Venting gases from a volcano
Although weather forecasts are common, forecasts of geologic events like volcanic eruptions present a greater challenge because each volcano has its own characteristics. Scientists use a combination of common pre-eruption activity, data and monitoring tools to predict eruptions. According to Emily Brodsky, a University of California, Santa Cruz associate professor of Earth and planetary sciences, a volcano's personality complicates forecast methodology by requiring each observatory to tweak the forecast model for its specific volcano.
Pre-Eruption Activity
Scientists study a volcano's pre-eruption activity to determine its personality. Before eruption, volcanoes cause ground vibrations, small earthquakes and rumbling noises. The magma movement beneath the volcano collects in a reservoir or chamber, and as it comes to the surface, the magma releases measurable gases. The magma collection also causes the volcano's slopes to swell slightly. Forecasters take into account these indicators, as well as information such as the length of the earthquakes, the time interval between eruptions, the silica content from lava of each eruption and rising temperatures in nearby gas vents, hot springs and surface rocks.
One deciding factor that varies volcano to volcano is the speed of the ascent of the magma, which is based upon the viscosity of the magma. The viscosity depends on the magma's silica content. The lower the silica content, the thinner the magma. Runny, thin magma fills a volcanic chamber more quickly causing a quicker eruption. Thick, sticky, high silica-containing magma takes longer to fill the chamber, causing a longer time to pass until eruption.
Tools
A number of measuring and monitoring tools help scientists compile necessary modeling data. Tiltmeters and geodimeters measure the volcano's pre-eruption swelling, while seismographs monitor earthquakes. Scientists use correlation spectrometers to measure sulfur dioxide, a gas released in increased quantities pre-eruption. Airborne magnetometers measure magma-caused changes in the Earth's magnetic field, while sensitive resistivity meters measure changes in the Earth's electrical currents. Geographic information systems (GIS) help scientists map lava, pyroclastic and debris flows to aid in eruption response planning.
Prediction Periods
The scientific community has had better luck with long-term volcano forecasts than with short-term forecasts. Part of this is due to inconsistencies between volcanoes' pre-eruption behavior. For instance, with respect to pre-eruption rumbling noises, some volcanoes rumble constantly while others stop and start. For some, the rumbling indicates a same-day eruption; for others it's months or longer before the eruption. Also, the longer a volcano remains dormant, the greater the time between pre-eruption earthquakes and eruption. A volcano with frequent eruptions provides little warning pre-eruption. Due to this erratic behavior between systems, forecasters at local observatories issue alerts based on the volcano they study and observe. These alerts can vary in length but commonly remain open for a period of 10 to 100 days, according to Brodsky.
Research Problems
False alarms present a major problem for observatories. Although volcanoes make noise before erupting, "they don't erupt every time they make noise," Brodsky states. Another of the indicators, magma, can also behave erratically. Sometimes its movement doesn't lead to an eruption because it cools beneath the surface. Scientists also need to test their assumptions and models against reality. This means future eruptions provide the best test of the accuracy of analytical models of magma movement through a volcano's channels. Volcanoes vary in structure and mineral composition, requiring observatories to tweak forecast models to suit the volcanic system. Finally, the expense of monitoring equipment and personnel precludes studying every volcano in real time.
Tags: silica content, changes Earth, help scientists, magma movement, make noise
