Volcanoes are dynamic systems, and monitoring them is a complex scientific endeavor dedicated to public safety and understanding Earth’s inner workings. The goal of volcanic measurement is to quantify the size and intensity of past eruptions and, more importantly, to detect subtle changes in the volcano’s behavior that might signal an impending eruption. This process requires a multidisciplinary approach, utilizing instruments that measure everything from ground swelling to gas emissions, allowing scientists to build a comprehensive picture of the activity beneath the surface.
Measuring the Force of an Eruption
To compare the size and intensity of explosive eruptions globally, scientists use the Volcanic Explosivity Index (VEI). This index is a relative scale, running from 0 for non-explosive events up to 8 for the most colossal eruptions in Earth’s history. The numerical value assigned to an eruption is primarily based on the total volume of material ejected, such as ash and tephra.
The scale also incorporates the height of the eruption column, which correlates with the intensity or speed at which the material is released. An increase of one unit on the VEI scale above two generally represents a tenfold increase in the volume of ejected material, making it a logarithmic scale. For instance, a VEI 5 eruption is roughly ten times more explosive than a VEI 4.
Non-explosive, or effusive, eruptions like those often seen in Hawaii are typically assigned a VEI of 0, as the index is not designed to measure the flow of lava. By quantifying explosive events in this standardized way, volcanologists can compare events ranging from a minor eruption ejecting 10,000 cubic meters of material to a supervolcanic eruption ejecting over 1,000 cubic kilometers. This allows for a historical context of volcanic activity and helps in assessing long-term hazards.
Detecting Internal Pressure Changes (Ground Deformation)
The movement of magma beneath a volcano’s surface often causes the ground to swell or deflate, a process known as ground deformation. Measuring these subtle changes is a powerful way to monitor the pressure within the magma reservoir. Scientists use specialized instruments and satellite technologies to track this physical shifting of the volcano’s edifice.
One ground-based tool is the tiltmeter, which is highly sensitive to small changes in the slope or angle of the ground surface. Tiltmeters work much like a carpenter’s level and can detect changes in tilt measured in microradians. When magma accumulates in a chamber, it causes the surface to inflate, and the surrounding slopes tilt away from the center of uplift.
Global Positioning System (GPS) stations are also deployed across the volcano’s surface. These permanent stations continuously track their position with millimeter-to-centimeter accuracy, providing real-time data on both horizontal and vertical ground movement. This data helps model the location and depth of the magma source causing the deformation.
For a wider view, scientists use a satellite technique called Interferometric Synthetic Aperture Radar (InSAR). InSAR involves comparing two radar images taken from space at different times to create an interferogram that maps ground displacement. This method is useful for monitoring remote or inaccessible volcanoes, as it can detect sub-centimeter changes in ground height over a broad area.
Monitoring Underground Movement (Seismic Activity)
Seismometers are deployed on and around volcanoes to monitor the underground movement of magma and gases. As magma forces its way through the crust, it fractures the surrounding rock, causing high-frequency events known as volcano-tectonic (VT) earthquakes. These VT events are similar to normal tectonic earthquakes but are directly related to the stress changes from magma movement.
A different type of seismic signal is the long-period (LP) or low-frequency earthquake, which is caused by the resonance of cracks as magma and gas move toward the surface. These LP events are often seen before eruptions, indicating the movement of volcanic fluids. When a number of these events occur close together, they form an earthquake swarm, which signals magma migration.
A continuous, low-amplitude signal known as volcanic tremor is another indicator of fluid movement. This continuous shaking can be caused by the sustained flow of magma or gas through conduits and cracks. By analyzing the type, location, and intensity of these seismic signals, seismologists can track the path of magma and assess the urgency of the volcanic unrest.
Analyzing Volcanic Gases and Heat
Volcano monitoring includes measuring the chemical composition of gases escaping from the magma, as changes in gas ratios can signal an impending eruption. Gases like Sulfur Dioxide (\(\text{SO}_2\)) and Carbon Dioxide (\(\text{CO}_2\)) are released from the magma as it rises and depressurizes. A significant increase in \(\text{SO}_2\) emission, for example, suggests that magma has moved to a relatively shallow depth.
Scientists use remote sensing techniques like the Differential Optical Absorption Spectroscopy (DOAS) to measure the \(\text{SO}_2\) concentration in the volcanic plume from a safe distance. Instruments such as the Multi-GAS system provide real-time, continuous measurement of multiple gas species, including \(\text{CO}_2\) and \(\text{SO}_2\), deployed near fumaroles. The ratio of \(\text{CO}_2\) to \(\text{SO}_2\) can indicate the depth of the magma, with high ratios suggesting the presence of deeper, undegassed magma.
Thermal monitoring complements gas analysis by detecting an increase in heat flow on the volcano’s surface, which can precede an eruption. Infrared cameras and satellite-based sensors are used to measure surface temperatures and identify hot spots like active lava flows or fumarole fields. Satellite systems can detect thermal anomalies from thousands of kilometers away, providing essential data for monitoring remote volcanoes and tracking released heat energy.