How Volcanoes Are Monitored
Why volcanoes usually give warning, and how scientists read it
Why volcanoes give warning
Unlike an earthquake fault, a volcano cannot erupt without first moving magma from a reservoir several kilometres down toward the surface. That movement is physical and detectable: the magma cracks the rock it pushes through, it inflates the ground above it like a slowly filling balloon, and it releases dissolved gas that seeps to the surface. These signs typically build over days to months before an eruption.
This is why eruption forecasting works far better than earthquake prediction. It is not perfect — unrest sometimes fades without an eruption, and the size and exact timing are hard to call — but a well-monitored volcano rarely erupts with no warning at all.
The three core measurements
Thermal cameras, webcams, satellite hotspot detection, and measurements of any lakes or hot springs fill in the picture. No single signal is decisive; observatories look for several changing together.
- Seismicity. A dense network of seismometers on the volcano detects the small earthquakes caused by magma fracturing rock, plus a continuous shaking called tremor that often signals fluid on the move. Swarms that grow in number and migrate upward over days are a classic pre-eruption pattern.
- Ground deformation. GPS/GNSS receivers, tiltmeters, and satellite radar (InSAR) measure the ground rising, falling or tilting by centimetres as magma accumulates or drains. Satellite radar can survey volcanoes that have no instruments on the ground at all.
- Gas. Instruments on the ground and on satellites track sulfur dioxide and carbon dioxide. A rise in SO₂, or a shift in the ratio of gases, indicates fresh magma is degassing near the surface.
Alert levels and aviation
Observatories translate their data into standardised public alerts. The U.S. Geological Survey uses a Volcano Alert Level (Normal, Advisory, Watch, Warning) for ground hazards and a separate Aviation Color Code (Green, Yellow, Orange, Red) for the ash threat to aircraft; most countries use a similar colour scheme.
Volcanic ash is a serious hazard to jet engines, so nine Volcanic Ash Advisory Centres around the world issue forecasts of where an ash cloud will drift. The 2010 Eyjafjallajökull eruption in Iceland — only VEI 4 — closed European airspace for days on the strength of those forecasts.
When it works, and when it doesn’t
In April–June 1991, scientists from the Philippine Institute of Volcanology and the USGS tracked escalating earthquakes and gas at Pinatubo, a volcano that had not erupted in 500 years. They raised the alert in stages and evacuated more than 60,000 people. The VEI 6 eruption on 15 June destroyed the surrounding area; the evacuation is credited with saving thousands of lives.
The counter-example is Nevado del Ruiz, Colombia, in 1985. A hazard map correctly showed the town of Armero in the path of a potential mudflow, and the volcano had been restless for a year, but the warning did not reach or convince local authorities in time. A relatively small eruption melted summit ice and sent a lahar down the valley that killed about 23,000 people.
Of the roughly 1,300 potentially active volcanoes on the map, only a few hundred have real ground-based monitoring. Many of the least-monitored are in lower-income countries with dense populations nearby.
Sources
- U.S. Geological Survey — Volcano Hazards Program: “Monitoring volcanoes”, “Volcano alert levels”, “Aviation Color Codes”, “Pinatubo 1991.”
- Global Volcanism Program, Smithsonian Institution — Weekly Volcanic Activity Report; “How are volcanoes monitored?”
- Newhall, C. G. & Punongbayan, R. S. (eds.) (1996). Fire and Mud: Eruptions and Lahars of Mount Pinatubo, Philippines. PHIVOLCS / University of Washington Press.
- Voight, B. (1990). “The 1985 Nevado del Ruiz volcano catastrophe: anatomy and retrospection.” Journal of Volcanology and Geothermal Research 44, 349–386.