Several measurable changes occur just before a volcanic eruption, including swarms of small earthquakes, ground swelling, shifts in gas emissions, rising temperatures near the surface, and low-frequency sound waves from pressurized gas. These precursors can appear weeks or months in advance, though some intensify dramatically in the final hours. Not every warning sign leads to an eruption, and some eruptions, particularly steam-driven ones, strike with almost no notice at all.
Earthquake Swarms and Harmonic Tremor
The most reliable early warning is a surge in seismic activity beneath the volcano. As magma forces its way upward through rock, it fractures surrounding material, producing clusters of small earthquakes. These swarms typically begin at depths of 15 to 25 kilometers and migrate upward over days or weeks. At one well-studied eruption, earthquake depths rose steadily from 8 to 9 kilometers to about 5 kilometers, then jumped rapidly to shallower levels roughly one day before lava broke the surface.
Alongside these discrete quakes, volcanoes produce a continuous vibration called harmonic tremor. Unlike a regular earthquake with a sharp start and stop, harmonic tremor is a sustained, rhythmic shaking driven by magma and gas moving through underground channels. These tremors typically pulse at frequencies between 1 and 4 Hz. At Japan’s Sakurajima volcano, researchers found that the spectral character of tremor shifts before an explosion: the dominant frequency peak near 1.25 Hz broadens noticeably, and the source of the tremor moves from deeper to shallower zones as the eruption approaches.
Ground Swelling and Deformation
When magma accumulates beneath a volcano, the surface physically inflates, sometimes by centimeters. GPS stations and tiltmeters placed around a volcano can detect this swelling in real time. The changes can be remarkably subtle. At Italy’s Stromboli volcano, tiltmeters recorded shifts of just 0.1 microradians (a nearly imperceptible tilt) about 2 to 4 minutes before explosive paroxysms, followed by larger shifts of 1 to 2 microradians during the event itself. Satellite-based radar measurements can also track ground deformation from orbit with sub-centimeter precision, making it possible to monitor remote volcanoes that lack ground instruments.
Changes in Volcanic Gas
Volcanoes continuously release gases, but the chemical makeup of those emissions changes as magma rises. The ratio of carbon dioxide to sulfur dioxide is one of the most closely watched indicators. Carbon dioxide escapes from magma at greater depths than sulfur dioxide does, so when magma is still deep underground, the gas plume tends to be carbon dioxide-heavy. As magma climbs closer to the surface and pressure drops, sulfur dioxide increases and the ratio shifts. A rising proportion of sulfur dioxide in the gas plume signals that fresh magma is approaching the surface.
Monitoring teams use two main tools to track this. Scanning ultraviolet spectrometers measure how much sulfur dioxide a volcano is releasing into the atmosphere. Portable gas analyzers called MultiGAS instruments sit closer to the vent and directly sample the ratio of sulfur dioxide to carbon dioxide and other gases, revealing not just how much gas is escaping but from what depth the magma is degassing.
Thermal Anomalies
Rising magma heats the ground, crater lakes, and fumaroles above it. Satellites equipped with thermal infrared sensors can detect temperature increases as small as 1 to 2 degrees Kelvin at the surface. These subtle thermal changes are easy to miss with lower-resolution instruments, but a two-decade analysis of satellite thermal data found that these small warming signals preceded roughly 81% of eruptions. The warming often appears well before any visible activity, giving scientists an additional tool for identifying volcanoes entering a dangerous phase.
Infrasound: The Sounds You Can’t Hear
Volcanoes produce powerful sound waves below the range of human hearing, typically around 1 Hz. These infrasound signals are generated by gas pressurization, explosions, and the oscillation of lava within the crater. At Chile’s Villarrica volcano before its 2015 eruption, researchers tracked changes in the pitch and damping of resonant infrasound tones and used them to determine that the lava lake inside the crater had begun rising six days before the paroxysm. The lake reached the flared upper part of the crater and then oscillated for two days before a 1.5-kilometer-high lava fountain erupted. Infrasound monitoring is particularly valuable for open-vent volcanoes that degas continuously, because the acoustic signature is directly shaped by the geometry of the crater and the position of the lava within it.
How Fast Magma Rises
The speed of magma ascent varies enormously depending on the volcano and the type of magma involved. Measured ascent velocities, including time the magma spends stalled at intermediate depths, range from about 0.01 to 0.1 meters per second. At deeper levels (22 to 27 kilometers), magma moves more slowly, around 0.01 to 0.04 meters per second. From shallower storage zones at 4 to 7 kilometers, it accelerates to 0.05 to 0.1 meters per second. Magma doesn’t rise in a single smooth journey. It accumulates at various depths, sometimes for weeks, before continuing upward. During one eruption sequence, distinct batches of magma were tracked from source zones progressively deeper than 27 to 31 kilometers, each pausing at intermediate reservoirs before reaching the surface.
Steam-Driven Eruptions Are Harder to Predict
Not all eruptions are triggered by rising magma. Phreatic eruptions are driven by superheated water flashing to steam underground, and they can occur with little or no warning. Because no fresh magma reaches the surface, many of the classic precursors (gas ratio changes, deep earthquake migration) are muted or absent. The ejected material consists mostly of old, altered rock rather than fresh volcanic glass.
There are some precursory signals unique to phreatic events. At Colombia’s Nevado del Ruiz volcano, a distinctive pattern called banded tremor appeared several days before a phreatic eruption. At Japan’s Aso volcano, a very long period displacement lasting 350 to 400 seconds, starting with accelerating inflation followed by a slow deflation, preceded an eruption. Shifts in the ratio of hydrogen sulfide to sulfur dioxide can also signal changes in the shallow hydrothermal system. Still, phreatic eruptions remain among the most difficult to forecast, and the USGS notes that steam-blast eruptions can happen with essentially no advance notice.
Precursors Don’t Always Mean an Eruption
One of the most important things to understand about volcanic warning signs is that they frequently occur without an eruption following. Precursors can persist for weeks, months, or even years and then simply fade. A study of 19 volcanoes in Central America found that after a major earthquake sequence in 2012, many volcanoes showed increased unrest, including heightened seismicity and other signs. But only the 11 volcanoes that were already in a critical state of unrest before the earthquakes actually erupted. Eight others showed increased activity but never reached eruption. Five volcanoes with no prior unrest briefly switched to low-level seismicity after the earthquakes, then settled back down.
The pattern is clear: external triggers like nearby earthquakes can push a volcano’s unrest level higher, but they aren’t enough on their own to cause an eruption. A volcano generally needs to already have a pressurized, mobile magma supply before precursory signals escalate into an actual eruption. This is why volcanologists monitor multiple indicators simultaneously rather than relying on any single signal. The convergence of rising seismicity, ground inflation, gas changes, and thermal warming together provides the strongest evidence that an eruption is approaching.