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Volcanic Hazards

Many types of hazards are associated with volcanoes

Article by Jessica Ball

Volcanic Hazards

Volcanoes can be exciting and fascinating, but also very dangerous. Any kind of volcano is capable of creating harmful or deadly phenomena, whether during an eruption or a period of quiescence. Understanding what a volcano can do is the first step in mitigating volcanic hazards, but it is important to remember that even if scientists have studied a volcano for decades, they do not necessarily know everything it is capable of. Volcanoes are natural systems, and always have some element of unpredictability.

Volcanologists are always working to understand how volcanic hazards behave, and what can be done to avoid them. Here are a few of the more common hazards, and some of the ways that they are formed and behave. (Please note that this is intended as a source of basic information only, and should not be treated as a survival guide by those who live near a volcano. Always listen to the warnings and information issued by your local volcanologists and civil authorities.)

Lava Flows

Lava is molten rock that flows out of a volcano or volcanic vent. Depending on its composition and temperature, lava can be very fluid or very sticky (viscous). Fluid flows are hotter and move the fastest; they can form streams or rivers, or spread out across the landscape in lobes. Viscous flows are cooler and travel shorter distances, and can sometimes build up into lava domes or plugs; collapses of flow fronts or domes can form pyroclastic density currents (discussed later).

Most lava flows can be easily avoided by a person on foot, since they don't move much faster than walking speed, but a lava flow usually cannot be stopped or diverted. Because lava flows are extremely hot - between 1,000-2,000C (1,800 - 3,600 F) - they can cause severe burns and often burn down vegetation and structures. Lava flowing from a vent also creates enormous amounts of pressure, which can crush or bury whatever survives being burned.

Pyroclastic Density Currents

Pyroclastic density currents are an explosive eruptive phenomenon. They are mixtures of pulverized rock, ash, and hot gases, and can move at speeds of hundreds of miles per hour. These currents can be dilute, as in pyroclastic surges, or concentrated, as in pyroclastic flows. They are gravity-driven, which means that they flow down slopes.

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A pyroclastic surge is a dilute, turbulent density current that usually forms when magma interacts explosively with water. Surges can travel over obstacles like valley walls, and leave thin deposits of ash and rock that drape over topography. A pyroclastic flow is a concentrated avalanche of material, often from a collapse of a lava dome or eruption column, which creates massive deposits that range in size from ash to boulders. Pyroclastic flows are more likely to follow valleys and other depressions, and their deposits infill this topography. Occasionally, however, the top part of a pyroclastic flow cloud (which is mostly ash) will detach from the flow and travel on its own as a surge.

Pyroclastic density currents of any kind are deadly. They can travel short distances or hundreds of miles from their source, and move at speeds of up to 1,000 kph (650 mph). They are extremely hot - up to 400C (750F). The speed and force of a pyroclastic density current, combined with its heat, mean that these volcanic phenomena usually destroy anything in their path, either by burning or crushing or both. Anything caught in a pyroclastic density current would be severely burned and pummeled by debris (including remnants of whatever the flow traveled over). There is no way to escape a pyroclastic density current other than not being there when it happens!

One unfortunate example of the destruction caused by pyroclastic density currents is the abandoned city of Plymouth on the Caribbean island of Montserrat. When the Soufrire Hills volcano began erupting violently in 1996, pyroclastic density currents from eruption clouds and lava dome collapses traveled down valleys in which many people had their homes, and inundated the city of Plymouth. That part of the island has since been declared a no-entry zone and evacuated, although it is still possible to see the remains of buildings which have been knocked over and buried, and objects that have been melted by the heat of the pyroclastic density currents.

Pyroclastic Falls

Pyroclastic falls, also known as volcanic fallout, occur when tephra - fragmented rock ranging in size from mm to tens of cm (fractions of inches to feet) - is ejected from a volcanic vent during an eruption and falls to the ground some distance away from the vent. Falls are usually associated with Plinian eruptive columns, ash clouds or volcanic plumes. Tephra in pyroclastic fall deposits may have been transported only a short distance from the vent (a few meters to several km), or, if it is injected into the upper atmosphere, may circle the globe. Any kind of pyroclastic fall deposit will mantle or drape itself over the landscape, and will decrease in both size and thickness the farther away it is from its source.

Tephra falls are usually not directly dangerous unless a person is close enough to an eruption to be struck by larger fragments. The effects of falls can be, however. Ash can smother vegetation, destroy moving parts in motors and engines (especially in aircraft), and scratch surfaces. Scoria and small bombs can break delicate objects, dent metals and become embedded in wood. Some pyroclastic falls contain toxic chemicals that can be absorbed into plants and local water supplies, which can be dangerous for both people and livestock. The main danger of pyroclastic falls is their weight: tephra of any size is made up of pulverized rock, and can be extremely heavy, especially if it gets wet. Most of the damage caused by falls occurs when wet ash and scoria on the roofs of buildings causes them to collapse.

Pyroclastic material injected into the atmosphere may have global as well as local consequences. When the volume of an eruption cloud is large enough, and the cloud is spread far enough by wind, pyroclastic material may actually block sunlight and cause temporary cooling of the Earth's surface. Following the eruption of Mount Tambora in 1815, so much pyroclastic material reached and remained in the Earth's atmosphere that global temperatures dropped an average of about 0.5 C (~1.0 F). This caused worldwide incidences of extreme weather, and led 1816 to be known as 'The Year Without A Summer'.


Lahars are a specific kind of mudflow made up of volcanic debris. They can form in a number of situations: when small slope collapses gather water on their way down a volcano, through rapid melting of snow and ice during an eruption, from heavy rainfall on loose volcanic debris, when a volcano erupts through a crater lake, or when a crater lake drains because of overflow or wall collapse.

