicate that the average global volcanic output is insignificant when compared to emissions from human activity.

Sulfur dioxide (SO₂) is irritating to eyes, skin and respiratory system

Sulfur dioxide is a colorless gas with a pungent odor that irritates skin and the tissues and mucous membranes of the eyes, nose, and throat. SO₂ emissions can cause acid rain and air pollution downwind of a volcano—at Kīlauea volcano in Hawaii, high concentrations of sulfur dioxide produce volcanic smog (VOG) causing persistent health problems for downwind populations. During very large eruptions, SO₂ can be injected to altitudes of greater than 10km into the stratosphere. Here, SO₂is converted to sulfate aerosols which reflect sunlight and therefore have a cooling effect on the Earth's climate. They also have a role in ozone depletion, as many of the reactions that destroy ozone occur on the surface of such aerosols.

Please see our discussion of volcanic gases and climate change for additional information.

Hydrogen sulfide (H₂S) is very toxic in high concentrations

Hydrogen sulfide is a colorless, flammable gas with a strong, offensive odor. It is sometimes referred to as sewer gas. Interestingly, the human nose is more sensitive to H₂S than any gas monitoring instrument we have today: air mixtures with as little as 0.000001% H₂S are associated with a rotten egg smell. Unfortunately, however, our sense of smell is not a reliable alarm - at mixing ratios above about 0.01%, H₂S becomes odorless and very toxic, causing irritation of the upper respiratory tract and, during long exposure, pulmonary edema. Exposure to 500 ppm can cause a human to fall unconscious in 5 minutes and die in an hour or less.

Hydrogen halides (HF, HCl, HBr) are toxic acids

When magma ascends close to the surface, volcanoes can emit the halogens fluorine, chlorine and bromine in the form of hydrogen halides (HF, HCl and HBr). These species have high solubility; therefore they rapidly dissolve in water droplets within volcanic plumes or the atmosphere where they can potentially cause acid rain. In an ash-producing eruption, ash particles are also often coated with hydrogen halides. Once deposited, these coated ash particles can poison drinking water supplies, agricultural crops, and grazing land.

Heed evacuation warnings if a volcano is known to be active. If you witness a pyroclastic flow, run in the opposite direction as quickly as possible.

Pyroclastic flows contain a high-density mix of hot lava blocks, pumice, ash and volcanic gas. They move at very high speed down volcanic slopes, typically following valleys. Most pyroclastic flows consist of two parts: a lower (basal) flow of coarse fragments that moves along the ground, and a turbulent cloud of ash that rises above the basal flow. Ash may fall from this cloud over a wide area downwind from the pyroclastic flow.

Pyroclastic flows form in different ways:

• Collapse of eruption column: during a highly explosive eruption, the column ejected upwards into the atmosphere cools and can become too cool and dense to maintain upward momentum.
• "Boiling over" from eruptive vent: during an explosive eruption, material is erupted without forming a high plume and rapidly moves down slope.
• Collapse of lava domes or flows: The fronts of lava flows or domes can become so steep that they collapse due to gravitational force.

Pyroclastic flows destroy nearly everything in their path

With rock fragments ranging in size from ash to boulders that travel across the ground at speeds typically greater than 80 km per hour (50 mph), pyroclastic flowsknock down, shatter, bury or carry away nearly all objects and structures in their path. The extreme temperatures of rocks and gas inside pyroclastic flows, generally between 200°C and 700°C (390-1300°F), can ignite fires and melt snow and ice.

Pyroclastic flows vary considerably in size and speed, but even relatively small flows that move less than 5 km (3 mi) from a volcano can destroy buildings, forests, and farmland. 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.

Pyroclastic flows generally follow valleys or other low-lying areas and, depending on the volume of rock debris carried by the flow, they can deposit layers of loose rock fragments to depths ranging from less than one meter to more than 200 m (up to about 700 ft).

Pyroclastic flows can also lead to secondary hazards, especially flooding and lahars by:

• Eroding, melting and mixing with snow and ice, thereby sending a sudden torrent downstream.
• Damming or blocking streams in volcanic valleys, which may create lakes behind the blockage that eventually overtop and erode the blockage producing a rush of water and volcanic material downstream.
• Increasing the rate of stream runoff and erosion during rainstorms due to the creation of an easily eroded landscape with sparse vegetation.

Landslides are large masses of wet or dry rock and soil that fall, slide, or flow very rapidly under the force of gravity. Landslides on volcanoes can occur without any associated volcanic activity.

