4.4 Monitoring Volcanoes

Charlene Estrada

The Science of Predicting Volcanic Eruptions

Upper part of the image is a lake of bright lava in the bottom. On the bottom, a woman wearing yellow coat and a helmet smiles looking at the camera. She seems be perched on a slope.
Figure 4.4.1. Volcanologists get as close as they can to study volcanoes. But monitoring volcanoes from a space is less risky and could be even more impactful. Jani Radebaugh, CC BY-SA 4.0

Scorching temperatures, toxic gases, and let’s not forget the volcanic bombs! Monitoring volcanoes is an extremely dangerous job, and it comes as no surprise that volcanology routinely appears among the top ten deadliest professions in the world! Nevertheless, as seen in video 4.4.1, scientists take risks to monitor volcanoes because it may reduce the chance of a potential eruption taking a populated region by surprise.

Video 4.4.1. Every volcanic eruption is an opportunity to learn about Earth’s interior and to refine our prediction models (1:16).

[Video Description: Images of molten lava at active volcanic craters.  Text: “Volcanoes are SO HOT right now. SO HOT that globally around 40 volcanoes erupt each month. Clip of hydrothermal vent. “Along with hundreds more erupting on the seafloor.” Video of lava. “EACH MONTH. And every eruption is an opportunity to learn about what’s happening deep, deep within the Earth.” Video of white volcanic gases. “In addition to lava and ash, these portals to the deep emit gases. Water, carbon and sulfur from within Earth provide valuable clues about how our planet works. Collecting these gases might look cool, but it’s the HOTTEST job in science.” Video of Jeep driving down the road. “By 2019, the Deep Carbon Observatory will TRIPLE the number of continuous volcanic gas monitoring stations worldwide, collecting real-time data in some of the most remote places on Earth.” World map of earthquakes, eruptions, and gas emissions. “When pieced together, this information helps reveal the story of deep carbon. Measuring these gases may even help us forecast future volcanic eruptions, and that’s not just a bunch of HOT AIR.” Video of erupting stratovolcano by a town.]


It is difficult to predict whether or when a volcano will erupt, but scientists closely monitor a volcano over time to look for changes that could show geologic activity. Scientists will measure characteristics such as seismic activity surrounding the volcano, the deformation of the cone, gas emissions, and past history of volcanic eruptions [1,6]. These measurements will lead scientists to advise officials to take decisive action that can lead to evacuations, and hopefully, the prevention of a catastrophe.


Monitoring techniques shown include remote sensing, camera imaging, earthquake detection, surveying ground tilt and GPS, and gas monitoring.
Figure 4.4.2. The wide array of available monitoring techniques that scientists use to characterize and predict volcanic eruptions.

History of Volcanic Activity

How active is a volcano? Scientists compile historical records from cities, towns, and villages surrounding a volcano to learn when, over the course of written history, it last erupted. Geologists can also examine the cooled lava fields along the slopes of a volcano, and even date some of the recently deposited ash and rock to determine when it was last ejected from the vent.

Video 4.4.2. What can scientists see in a particle of ash? Geologists, John Wolff and Michael Rowe, discuss the use of geochemistry to aid in the prediction of volcanic eruptions. Their work centers on geologic activity in the Cascade Range, where a powerful subduction zone off the coast of North America produces constant volcanic activity in the area (3:44). Source: Washington State University, CC BY.

One way to classify volcanoes is by their activity: erupting, dormant, active, and extinct. An erupting volcano, as the name suggests, is currently erupting. A volcano that is not erupting, but remains connected to a magma chamber that might erupt, is called dormant. If a volcano has erupted just once in the past 10,000 years, scientists consider it to be active, even if it is in a period of dormancy or currently erupting. The only “safe” volcano is an extinct volcano, which means that we do not expect it to erupt in the foreseeable future. Nonetheless, even extinct volcanoes may surprise scientists and they are still prone to mass wasting events.

Scientists usually focus their monitoring efforts on active volcanoes, especially those that might affect populated areas. By documenting the eruption history of active volcanoes, scientists could construct a broad timeframe in which it is reasonable to expect an eruption.


When magma moves beneath an active volcano, it can shake the ground nearby. The sudden shaking releases energy in the form of seismic waves, that is, it produces seismic movements or earthquakes.

A good indicator of a volcano that is just about to erupt is a series or “swarm” of earthquakes. Scientists measure these with instruments called seismographs, which capture the seismic waves released by the movement of the volcanic slope [7].

Swarms of Earthquakes, recorded as Seismograms, caused by Volcanic Activity
Figure 4.4.3. Swarms of earthquakes, recorded as seismograms, caused by volcanic activity.

Ground Deformation

In addition to producing earthquakes, the movement of magma under a volcano can also bulge the flanks of the mountainside. The deformation of the ground along the volcanic slope might not very apparent to the naked eye. Scientists use instruments called tiltmeters, which precisely measure the angle of a volcano’s slope. When that slope starts to deform, due to pressure from the underlying melted rock and gas, it will change the slope angle of the volcano [8]. The tiltmeter is a sensitive instrument that can detect infinitesimal changes. The deformation process could even cause mass wasting events.

Mount St Helens' northern peak is bulged outward in 1980 before its massive eruption
Figure 4.4.4. Before its eruption in 1980, the top of Mount St Helens appeared to be bulging. Today, well after its eruption, this area has collapsed or caved-in!

Some scientists now use the modern technique of remote sensing to detect subtle changes in the volcano’s shape and slope. Scientists use modern drones and satellites to track slight changes in elevation and temperature in areas that are inaccessible to scientists. Some of these technologies include Light Detection and Ranging (LiDAR), Global Positioning System stations (GPS), and Interferometric Synthetic Aperture Radar (InSAR). Figure 4.4.4 shows the working principle for inSAR.

Image shows how inSAR works: A Satellite passes over an area at least 2-3 times to record information about how the ground changes during volcanic activity.
Figure 4.4.5. How does inSAR work? A satellite passes over an area and makes a map of the relative elevation. When the satellite passes over the same place multiple times again, it will notice differences in elevation brought about by volcanic activity.

Gas Emissions

Some active volcanoes release gases before magma. These gases include sulfur dioxide (SO2), carbon dioxide (CO2), water vapor (H2O), and hydrochloric acid (HCl). Scientists will detect these gases near the vent of the volcano or sample them and later analyze the gas concentration with sophisticated instruments, called spectrometers. The increase in the concentration of certain gases may indicate an imminent eruption [9].

Large gas monitoring station with solar panels at Mount St. Helens with a geologists sitting on top of it for scale.
Figure 4.4.6.  A gas monitoring station placed at Mount St. Helens can collect water vapor, carbon dioxide, sulfur dioxide, and hydrogen sulfide.


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