What are the warning signs of a volcanic eruption?
What are the warning signs of a volcanic eruption?
To learn more about the possibility of a volcanic eruption, we are going to look at Lassen Volcano National Park as our example. Before Mt. St. Helens erupted, Lassen was the last volcano to erupt in the Cascade Mountain range.
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.
Because geologically recent volcanic activity in an area is the best guide to forecasting future eruptions, scientists study the lava flows, ash, and other deposits from past eruptions. Volcanoes in the Lassen area tend to erupt infrequently, and may be inactive for periods lasting centuries or even millennia. The most recent eruptions in the Lassen area were the formation of The Chaos Crags, a group of six lava domes, formed about 1,100 years ago and the latest large eruptions to occur in the area occurred at Lassen Peak between 1914 and 1917. Such large eruptions in the Lassen area have an average recurrence interval of about 10,000 years. However, the geologic history of the Lassen area indicates that volcanism there is episodic, having periods of relatively frequent eruptions separated by long quiet intervals. For example, the last large event before the Chaos Crags eruption was the one that built Lassen Peak 27,000 years ago.
After the eruption of Mount St. Helens in 1980, the U.S. Geological Survey (USGS) intensified its monitoring of active and potentially active volcanoes in the Cascade Range. Monitoring of the Lassen area includes periodic measurements of ground deformation and volcanic gas emissions and continuous transmission of data from a local network of nine seismometers to USGS offices in Menlo Park, California. Should indications of a significant increase in volcanic activity be detected, the USGS will immediately deploy scientists and specially designed portable monitoring instruments to evaluate the threat. In addition, the National Park Service (NPS) has developed an emergency response plan that would be activated to protect the public in the event of an impending eruption.
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