Landslides commonly originate as massive rock falls or avalanches, which disintegrate during movement into fragments ranging from small particles to blocks hundreds of meters across. If the landside is large enough and contains a high-enough percentage of water and fine material (typically, >3-5 percent of clay-sized particles), it may transform into a lahar that can travel as much as 200 km (120 mi) downstream.

Landslides are common on volcanic cones because they are tall, steep, and weakened by the rise and eruption of molten rock. Magma releases volcanic gases that partially dissolve in groundwater, resulting in a hot acidic hydrothermal system that weakens rock by altering minerals to clay. Furthermore, the mass of thousands of layers lava and loose fragmented rock debris can lead to fault zones that move frequently.

Volcano landslides (debris avalanches) range in size from less than 1 km³ (0.24 mi ³) to more than 100 km³ (24 mi³. The high velocity and great momentum of landslides allows them to cross valley divides and run up slopes several hundred meters high. For example, the landslide at Mount St. Helens on May 18, 1980, had a volume of 2.5 km³(0.6 mi³), reached speeds of 50-80 m/s (100-180 mi/hr), and surged up and over a 400-m-tall (1300 ft) ridge located about 5 km (3 mi) from the volcano.

Volcanoes are geographical formations where lava, small rocks, and steam are released onto the surface of the earth. There are hundreds of active volcanoes around the world and over 160 active volcanoes in the United States. Twelve U.S. states—Alaska, Arizona, California, Colorado, Hawaii, Idaho, Nevada, New Mexico, Oregon, Utah, Washington, and Wyoming—have active volcanoes. Volcanic eruptions can result in serious health hazards.

Volcanos and Air Pollution

When a volcano erupts it can release harmful particles, including volcanic gases and ash, into the air.

Important volcanic gases that may impact your health include the following:

• Carbon dioxide (CO2)
• Sulfur dioxide (SO2)
• Hydrogen chloride (HCl)
• Hydrogen sulfide (H2S)
• Radon (Rn)
• Hydrogen fluoride (HF)
• Sulfuric acid (H2SO4)

Many volcanic gases are odorless or invisible, so there is often little or no warning to help people avoid dangerous exposure.

Volcanos and Your Health

Inhaling volcanic gases and ash can be harmful to your health. Breathing in volcanic gases at high concentrations can cause mild symptoms, such as irritation of the eyes, and more severe symptoms, such as difficulty breathing or even death.

Most of the effects from volcanic particles and gases are short term. Short-term effects of gas inhalation include the following:

• Irritation of eyes or airways
• Vomiting
• Dizziness
• Headaches
• Rapid or difficult breathing
• Visual disturbances
• Tremors

Longer exposure to volcanic gases and ash has been linked to the development of other conditions, such as:

• Bronchitis and infection of the lung
• Respiratory disorders, such as lung disease and lung cancer

Acute exposures to certain volcanic gases can also lead to unconsciousness within minutes and eventually death.

Who is Most at Risk for Health Problems Associated with Exposure to Volcanic Gases?

People most likely to experience negative health effects from volcanic gases and ash include the following:

• People with asthma
• Children and babies
• People with chronic respiratory or cardiac conditions

If you have asthma, exposure to volcanic gases and ash can make your symptoms worse. Carefully follow your asthma action plan on days when pollution levels are high. Avoid exposure as much as possible and carry your medication with you.

What Can You Do to Protect Yourself?

When a volcanic eruption occurs, follow local guidance on how to respond.

If you start to have symptoms, move away from the area immediately. Your symptoms should improve or stop when you are away from the gases. If your symptoms continue, contact your healthcare provider.

To limit exposure to gases and ash, go indoors and keep your indoor air clear by following these steps:

• Close and lock all outside doors and windows.
• Turn off all heating and air conditioning units and fans.
• Close any fireplace or woodstove dampers.
• Replace or clean furnace filters.

Note that during heavy ashfall you may not be able to stay indoors for more than a couple hours as the weight of ash on the roof of your building may cause the roof to collapse or air intake to be blocked.

If you must be outside, take these steps:

• Avoid ashfall as much as you can.
• Reduce your chances of inhaling ash by wearing an N95 respirator while outdoors or when cleaning up. Check that it is sealed tightly to your face.
• Carry your medication with you if you have a respiratory condition.
• Avoid low-lying areas.
• Wear long sleeves and long pants to avoid skin irritation.
• Use goggles to protect your eyes.

Learn more about steps you can take to protect yourself before, during, and after a volcanic eruption.

