4.3 Deadly Hazards

Charlene Estrada

Volcanic Hazards

Volcanoes are responsible for a large number of deaths, but lava is not the only danger associated with these hazards. Mount Vesuvius (Naples, Italy) is infamous for its violent explosion thousands of years ago in 79 AD when a pyroclastic flow travelled over the Roman countryside and engulfed the cities of Pompeii and Herculaneum [1]. It was not until the 18th century that we uncovered the shocking remains of these towns beneath over 10 feet of ash and the casts of people preserved within it.

Cast of a sitting victim of the Vesuvius eruption, Pompeii 79 AD
Figure 4.3.1. Cast of a sitting victim of the Vesuvius eruption, Pompeii 79 AD.

We know more about volcanic hazards over the past century because they have been better monitored and documented. We have seen enormously violent explosions, effusive lava, fast pyroclastic flows, ash, landslides, toxic gases, and more!

Pyroclastic Flows

The most dangerous type of volcanic hazard are pyroclastic flows. These flows are a mix of lava blocks, pumice, ash, and hot gases between 400 to 1,300°F! The turbulent cloud of ash and gas races down the steep flanks at high speeds at an average of 60 mph (much faster than people can run) into the valleys where farmlands grow and cities thrive [1].

Figure 4.3.2. “Pyroclastic Flow at Mayon Volcano” by the United States Geologic Survey, Public Domain.

Pyroclastic flows can often be expected of stratovolcanoes that contain felsic or intermediate magma. This magma is silica-rich and contains volatile gases that make it highly viscous. Therefore, these volcanoes often have very violent eruptions that are accompanied by a pyroclastic flow.

There are numerous examples of deadly pyroclastic flows. In 2014, the Mount Ontake pyroclastic flow in Japan killed 47 people. The flow was caused by magma heating groundwater into steam, which then rapidly ejected with ash and volcanic bombs. Some were killed by inhalation of toxic gases and hot ash, while volcanic bombs struck others [1]. In 1902, on the Caribbean Island Martinique, Mount Pelee erupted with a violent pyroclastic flow that destroyed the entire town of St. Pierre and killing 28,000 people in moments [3].


Video 4.3.1 What are lahars and how are lahars formed? How does the composition influence the lahar behavior? (3:34)

A lahar is an Indonesian word for a mudflow that is a mixture of water, ash, rock fragments, and other debris that moves down the mountainside of a volcano (or other nearby mountains covered with fresh ash). They form from the rapid melting of snow or glaciers on volcanoes or sometimes in combination with a new eruption and heavy thunderstorm, as seen at Mt. Pinatubo.

Lahars move like a slurry of concrete, but they can move extremely fast at speeds up to 50 mph. Part of the reason they are so deadly is that they are slurry-like; they easily capture materials in their wake and they can travel very long distances like a flash flood [1].

During the 1980 Mount St. Helens eruption, lahars reached 17-miles (27 km) down the North Fork of the Toutle River. Another scenario played out when a lahar from the volcano Nevado del Ruiz, Colombia, buried a town in 1985 and killed about 25,000 people [1].

Figure. 4.3.3. “Lahar off Mount St. Helens” by the United States Geologic Survey  Public Domain.

Tephra and Ash

A building lies in ruins under the weight of ash from the eruption of Mount Pinatubo, a volcano which came to life in June 1991 for the first time in over 600 years.
Figure 4.3.4. A building is crushed under the weight of ash from the eruption of Mount Pinatubo, a volcano which came to life in June 1991 for the first time in over 600 years.

Volcanoes­—mainly stratovolcanoes—eject substantial amounts of tephra and ash [1].

Tephra is heavier than ash, so it will fall closer to the volcano’s crater and vent. Large masses of tephra sometimes erupt from volcanoes and can pose deadly hazards to anyone nearby. These are called volcanic bombs [1].

Figure 4.3.5.Ash Plume from Mount Cleveland” by NASA’s Earth Observatory is licensed under Public Domain.

Ash is much finer, but it is dangerous. It can travel much longer distances away from the volcano, and that can cause more widespread issues in nearby towns and cities. A build-up of ash can collapse the roof of a building or home, and the microscopic minerals within ash will cause respiratory illnesses such as silicosis. Inhaling ash is extremely hazardous because much of it contains microscopic volcanic glass particles [4]. Imagine inhaling tiny shards of glass!

Ash will also interfere with transportation services farther away from an eruption. For example, the 2010  Eyjafjallajökull volcanic eruption in Iceland created a gigantic ash cloud that caused a significant air travel disruption in northern Europe. No one was hurt, but the cost to the world economy was estimated in the billions of dollars [3].

Volcanic Gases

Magma contains dissolved volatile gases. As magma rises toward the surface, the pressure that keeps it in the magma chamber will start to decrease, which allows those gases to escape. Think of this process like twisting the cap of a soda bottle; the first thing to rise and escape in that initial hissing noise is gas! [1]

Volcanic gases react with the atmosphere in various ways; the conve...
Figure 4.3.6. Volcanic gases react with the atmosphere in various ways; the conversion of sulfur dioxide (SO2) to sulfuric acid (H2SO4) has the most significant impact on climate. (Public domain, USGS Volcanic Hazards Program)


The types of gases that are commonly released from volcanoes include greenhouse gases such as carbon dioxide (CO2), sulfur dioxide (SO2), hydrogen sulfide (H2S), methane (CH4), and water vapor (H2O). There are also toxic and acidic gases present at volcanoes such as HF, HBr, and HCl. After gigantic volcanic explosions, some volcanic gases such as sulfur dioxide will become sulfate aerosols in the atmosphere. These aerosol particles block sunlight coming toward Earth’s surface and cause the planet to become cooler [5].

The volcanic gases can be both toxic and suffocating, and these gases sometimes are released from a volcano without an accompanying eruption. Gases were released from the Oku Volcanic Plain in Lake Nyos, Cameroon, and the carbon dioxide suffocated almost 2,000 people in 1986 [1].



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