Scientists disagree about the degree to which earthquakes factor as precursors to a volcanic eruption, but in numerous documented cases, seismic activity acts as an indicator for this type of event. As magma moves from chambers deep in the earth towards the surface, pressure and temperature change may move the ground under which it flows. This movement results in earthquakes, sometimes so small they are only detectable with sensitive equipment. Called volcano-tectonic events, these quakes do not result from the movement of tectonic plates along fault lines as in typical earthquakes.
Since the components of magma include dissolved gases, emissions exit the surface at many points during a volcano's history. Gases leak from fumaroles, or cracks along a volcano, at increased levels before an eruption. Chemical makeup of these gases includes steam, chlorine, carbon dioxide and sulfur, among others. Because fumaroles emit gas for years after an eruption, the presence of gas or steam does not necessarily mean that a volcano is imminent. These gas emissions require measurement by scientists to determine the likelihood of a volcanic event.
As magma charts its path toward the surface of a volcano, it produces temperature change in the form of heat. This heat affects materials in its vicinity. Patches of ground on the volcano may become noticeably hot. Water streams near the volcano may also raise in temperature. Finally, the air above the volcano experiences this heating process. As with many precursors of a volcano, this heating can be so subtle that special instruments, like infrared surveys, may prove necessary to detect it.
Although one often thinks of the ground as solid and unmoving, earth around a volcano actually swells outward preceding a volcanic eruption. This swelling results from the increased flow of magma beneath the surface. Creating bulges on the volcano, this swelling may be almost unnoticeable. Scientists must use sensitive equipment to document the elevation of the ground around a volcano over a period of years. Looking for signs such as a change in tilt or swelling, they can measure minute changes over time.
