Mount Adams – Which Hasn’t Erupted Since The Stone Age – May Be Stirring

The largest volcano in Washington state has shown a flurry of seismic activity in recent weeks, having previously remained virtually silent for thousands of years. Officials from the US Geological Survey (USGS) are now installing temporary seismic stations around Mount Adams in order to monitor the situation and determine whether the sleepy giant is likely to erupt.

Despite being second in height to Mount Rainier, Mount Adams is Washington’s largest active volcano in both area and volume. According to a USGS statement , the most recent eruption at the site occurred some time between 3,800 and 7,600 years ago, when humanity was still in the Stone Age.

Since scientists began monitoring the volcano in 1982, earthquakes have been observed at a rate of about one every two to three years. However, during the month of September, the Cascades Volcano Observatory (CVO) and Pacific Northwest Seismic Network (PNSN) detected six earthquakes at Mount Adams – the most ever seen in a single month.

All of the quakes were small – ranging between magnitudes of 0.9 and 2.0 on the Richter scale – and none were felt at the surface. Satellite imagery has also confirmed that no ground deformation occurred in the vicinity of the volcano following the tremors.

“Currently, there is no indication that the level of earthquake activity is cause for concern,” explains the USGS, adding that “the alert level and color code for Mount Adams remain at GREEN / NORMAL.”

Given the volcano’s unusual chattiness, however, the agency is wisely increasing its monitoring capacity around Mount Adams with the installation of extra recording equipment. “This will improve our ability to locate smaller earthquakes with more certainty and assist in understanding the cause of these earthquakes. The results of our findings will determine if any additional actions are needed,” writes the USGS.

At present, it’s unclear if the earthquakes are portents of a more significant awakening or just a random anomaly. If Mount Adams does erupt, however, it’s unlikely to be an explosive event as the majority of the volcano’s historic activity has involved short-traveling lava flows rather than spectacular blow-outs.

Four such lava flows are believed to have occurred in the past 12,000 years, none of which made it beyond a few miles of Mount Adams. According to the USGS, the biggest threat to local residents are muddy flows of rock, ash, and ice called lahars that can occur during both eruptive and non-eruptive episodes.

Occasionally, these lahars can travel a great distance from the volcano itself, with far-reaching flows thought to have occurred 6,000 and 300 years ago. “The ice-capped summit conceals large volumes of hydrothermally weakened rock, and future landslides of this weakened rock could generate far-traveled lahars,” continues the USGS statement.

It's for this reason that Mount Adams is categorized as a high-threat volcano, although this ranking refers more to the damage that an eruption could cause to local populations than to the actual risk of activity.

For the time being, the CVO and PNSN say they will “continue to monitor earthquake activity and release further updates as the situation warrants.”