One volcano can produce a variety of volcanic hazards when erupting. Some volcanic hazards can happen when there isn’t even an eruption. The more that is known about volcanic hazards, the better prepared communities, who are at risk from these hazards, can be.
There are seven types of hazards associated with volcanoes:
- Toxic gas emissions
- Lahars
- Ash columns and flows
- Lava flows
- Earthquakes
- Tsunamis
- Landslides
Volcanic Earthquakes
Volcanoes produce earthquakes through the shifting and breaking of rock as magma rises through the crust of the earth. Earthquakes near volcanoes are almost always associated with volcanic activity and magma intrusion. In fact, seismic monitoring of volcanoes is one of the main methods that volcanologists use for observing volcanic activity.
Volcanologists will set up an array of seismic monitoring equipment around a volcano to measure the frequency and magnitude of magma-related earthquakes. With these instruments, they are able to identify whether or not the epicenter of an earthquake is below the volcano and at what depth it is – all information that is required to know if magma is rising through the volcano’s conduit.
Earthquakes as Alert Systems
When volcanology as a science was still in its beginning stages, earthquakes near volcanoes were one of the first warning signs communities had that there may soon be a volcanic eruption. For example, the 79 A.D. eruption of Vesuvius volcano began with swarms of earthquakes originating from the volcano years before. Unfortunately, the community of Pompeii did not realize the significance of these earthquakes and were caught off guard when Vesuvius erupted, coating the entire city with a thick blanket of boiling hot ash.
Volcanic Landslides
The constant movement of material both within and outside of volcanoes causes slope instability that can have devastating effects. Landslides on volcanoes are commonly produced from either earthquake activity, which loosens up the mountain, or the deformation of the volcano from within, causing fatigue on the slopes. Thousands of years of eruptions cause volcano slopes to be made up of blocky lava flows and layers of loose ash. When the stresses of earthquakes or expanding slopes prove to be too much, flank collapses occur where entire portions of a volcano can give way.
Deadly Volcanic Landslides
The eruption of Mount St. Helens in 1980 was initiated by a monstrous landslide that tore away most of the mountain’s northern slope. Pressure from ascending magma had caused a bulge on the northern slope, deforming and weakening this side of the mountain. This bulge soon became a major concern for geologists and volcanologists who understood the implications of an active volcano with unstable slopes.
Sure enough, on the morning of May 18, 1980, the pressure from within proved to be too much for the volcano’s weakened northern slope. What would have normally been a small and insignificant earthquake was just large enough to shake loose the entire northern side of the volcano. Immediately, pressure on the magma building up inside the volcano was released like a champagne bottle being uncorked. Mount St. Helens erupted with enough energy to send an ash column 25 kilometers (about 15 miles) into the sky in less than 15 minutes.
The landslide and subsequent eruption claimed 57 lives – those closer to the volcano were overcome by millions of cubic meters of mountain material while those farther away were killed by fast moving pyroclastic flows.
Source: U.S. Geological Survey; Scarth, Arwyn. Vulcan’s Fury: Man Against Volcano. London: Yale University Press, 1999.
Alexandra Matiella Novak - With a PhD in Geology and expertise in science education, Alexandra is passionate about increasing the public's Earth science literacy.
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