The most important sign of an impending volcanic eruption is seismic activity beneath the volcanic area. Seismologists can interpret subtle differences between earthquakes related to the rise of magma and the more familiar quakes caused by tectonic faulting. Other warning signs of magma rising into the shallow subsurface might include increased release of volcanic gases from small openings called fumaroles, such as those found in the Bumpass Hell area of Lassen Volcanic National Park, and changes in the gas composition. Deformation of the ground surface in the vicinity of a volcano may also indicate that magma is approaching the surface. Typically, these warning signs appear a few weeks to months before an eruption, but can last for decades or even centuries without leading to an eruption.
VOLCANO HAZARD ZONES OF THE LASSEN VOLCANIC NATIONAL PARK REGION
