In 2005 USGS geologist Chris Newhall 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 into the atmosphere through cracks in the overlying rock
2. Bit of a bulge — the deformation of part of the volcano, indicating that a magma chamber at depth is swelling or becoming 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, which may indicate that magma has stalled, which could mean that something is about to give way
5. Big bump — a pronounced bulge on the side of the volcano (like the one at Mount St. Helens in 1980), which may indicate that magma has moved close to surface
6. Blowing off steam — steam eruptions (a.k.a. phreatic eruptions) that happen when magma near the surface heats groundwater to the boiling point. The water eventually explodes, sending 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 can take to monitor a volcano and predict when it might erupt.

Assessing Seismicity

The simplest and cheapest way to monitor a volcano is with seismometers. The University of Utah Seismograph Stations (UUSS) operates the Yellowstone Seismic Network, and earthquake data are transferred from Yellowstone to the UUSS in real time using a radio and satellite telemetry system. UUSS scientists analyze the earthquake data and report information out via their website.

The United State Geological Survey operates 5 volcano observatories in the U.S., and monitors volcanic activity worldwide. Volcano monitoring and research conducted at five U.S. volcano observatories advances our understanding of active volcanism and its impacts, and enables the USGS Volcano Hazards Program to provide information about and warnings of volcanic activity in the United States. Each observatory is assigned a geographic area of responsibility and issues formal notices of activity for volcanoes in those regions. The observatories and their partner organizations operate real-time volcano monitoring networks, disseminate forecasts and notifications of significant activity, assess volcano hazards, conduct scientific research into volcanic processes, and work with communities to prepare for volcanic eruptions. The seismic networks operated by the USGS (and partners) 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 vapor quickly turns into clouds of liquid water droplets and is relatively easy to detect just by looking, but CO₂ and SO₂ are not as 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 monitored from a distance (from the ground or even from the air) using infrared devices, but to obtain more accurate data, we need to 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 13.6.12), or by collecting samples of the air and analyzing them in a lab.

Measuring Deformation

Measurement of deformation allows us to determine if a volcano is expanding or contracting. That can typically be related to the movement of magma into or out of a magma chamber near to surface and so is an indicator of the potential for an eruption.  There are two main ways to measure ground deformation. One is known as a tiltmeter, which is a sensitive three-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 system) technology (Figure 13.6.1). Both are effective, but GPS provides more information than a tiltmeter because it can tell us how far the ground has actually moved in the east-west, north-south, and up-down directions.

By combining information from these types of sources, along with careful observations made on the ground and from the air, and a thorough knowledge of how volcanoes work, 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, there is no longer a large risk of surprise eruptions, 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 like 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 Earth.

Media Attributions

• USGS operates five U.S. Volcano Observatories
• Figure 4.5.1: A Typical GPS Monitoring Station by USGS. Public domain.