Volcanic hazards

Volcanoes produce many types of hazard, both during eruptions and when they are quiet. Multiple hazards may be produced in a single eruption.

Pyroclastic flow in Montserrat. Photovolcanica/Shutterstock.

Pyroclastic flows

Pyroclastic flows are flows of ash, rocks and gas which move very quickly, much faster than you can run, and are often extremely hot. They move rapidly downhill, normally following valleys on the slopes of a volcano. They can form in several ways, either through the collapse of an eruption column during an explosive eruption, or when a lava dome or flow suddenly collapses. They are the most dangerous and destructive of volcanic hazards, with death or severe injury likely for anyone in their path.

Pyroclastic flow is the colloquial term used by most people to refer to the whole pyroclastic density current (PDC), but technically is only the gravitationally-controlled lower part of the flow. The upper cloud of hot ash and gas riding over the flow is called a surge. This can detach and move up and over topography.


Lahars are volcanic mudflows or debris flows. They are a flood of water, ash, rocks, mud, boulders and other debris that is picked up as the flow rapidly moves downstream. Lahars can form when snow or ice is rapidly melted by hot eruptive products or when loose volcanic deposits are remobilised by heavy rainfall. Lahars can occur during eruptions or even years later. They normally effect river valleys and the flat ground beyond. They can be very destructive, and their speed, debris and with a consistency of wet concrete, also deadly. 


The molten rock beneath volcanoes is called magma, but when it erupts onto the surface it is called lava. Lava flows form as rivers of lava, sometimes kilometres long. These lava flows normally move slowly enough that you can walk away, but can sometimes move very quickly. Lava can also sometimes be seen as glowing red-hot lava fountains at the vent. When lava cools it turns black and solid. Lava domes form when sticky, viscous lava is squeezed out of the vent slowly, like toothpaste, and build up into a pile over the vent. Lava domes can sometimes collapse to form pyroclastic flows.


Volcanic ballistics are also called blocks and volcanic bombs. They are fragments of rock and lava that are thrown out of the volcano at great speeds. They usually fall within about 5 km of the vent, and can range in size from a few centimetres to metres across. Some ballistics are red-hot and cause fires in dry vegetation or buildings. Their speed means that even small volcanic bombs can cause serious injury or death.


Eruptions produce rock fragments of different sizes, collectively called tephra. Ballistics are the largest form of tephra, whilst volcanic ash is the smallest, at less than 2 mm across each grain. Volcanic ash is quite different to the ash from a fire, because it is in effect tiny grains of rock and glass, making it very abrasive. Ash is produced in huge quantities in eruptions, pushed high into the atmosphere in the most powerful eruptions, where it is then carried downwind for hundreds or even thousands of kilometres. It gradually falls out of the cloud, with the thickest deposits normally closest to the volcano. Ash can cause breathing difficulties and skin and eye irritation, and thick deposits can cause roofs, buildings and trees to collapse. Farmland can be buried, with crops destroyed and livestock left without access to food and water. Ash is often very disruptive to everyday life in towns and cities, with disruption to transport and services.

Volcanic gas

The magma beneath volcanoes contains large volumes of dissolved gases, which escape during eruptions and between eruptions from gas vents called fumaroles, bubbling through lakes and rivers, and through the ground. The most common gas is water vapour – steam, and steam plumes can often be seen above volcanoes. Many harmful gases are also released, including carbon dioxide, sulphur dioxide and hydrogen sulphide. At some concentrations these gases can be hazardous to the health of people, animals and plants, and corrosive to property.

Carbon dioxide is colourless and odourless and can collect in low areas like basements, caves and other depressions, where the concentration can build to a deadly level.  

Sulphur dioxide is colourless and smelly. It irritates the skin, eyes, nose, throat and respiratory system. It can cause acid rain, and can affect the climate.

Hydrogen sulphide is colourless, and very smelly, like rotten eggs. The smell disappears at high concentrations when it becomes very toxic, affecting the respiratory system and can lead to death.

Lahar deposit laden with trees at Mount St Helens. USGS.


Red-hot lava flow in Hawaii. Yvonne Baur/Shutterstock.

Bombs being thrown from a small explosive eruption at Krakatoa. Tanguy de Saint-Cyr/Shutterstock.


Tsunami are large waves that form in any body of water, when the water is displaced by actions like earthquakes, submarine explosions, when part of the volcanic edifice collapses into the water, or when there is the rapid entrance of volcanic products – like a pyroclastic flow – into the water. Tsunamis can be immensely powerful and can travel great distances across oceans. They can wash inland for kilometres, destroying everything in their path.

USGS_public_Ash Papua New Guinea 1984.jp

Volcanic ash burying cars and houses in Papua New Guinea. USGS.

Clouds of gas emitted from Yasur volcano, Vanuatu. Sarah Brown.