Volcanology is the science dedicated to monitoring and understanding the inner workings of volcanoes to forecast eruptions and mitigate potential hazards. This field relies on specialized tools that detect subtle shifts in a volcano’s behavior, which often precede an eruption. Volcanologists gather data by measuring changes in ground shape, internal vibrations, and chemical emissions to interpret the volcano’s current state. Continuous data collection establishes baselines of normal activity and helps recognize when a volcano enters a period of unrest.
Detecting Ground Movement (Deformation)
The physical distortion of a volcano’s surface, known as deformation, is a reliable indicator of magma movement beneath the surface. Deformation occurs because magma filling or draining a subterranean reservoir changes the pressure on the surrounding rock, causing the ground to swell or subside.
High-precision Global Positioning System (GPS) receivers are a primary tool for measuring these changes. Receivers are installed across a volcano’s flanks and summit, continuously monitoring their exact position to detect horizontal and vertical movement with millimeter accuracy.
Another instrument, the tiltmeter, measures minute changes in the ground’s slope at a single point. Tiltmeters are highly sensitive and are often buried in shallow boreholes to insulate them from surface noise, providing a clearer reading of underlying magmatic pressure. Both GPS and tiltmeters provide dense temporal data, but they only measure specific points on the volcano’s surface.
To obtain a broad spatial map of the entire volcanic surface, volcanologists rely on Interferometric Synthetic Aperture Radar (InSAR). This satellite-based technique compares two radar images of the volcano taken at different times from orbit. By measuring the difference in the radar signal’s phase, InSAR produces a detailed map of surface displacement over a large area with centimeter-scale accuracy. This capability is useful for monitoring remote or hard-to-access volcanoes where ground-based equipment is impractical.
Listening for Internal Activity (Seismic and Acoustic Monitoring)
Seismometers are the bedrock of internal activity monitoring, detecting vibrations caused by subterranean processes. These instruments detect distinct types of seismic signals that provide clues about the location and type of magma movement. Volcano-Tectonic (VT) earthquakes are high-frequency signals resulting from the brittle fracturing of rock as magma forces its way toward the surface.
Conversely, Long-Period (LP) earthquakes are lower-frequency signals caused by the resonance of cracks and conduits as magmatic fluids move through them. The continuous, rhythmic ground vibration known as harmonic tremor is a sustained release of seismic energy associated with the prolonged movement of magma or the venting of volcanic gas. Changes in the frequency and intensity of these seismic patterns are often the earliest signs used to forecast an eruption.
Acoustic monitoring uses infrasound sensors to detect extremely low-frequency sound waves in the atmosphere (less than 20 Hz). These waves are generated by explosive activity, sustained gas jetting, or the movement of a volcanic plume. Infrasound sensors are often deployed in arrays to triangulate the source of the acoustic energy. Because these waves travel long distances and are unaffected by cloud cover, they are useful for monitoring remote or ash-obscured volcanoes.
Analyzing Chemical Signatures and Heat
The composition and flux of gases escaping from a volcano offer a direct window into the depth and condition of the underlying magma body. Remote-sensing gas spectrometers, such as the Correlation Spectrometer (COSPEC) and Differential Optical Absorption Spectrometer (DOAS), measure the emission rate of sulfur dioxide (\(\text{SO}_2\)) in the volcanic plume. These instruments analyze the absorption of ultraviolet light by the gas molecules as the instrument is driven or flown beneath the plume.
Other instruments like the Multi-GAS system use infrared sensors to measure gases such as carbon dioxide (\(\text{CO}_2\)) and hydrogen sulfide (\(\text{H}_2\text{S}\)). The ratio of carbon to sulfur gases (\(\text{CO}_2/\text{SO}_2\)) is a diagnostic indicator. An increase in this ratio can signal the arrival of fresh, deeper magma because \(\text{CO}_2\) exsolves at greater depths than \(\text{SO}_2\). High and rising \(\text{SO}_2\) emission rates suggest that magma has moved to shallow levels, allowing the gas to escape more easily.
Thermal cameras and pyrometers provide non-contact measurement of surface temperatures, highlighting areas of increased heat flow. Thermal infrared cameras operate in the longwave infrared spectrum (8–14 \(\mu \text{m}\)) to detect temperature anomalies in lava flows, active vents, and fumaroles. Satellite-based systems routinely detect high-temperature thermal anomalies globally, providing a continuous record of heat output that can indicate magma near the surface or the onset of an effusive eruption.
Fieldwork and Sample Collection
While remote sensing provides continuous data, direct fieldwork is necessary for calibration and detailed laboratory analysis. Volcanologists collect rock, liquid, and gas samples to determine the precise chemical and isotopic composition of the materials. Basic geological tools include rock hammers for collecting rock fragments and specialized heat-resistant containers for collecting lava samples.
For gas collection, scientists use pre-evacuated glass bottles, often containing an alkaline solution to trap specific acidic gases. These bottles are sealed over a fumarole using specialized tubing and inverted funnels. Due to the inherent danger of unstable ground, corrosive fumes, and high heat, volcanologists wear protective equipment. This equipment includes gas masks with filters for acidic fumes, hard hats, and specialized heavy-duty boots and gloves.
Unmanned Aerial Vehicles (UAVs), or drones, have revolutionized fieldwork by allowing the remote deployment of miniature gas sensors and sampling devices directly into hazardous plumes or inaccessible vents. Drones collect gas and aerosol samples from within the plume, providing close-range measurements of composition, temperature, and humidity that are impossible to obtain manually. The use of drones significantly reduces the risk to field crews while enabling high-resolution data collection in dangerous areas.