The primary goal of volcano monitoring is to track the movement of magma beneath a volcano and thereby predict when, and how violently, it will erupt at the surface. Three common signals are used to monitor magma movement:
Occurrence of earthquakes, which are generated when magma (and/or associated gases) migration causes rocks to break. These types of earthquakes are common in the weeks to months before a volcanic eruption. Earthquakes triggered by the sudden release of pressurized gases are typically shallow and are common in the days to hours before an eruption.
Swelling of the surface of the volcano, which is caused by rising magma. The deformation can be detected by ground-based surveying techniques, GPS stations installed around a volcano, and aerial and satellite-based surveys.
Release of volcanic gases through fractures during or before magma ascent. These gases can be measured within active fumaroles (gas vents) or spectroscopically. Both the absolute abundance and the ratio of different gas species provide information on the location of gas release and the extent to which the magmatic system might be accumulating excess gases.
One of the most promising new techniques for improving our understanding of magma transfer into the upper crust is interferometric synthetic aperture radar (InSAR). By combining images taken months to years apart, we can detect subtle changes in elevation. For example, images of South Sister volcano, in Oregon, taken in August 1996 and October 2000 show that an area to the west of the volcano inflated by about 10 cm. This observation stimulated closer monitoring of the area by other methods, and it has been documented that the uplift has since continued more slowly, at about 2.5 cm/year, and most likely indicates intrusion of magma about 7 km below the surface. Because the intrusion has been accompanied by very little seismicity, it would not have been discovered without InSAR. Although this intrusion is unlikely to produce a volcanic eruption in the near future, documentation of both the temporal and spatial patterns of magma intrusion over the next decades will greatly improve our knowledge of crustal processes related to magma transfer and storage. InSAR has proven extremely useful for detecting subtle changes in volcanoes but has limitations as a monitoring tool in active regions because of infrequent data acquisitions.
Interferogram showing uplift west of the South Sister volcano. Each full-color cycle represents 2.83 cm of range change between the ground and the satellite. SOURCE: Charles Wicks, U.S. Geological Survey.