Volcanoes/Volcanic Eruptions – Associated Hazards


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 various associated hazards:[1]

Volcanic Gases of Kīlauea, Hawai‘i[3]

Volcanic gases can rise tens of kilometers into Earth's atmosphere during large explosive eruptions. Once airborne, the prevailing winds may blow the eruption cloud hundreds to thousands of kilometers from a volcano.

The volcanic gases that pose the greatest potential hazard to people, animals, agriculture, and property are sulfur dioxide, carbon dioxide, and hydrogen fluoride. Locally, sulfur dioxide gas can lead to acid rain and air pollution downwind from a volcano. Because carbon dioxide gas is heavier than air, the gas may flow into in low-lying areas and collect in the soil. The concentration of carbon dioxide gas in these areas can be lethal to people, animals, and vegetation.[2]


Lahar in Guatemala - 1989[5]

Lahar is an Indonesian term that describes a hot or cold mixture of water and rock fragments flowing down the slopes of a volcano and (or) river valleys.

Eruptions may trigger one or more lahars directly by quickly melting snow and ice on a volcano or ejecting water from a crater lake. More often, lahars are formed by intense rainfall during or after an eruption--rainwater can easily erode loose volcanic rock and soil on hillsides and in river valleys.

Lahars and mudflows can damage buildings, wildlife and cars and can prove difficult to escape once caught in them. The lahars can coat objects, wash objects away and can knock objects down by their force. Lahars, debris flows and mudflows that travel into a river or stream run the potential for crowding the waterway, forcing the water to flow outward and causing a flood. The volcanic matter could also pollute the water, making it unsafe to drink.

Lahars almost always occur on or near stratovolcanoes because these volcanoes tend to erupt explosively and their tall, steep cones are either snow covered, topped with a crater lake, constructed of weakly consolidated rock debris that is easily eroded, or internally weakened by hot hydrothermal fluids. Lahars are also common from the snow- and ice-covered shield volcanoes in Iceland where eruptions of fluid basalt lava frequently occur beneath huge glaciers.[4]


Mount St. Helens Landslide[7]

Landslides are common on volcanoes because their massive cones typically rise hundreds to thousands of meters above the surrounding terrain; and are often weakened by the very process that created them--the rise and eruption of molten rock.

A landslide typically destroys everything in its path and may generate a variety of related activity. Historically, landslides have caused explosive eruptions, buried river valleys with tens of meters of rock debris, generated lahars, triggered waves and tsunami, and created deep horseshoe-shaped craters.[6]


Lava Flow on Iceland's Krafla Volcano[9]

Lava flows destroy everything in their path, but most move slowly enough that people can move out of the way. 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.

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, asphyxiation due to accompanying toxic gases, pyroclastic flows from a collapsing dome, and lahars from meltwater.[8]


Pyroclastic flow Merapi, Indonesia[11]

Pyroclastic flows are high-density mixtures of hot, dry rock fragments and hot gases that move away from the vent that erupted them at high speeds.

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.

A pyroclastic flow will destroy nearly everything in its path. With rock fragments ranging in size from ash to boulders traveling across the ground at speeds typically greater than 80 km per hour, pyroclastic flows knock down, shatter, bury or carry away nearly all objects and structures in their way. The extreme temperatures of rocks and gas inside pyroclastic flows, generally between 200°C and 700°C, can cause combustible material to burn, especially petroleum products, wood, vegetation, and houses.[10]


Mount St. Helens Tephra[13]

Tephra is a general term for fragments of volcanic rock and lava regardless of size that are blasted into the air by explosions or carried upward by hot gases in eruption columns or lava fountains. Such fragments range in size from less than 2 mm (ash) to more than 1 m in diameter.

Tephra consists of a wide range of rock particles (size, shape, density, and chemical composition), including combinations of pumice, glass shards, crystals from different types of minerals, and shattered rocks of all types.

A variety of terms are used to describe the range of rock fragments erupted into the air by volcanoes. The terms classify the fragments according to size, shape, composition, or the way in which they form and travel.

  • Ash - Tephra less than 2 millimeters in diameter.
  • Lapilli - Tephra between 2 and 64 millimeters in diameter.
  • Blocks - Tephra greater than 64 millimeters in diameter.

Volcanic ash is highly disruptive to economic activity because it covers just about everything, infiltrates most openings, and is highly abrasive. Airborne ash can obscure sunlight to cause temporary darkness and reduce visibility to zero. Ash is slippery, especially when wet; roads, highways, and airport runways may become impassable. Automobile and jet engines may stall from ash-clogged air filters and moving parts can be damaged from abrasion, including bearings, brakes, and transmissions.[12]



  1. USGS – Volcano Hazards Program – Types of Volcano Hazards: http://volcanoes.usgs.gov/hazards/index.php
  2. USGS – Volcanic Gases and Their Effects: http://volcanoes.usgs.gov/hazards/gas/index.php
  3. Image Source: http://pubs.usgs.gov/fs/2002/fs144-02/ [Accessed March 30, 2014]
  4. USGS – Lahars and their Effects: http://volcanoes.usgs.gov/hazards/lahar/index.php
  5. Image Source: http://volcanoes.usgs.gov/hazards/lahar/santiaguito89.php [Accessed March 30, 2014]
  6. USGS – Volcano Landslides and their Effects: http://volcanoes.usgs.gov/hazards/landslide/index.php
  7. Image Source: http://vulcan.wr.usgs.gov/Glossary/DebrisAval/images.html [Accessed March 30, 2014]
  8. USGS – Lava Flows and their Effects: http://volcanoes.usgs.gov/hazards/lava/index.php
  9. Image Source: https://www.nsf.gov/news/news_summ.jsp?cntn_id=118683 [Accessed March 30, 2014]
  10. USGS – Pyroclastic Flows and Their Effects: http://volcanoes.usgs.gov/hazards/pyroclasticflow/index.php
  11. Image Source: http://volcanoes.usgs.gov/vdap/images/merapi/index.php [Accessed March 30, 2014]
  12. USGS – Volcanic Hazards: Tephra, including volcanic ash: http://volcanoes.usgs.gov/hazards/tephra/index.php
  13. Image Source: http://pubs.usgs.gov/of/2008/1131/ [Accessed March 30, 2014]