Volcanoes erupt in different ways, pose multiple types of hazards, and the initiation and duration of eruptions is relatively uncertain. Therefore, authorities and populations at risk must be knowledgeable about regional volcano hazards so that they can be both prepared and flexible in their response. Location-specific volcano hazards information can be found on a volcano-by-volcano basis.

The USGS has published several volcano-specific hazard maps, which illustrate the potential for ground-based volcanic impacts—lava flows, hot rocks, volcanic gases, and more far-reaching hazards such as ashfall and lahars that flow in valleys that drain the volcano.

In the event of an eruption, scientists will evaluate current events and develop "scenario" based hazards maps that depict possible scenarios for the progress of volcanic activity.

Become familiar with evacuation routes.

Typical volcano hazard zones during an eruption are:

• Areas immediately around the erupting volcanic vent.
• Areas downwind of the vent if the eruption produces ash.
• Areas downslope of the vent if the eruption produces lava flows.
• The stream and river valleys that drain the volcano if the volcano is covered in snow and ice.

Depending up the type of volcano, downslope communities may need to prepare for lava flows or lahars. Lava flows that impact people occur from basaltic volcanoes, like those in Hawaii. Lahars impact river valleys and typically occur from snow- and ice-clad volcanoes like those in the Cascade Range.

Downwind communities may need to prepare for tephra and ash fall. When ash-producing eruptions are imminent or in process, the USGS and its partners will analyze wind conditions and develop maps that indicate areas most likely to receive ash fall.

Be informed. Make a plan. Build a kit.

Become aware of the volcanic hazards that might affect the areas where you live, work, or visit.

Make a plan and ensure everyone in your family knows what to do in case of an emergency.

• Identify responsibilities for each member of your household and plan to work together as a team.
• Choose places to meet and an out-of-area emergency contact person.
• Plan what to do if you have to evacuate, decide where you would go and what route you would take to get there. You may choose to go to a hotel/motel, stay with friends or relatives in a safe location or go to an evacuation shelter if necessary.
• In the event of imminent danger, it is important to find a safe place and stay there.

Build and maintain a disaster supplies kit.
Ensure the following non-standard items are in your kit to live safely and more comfortably around active volcanoes:

• A recommended paint or respiratory mask and goggles for working outside in volcanic ash.
• For people with existing respiratory difficulties, extra medication and oxygen required to live in dusty or ashy conditions.
• Spare contact lenses, cleaning solution, eye glasses.
• Plastic and tape to seal ash out of your house during extreme ash fall.

You can do many things to protect yourself and your family from the dangers a volcanic eruption can cause. The best way to protect yourself and your family is to follow the advice of local officials. Local authorities will give you information on how to prepare for a volcanic eruption, and if necessary, on how to evacuate (leave the area) or take shelter where you are.

How to prepare

Be prepared either to shelter or to evacuate. Develop an evacuation plan and a sheltering plan for yourself, your family, and others in your household. Review the plans and make sure that everyone understands them. If you haven’t already done so, put together an emergency supply kit. Supplies should include the following:

Exposure to ash can harm your health, particularly the respiratory (breathing) tract. To protect yourself while you are outdoors or while you are cleaning up ash that has gotten indoors, use an N-95 disposable respirator (also known as an “air purifying respirator”). N-95 respirators can be purchased at businesses such as hardware stores. It is important to follow directions for proper use of this respirator. If you don’t have an N-95 respirator, you can protect yourself by using a nuisance dust mask as a last resort, but you should stay outdoors for only short periods while dust is falling. Nuisance dust masks can provide comfort and relief from exposure to relatively non-hazardous contaminants such as pollen, but they do not offer as much protection as an N-95 respirator. Cleanup or emergency workers may need a different type of breathing protection.

If you are told to evacuate

Follow authorities’ instructions if they tell you to leave the area. Though it may seem safe to stay at home and wait out an eruption, doing so could be very dangerous. Volcanoes spew hot, dangerous gases, ash, lava, and rock that are powerfully destructive.

