In 2005, geologist Chris Newhall, US Geological Survey, made a list of the six most important signs of an imminent volcanic eruption. They are as follows:

1. Gas leaks: the release of gases (mostly H₂O, CO₂ and SO₂) from the magma, through cracks in the overlying rock and into the atmosphere.
2. Bit of a bulge: the deformation of part of a volcano, indicating that a magma chamber at depth is swelling or getting more pressurized.
3. Getting shaky: many (hundreds to thousands) of small earthquakes, indicating that magma is on the move. The quakes may be the result of the magma forcing the surrounding rocks to crack, or a harmonic vibration that is evidence of magmatic fluids moving underground.
4. Dropping fast: a sudden decrease in the rate of seismicity may indicate that magma has stalled, and this could mean that something is about to give way.
5. Big bump: a pronounced bulge on the side of the volcano (like at Mt. St. Helens in 1980) may indicate that magma has moved close to surface.
6. Blowing off steam: steam eruptions (a.k.a. phreatic eruptions) happen when magma near to surface heats groundwater to the boiling point. The water eventually explodes, sending steam and fragments of the overlying rock far into the air.

With these signs in mind, we can make a list of the equipment we should have and the actions we should take to monitor a volcano and predict when it might erupt.

Assessing Seismicity

Perhaps the most effective, simplest and cheapest way to monitor a volcano is with seismometers. In an area with a volcano that has the potential to erupt seismometers can provide us with an early warning that something is changing beneath the volcano. If there is seismic evidence that a volcano is coming to life, then more seismometers should be placed in locations within a few tens of kilometres of the source of the activity so as to provide an ongoing picture of how things are changing. (Figure 7.5.1). This will allow geologists to determine the exact location and depth of the seismic activity so that they can see where the magma is moving.

Detecting Gases

Water vapour quickly turns into clouds of liquid water droplets and it is relatively easy to detect just by looking, but CO₂ and SO₂ are not so obvious. It’s important to be able to monitor changes in the composition of volcanic gases, and we need instruments to do that. Some can be done from a distance (from the ground or even from the air) using infrared devices, but to get more accurate data we need to actually sample the air and do chemical analysis. This can be achieved with instruments placed on the ground close to the source of the gases (see Figure 7.3.10 in Section 7.3), or by collecting samples of the air and analyzing them in a lab.

Measuring Deformation

There are two main ways to measure ground deformation at a volcano. One is known as a tiltmeter, which is a sensitive 3-directional level that can sense small changes in the tilt of the ground at a specific location. Another is through the use of GPS (global positioning satellite) technology. GPS is more effective than a tiltmeter because it provides information on how far the ground has actually moved – east-west, north-south and up-down (Figure 7.5.2), but either instrument can be used to assess deformation that might be related to the movement of magma beneath the surface, and so could be indicative of imminent eruptive activity.

By combining information from these types of sources and a thorough knowledge of how volcanoes work, along with careful observations made on the ground and from the air, geologists can get a good idea of the potential for a volcano to erupt in the near future (months to weeks, but not days). They can then make recommendations to authorities about the need for evacuations and restricting transportation corridors. Our ability to predict volcanic eruptions has increased dramatically in recent decades because of advances in our understanding of how volcanoes behave and in monitoring technology. Providing that careful work is done, the risk of a surprise eruption is now much lower than it used to be, and providing that public warnings are issued and heeded, it is less and less likely that thousands will die from sector collapse, pyroclastic flows, ash falls, or lahars. Indirect hazards are still very real, however, and we can expect the next eruption, similar to the one at Laki in 1783, to take an even greater toll than it did then—especially since there are now roughly eight times as many people on the Earth.

Media Attributions

• Figure 7.5.1 Mount Baker, US Geological Survey, public domain, https://www.usgs.gov/volcanoes/mount-baker/earthquake-monitoring-mount-baker
• Figure 7.5.2 Volcano Watch, US Geological Survey/ Hawaiian Volcano Observatory, public domain, https://www.usgs.gov/center-news/volcano-watch-volcano-monitoring-lower-puna-recent-vandalism-and-a-way-help