Lahars flow like liquids, but because they contain suspended material, they usually have a consistency similar to wet concrete. They flow downhill and will follow depressions and valleys, but they can spread out if they reach a flat area. Lahars can travel at speeds of over 80 kph (50 mph) and reach distances dozens of miles from their source. If they were generated by a volcanic eruption, they may retain enough heat to still be 60-70C (140-160F) when they come to rest.

Lahars are not as fast or hot as other volcanic hazards, but they are extremely destructive. They will either bulldoze or bury anything in their path, sometimes in deposits dozens of feet thick. Whatever cannot get out of a lahar's path will either be swept away or buried. Lahars can, however, be detected in advance by acoustic (sound) monitors, which gives people time to reach high ground; they can also sometimes be channeled away from buildings and people by concrete barriers, although it is impossible to stop them completely.


Volcanic gases are probably the least showy part of a volcanic eruption, but they can be one of an eruption's most deadly effects. Most of the gas released in an eruption is water vapor (H2O), and relatively harmless, but volcanoes also produce carbon dioxide (CO2), sulfur dioxide (SO2), hydrogen sulfide (H2S), fluorine gas (F2), hydrogen fluoride (HF), and other gases. All of these gases can be hazardous - even deadly - in the right conditions.

Carbon dioxide is not poisonous, but it displaces normal oxygen-bearing air, and is odorless and colorless. Because it is heavier than air, it collects in depressions and can suffocate people and animals who wander into pockets where it has displaced normal air. It can also become dissolved in water and collect in lake bottoms; in some situations, the water in those lakes can suddenly 'erupt' huge bubbles of carbon dioxide, killing vegetation, livestock and people living nearby. This was the case in the overturn of Lake Nyos in Cameroon, Africa in 1986, where an eruption of CO2 from the lake suffocated more than 1,700 people and 3,500 livestock in nearby villages.

Sulfur dioxide and hydrogen sulfide are both sulfur-based gases, and unlike carbon dioxide, have a distinct acidic, rotten-egg smell. SO2 can combine with water vapor in the air to form sulfuric acid (H2SO4), a corrosive acid; H2S is also very acidic, and extremely poisonous even in small amounts. Both acids irritate soft tissues (eyes, nose, throat, lungs, etc.), and when the gases form acids in large enough quantities, they mix with water vapor to form vog, or volcanic fog, which can be dangerous to breathe and cause damage to the lungs and eyes. If sulfur-based aerosols reach the upper atmosphere, they can block sunlight and interfere with ozone, which have both short and long-term effects on climate.

One of the nastiest, although less common gases released by volcanoes is fluorine gas (F2). This gas is yellowish brown, corrosive and extremely poisonous. Like CO2, it is denser than air and tends to collect in low areas. Its companion acid, hydrogen fluoride (HF), is highly corrosive and toxic, and causes terrible internal burns and attacks calcium in the skeletal system. Even after visible gas or acid has dissipated, fluorine can be absorbed into plants, and may be able to poison people and animals for long periods following an eruption. After the 1783 eruption of Laki in Iceland, fluorine poisoning and famine caused the deaths of more than half the country's livestock and almost a quarter of its population.

About the Author

Jessica Ball is a graduate student in the Department of Geology at the State University of New York at Buffalo. Her concentration is in volcanology, and she is currently researching lava dome collapses and pyroclastic flows. Jessica earned her Bachelor of Science degree from the College of William and Mary, and worked for a year at the American Geological Institute in the Education/Outreach Program. She also writes the Magma Cum Laude blog, and in what spare time she has left, she enjoys rock climbing and playing various stringed instruments.

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Lava Flows
Lava Stream
This is one of several lava streams of the Prince Avenue flow slicing through the forest between the cross streets of Paradise and Orchid. The lava stream is about 3 meters (10 feet) wide. (Kalapana/Royal Gardens, Hawaii). Image by USGS. Enlarge Image

Pyroclastic Density Currents
Montserrat Volcano
Pyroclastic flow deposits covering the old city of Plymouth on the Caribbean island of Montserrat. Image © iStockphoto / S. Hannah. Enlarge Image

Pyroclastic Flows
Pyroclastic Flow
Pyroclastic flow at Mount St. Helens, Washington, August 7, 1980. Image by USGS. Enlarge Image

Pyroclastic Falls
Volcanic ash weight
Mount Pinatubo, Philippines. View of World Airways DC-10 airplane setting on its tail because of weight of June 15, 1991 ash. Cubi Point Naval Air Station. USN photo by R. L. Rieger. June 17, 1991. Enlarge Image

Lahar flow
Large boulder carried in lahar flow, Muddy River, east of Mount St. Helens, Washington. Geologists for scale. Photo by Lyn Topinka, USGS. September 16, 1980. Enlarge Image

Lake Nyos
Lake Nyos, Cameroon, Gas Release August 21, 1986. Dead cattle and surrounding compounds in Nyos village. September 3, 1986. Image by USGS. Enlarge Image

Sulfur Dioxide
Sulfur banks
Sulfur dioxide issuing from fumaroles of the Sulfur Banks at the summit of Kilauea volcano, Hawaii. Photo © Jessica Ball. Enlarge Image

Volcanic Hazard Resources
Bardintzeff, J.-M. and McBirney, A.R., 2000, Volcanology: Massachusetts, Jones & Bartlett Publishers, 268 p.

Schminke, H.-U., 2004, Volcanism: Berlin, Springer, 324 p.

McNutt, S.R., Rymer, H., and Stix, J. (editor), 1999, Encyclopedia of Volcanoes: San Diego, CA Academic Press, 1456 p.

Gates, A.E. and Ritchie, D., 2007, Encyclopedia of Earthquakes and Volcanoes, Third Edition: New York, NY, Checkmark Books, 346 p.

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