Preparing to evacuate

• Tune in the radio or television for volcano updates.
• Listen for disaster sirens and warning signals.
• Review your emergency plan and gather your emergency supplies. Be sure to pack at least a 1-week supply of prescription medications.
• Prepare an emergency kit for your vehicle with food, flares, booster cables, maps, tools, a first aid kit, a fire extinguisher, sleeping bags, a flashlight, batteries, etc.
• Fill your vehicle’s gas tank.
• If no vehicle is available, make arrangements with friends or family for transportation, or follow authorities’ instructions on where to obtain transportation.
• Place vehicles under cover, if at all possible.
• Put livestock in an enclosed area. Plan ahead to take pets with you, but be aware that many emergency shelters cannot accept animals.
• Fill your clean water containers.
• Fill sinks and bathtubs with water as an extra supply for washing.
• Adjust the thermostat on refrigerators and freezers to the coolest possible temperature. If the power goes out, food will stay cooler longer.

As you evacuate

• Take only essential items with you, including at least a 1-week supply of prescription medications.
• If you have time, turn off the gas, electricity, and water.
• Disconnect appliances to reduce the likelihood of electrical shock when power is restored.
• Make sure your automobile’s emergency kit is ready.
• Follow designated evacuation routes—others may be blocked—and expect heavy traffic and delays.

If you are told to take shelter where you are

• Keep listening to your radio or television until you are told all is safe or you are told to evacuate. Local authorities may evacuate specific areas at greatest risk in your community.
• Close and lock all windows and outside doors.
• Turn off all heating and air conditioning systems and fans.
• Close the fireplace damper.
• Organize your emergency supplies and make sure household members know where the supplies are.
• Make sure the radio is working.
• Go to an interior room without windows that is above ground level.
• Bring your pets with you, and be sure to bring additional food and water supplies for them.
• It is ideal to have a hard-wired (non-portable) telephone in the room you select. Call your emergency contact—a friend or family member who does not live near the volcano—and have the phone available if you need to report a life-threatening condition. Remember that telephone equipment may be overwhelmed or damaged during an emergency.

You can do many things to protect yourself and your family from the dangers a volcanic eruption can cause. The best way to do protect yourself and your family is to follow the advice of local officials. Local authorities will provide you with information on how to prepare for a volcanic eruption, and if necessary, on how to evacuate (leave the area) or take shelter where you are.

If a lahar, pyroclastic flow, or lava flow is headed toward you

• Leave the area immediately. If you are warned to evacuate because an eruption is imminent, evacuate.
• If you can drive rather than walk, use your vehicle to evacuate. When driving keep doors and windows closed, drive across the path of danger if you can or away from the danger if you can not, and watch for unusual hazards in the road.

If you are indoors

• Close all windows, doors, and fireplace or woodstove dampers.
• Turn off all fans and heating and air conditioning systems.
• Bring pets and livestock into closed shelters.

If you are outdoors

• Seek shelter indoors.
• If caught in a rockfall, roll into a ball to protect your head.
• If near a stream or river, be aware of rising water and possible mudflows in low-lying areas. Move up-slope as quickly as possible.
• Seek care for burns right away. Immediate care can be life saving.
• If your eyes, nose, and throat become irritated from volcanic gases and fumes, move away from the area immediately. Your symptoms should go away when you are no longer in contact with the gases or fumes. If the symptoms continue, consult your doctor.

Protecting yourself during ashfall

• Stay inside, if possible, with windows and doors closed.
• Wear long-sleeved shirts and long pants.
• Use goggles to protect your eyes.If ash is continually falling, you may not be able to shelter indoors for more than a few hours, because the weight of the ash could collapse the roof of your building and block air intakes into the building. Listen to authorities for advice on leaving the area when ashfall lasts more than a few hours.
• Exposure to ash can harm your health, particularly the respiratory (breathing) tract. To protect yourself while you are outdoors or while you are cleaning up ash that has gotten indoors, a disposable particulate respirator (also known as an “air purifying respirator”) may be considered. An N-95 respirator is the most common type of disposable particulate respirator and can be purchased at businesses such as hardware stores. It is important to follow directions for proper use of this respirator. If you don’t have a particulate respirator, you can protect yourself by using a nuisance dust mask as a last resort, but you should stay outdoors for only short periods while dust is falling. Nuisance dust masks can provide comfort and relief from exposure to relatively non-hazardous contaminants such as pollen, but they do not offer as much protection as a particulate respirator. Cleanup or emergency workers may need a different type of breathing protection based on their work activity. Note that disposable particulate respirators do not filter toxic gases and vapors.
• Keep your car or truck engine switched off. Avoid driving in heavy ashfall. Driving will stir up ash that can clog engines and stall vehicles. If you do have to drive, keep the car windows up and do not operate the air conditioning system. Operating the air conditioning system will bring in outside air and ash.

Pay close attention to emergency plans, follow instructions for closures and evacuations—it might save your life.

Officials will update hazard zones and disseminate new information as the eruption develops. Do your part to remain safe, and help others in need.

If you are ON a volcano during an eruption:

1. Move AWAY from hazard zones as fast as possible.
2. You may be exposed to falling ash and volcanic rocks, lava flows, lahars, volcanic gases, and fast-moving torrents of hot rock and gas (pyroclastic flows).
3. Your route to safety might be cut off. If you are on a snow and ice covered volcano, STAY OFF VALLEY FLOORS AND OUT OF LOW-LYING AREAS – lahars may occur.
4. Pay attention to closure signs; they can save your life.

If you are NEAR or DOWNWIND of a volcano during an eruption:

You may be at risk from LAVA FLOWS.

1. Follow evacuation advice from officials.
2. Stay off fresh lava flows – burning vegetation releases methane gas, which can accumulate in covered areas and be released as an explosion.

You may be at risk from LAHARS (volcanic mudflows).

1. Stay out of valleys and low lying areas that lead away from the mountain.
2. Listen carefully to official reports via emergency broadcasts.
3. If officials warn of an approaching lahar, seek high ground off the valley floor as quickly as possible, such as moving up a hillside. Then, seek shelter.
4. Evacuate if necessary.

You may be at risk from VOLCANIC ASHFALL.

1. Seek shelter and minimize your exposure to ash.
2. DO NOT DRIVE, unless absolutely necessary; ash can damage the engine, filters, and exterior of your vehicle, and driving on ash-slickened roads with reduced visibility can be hazardous. Reduce driving speed.
3. Keep outside air from getting inside. Close and seal (e.g. with duct tape) non-essential doors, windows, vents and other gaps. Place damp towels at the bottom of external doors. Minimize use of HVAC or heating and cooling systems that draw in outside air.
4. Listen for up to date reports – ash fall might be a significant hazard or only a nuisance, depending upon eruption size, style, and duration.

You can do many things to protect yourself and your family after a volcanic eruption:

• Pay attention to warnings, and obey instructions from local authorities. For example, stay indoors until local health officials tell you it is safe to go outside.
• Listen to local news updates for information about air quality, drinking water, and roads.
• Turn off all heating and air conditioning units and fans, and close windows, doors, and fireplace and woodstove dampers to help keep ash and gases from getting into your house.
• Exposure to ash can harm your health, particularly the respiratory (breathing) tract. To protect yourself while you are outdoors or while you are cleaning up ash that has gotten indoors, a disposable particulate respirator (also known as an “air purifying respirator”) may be considered. An N-95 respirator is the most common type of disposable particulate respirator and can be purchased at businesses such as hardware stores. It is important to follow directions for proper use of this respirator. If you don’t have a disposable particulate respirator, you can protect yourself by using a nuisance dust mask as a last resort, but you should stay outdoors for only short periods while dust is falling. Nuisance dust masks can provide comfort and relief from exposure to relatively non-hazardous contaminants such as pollen, but they do not offer as much protection as a particulate respirator. Note that disposable particulate respirators do not filter toxic gases and vapors.
• Stay away from ashfall areas, if possible. Avoid contact with ash as much as you can. Keep your skin covered to avoid irritation from contact with ash.
• Wear goggles to protect your eyes from ash.
• Do not travel unless you have to. Driving in ash is hazardous to your health and your car. Driving will stir up more ash that can clog engines and stall vehicles.
• Replace disposable furnace filters or clean permanent furnace filters frequently.
• If your drinking water has ash in it, use another source of drinking water, such as purchased bottled water, until your water can be tested.
• Clear roofs of ash. Ash is very heavy and can cause buildings to collapse. Be very cautious when working on a roof. Ash can be slippery and make it easy to fall.

Volcanic eruptions may result in floods, landslides and mudslides, power outages, and wildfires.

Eruptive activity can ramp up and ease down for weeks to years after the start of the eruptive cycle.

Actions to take after an eruptive event are dependent upon whether the eruption is continuing, the level of impact, and type of eruption. The most important thing to do is to watch and listen for updates from local authorities; they will determine when to lift or revise evacuation orders and organize relief response.

Ashfall

• Don't rush into the clean-up operation without making a plan; read resources for proper techniques of ash removal and disposal on the Volcanic Ashwebsite.
• Avoid cleanup until ash has stopped falling, and use a 'top down' and 'up-wind' method to prevent recontamination of cleaned areas.
• Minimize driving to avoid damage to vehicles. Driving on ash-slickened roads with reduced visibility can be hazardous.
• Be prepared with clean-up resources on hand such as dust masks, extra medications, plastic sheeting, and heavy-duty tape.

Lava Flows

Lava flows typically move slowly enough for local officials to have issued evacuation notices. Roads may be blocked by lava, so it is important to find alternative means to return to an inundated area once the all-clear has been issued. Pay attention to closures and instructions from local officials.

Lahars

Until a lahar deposit solidifies, the thick slurry may not support a significant amount of weight. Do not attempt to drive across a lahar by vehicle and be cautious in walking across the deposit. As it travels downstream, the lahar can incorporate sharp metal and other hazardous materials. Use gloves and tools to clean up lahar debris. Be aware that lahar sediments will be remobilized by rain and normal river drainage for years after a volcanic event, which may result in destructive flooding.

A volcano is an opening in the Earth’s crust that allows molten rock, gases and debris to escape to the surface. During a volcanic eruption, lava and other debris can flow at speeds of up to 100 mph, destroying everything in their path. Volcanic ash can travel hundreds of miles and cause severe health problems.

There are 161 potentially active volcanoes in the United States. Alaska, Hawaii, California, Washington and Oregon have the largest number of active volcanoes, but other states and territories also have active volcanoes.

A volcanic eruption can:

• Contaminate water supplies.
• Damage machinery.
• Reduce visibility through smog and harmful gases that may threaten low-lying areas.
• Make it hard to breathe and irritate the skin, eyes, nose and throat.

IF YOU ARE UNDER A VOLCANO WARNING:

• Listen for emergency information and alerts.
• Follow evacuation or shelter orders. If advised to evacuate, do so early.
• Reduce your ash exposure by doing the following:
  • Limit your time outdoors and use a dust mask or cloth mask as a last resort if you must be outside.
  • Avoid areas downwind and river valleys downstream of the volcano.
  • Take temporary shelter from volcanic ash in the location where you are.
  • Cover ventilation openings and seal doors and windows.
  • Avoid driving in heavy ash. If you must drive, keep the windows up and do not use the air conditioning system.
  • Do not get on your roof to remove ash.
  • Stay indoors until authorities say it is safe to go outside.

Prepare NOW

• Know your area’s risk from volcanic eruption.
• Ask local emergency management for evacuation and shelter plans, and for potential means of protection from ash.
• Learn about community warning systems by signing up for a free service called the Volcano Notification Service (VNS) that sends notifications about volcanic activity.
• Get necessary supplies in advance, including nonperishable foods, cleaning supplies and water for several days, in case you have to evacuate immediately or if services are cut off. Keep in mind each person’s specific needs, including medication. Do not forget pets’ needs for medication and food.
• Consult your doctor if you have existing respiratory difficulties.
• Practice a communication and evacuation plan with everyone in your family. Have a plan for pets and livestock.
• Have a shelter-in-place plan if your biggest risk is from ash.
• Keep important documents in a safe place. Create password-protected digital copies.
• Find out what your homeowner’s insurance policy will cover when a volcano erupts.

Survive DURING

• Listen to alerts. The Volcano Notification Service provides up-to-date information about eruptions.
• Follow evacuation orders from local authorities. Evacuate early.
• Avoid areas downwind, and river valleys downstream, of the volcano. Rubble and ash will be carried by wind and gravity.
• Take temporary shelter from volcanic ash in the location where you are, if you have enough supplies. Cover ventilation openings and seal doors and windows.
• If outside, protect yourself from falling ash that can irritate skin and injure breathing passages, eyes and open wounds. Use a well-fitting, certified face mask, such as an N95.
• Avoid driving in heavy ash fall.

Be Safe AFTER

• Listen to authorities to find out when it is safe to return after an eruption. Stay indoors until authorities say it is safe to go outside.

• Send text messages or use social media to reach out to family and friends. Phone systems often are busy after a disaster. Only make emergency calls.
• Avoid driving in heavy ash. Driving will stir up volcanic ash that can clog engines and stall vehicles.
• Avoid contact with ash if you have any breathing problems. People with asthma and/or other lung conditions should take precaution in areas with poor air quality, as it can worsen symptoms.
• Do not get on your roof to remove ash unless you have guidance or training. If you have to remove ash, then be very careful as ash makes surfaces slippery. Be careful not to contribute additional weight to an overloaded roof.
• Wear protective clothing and a mask when cleaning up. Children should not help with cleanup efforts.