According to http://www.lassen.volcanic.national-park.com/info.htm#geo, ”In May 1914, Lassen Peak burst into eruption, beginning a 7-year cycle of sporadic volcanic outbursts. The climax of this episode took place in 1915, when the peak blew an enormous mushroom cloud some 7 miles into the stratosphere.” This was the most recent eruption in the continental US until Mt. St. Helen’s erupted in 1980.
By dating rocks from varying flows, USGS geologists have determined that Mt. Shasta erupts every 600-800 years on average and the last eruption appears to have occurred in 1786.
Both stratovolcanoes are part of the Cascade Mountain Range, which is part of the Pacific Ring of Fire that denotes a plate boundary. Both have steam vents and fumoroles, but while Shasta is the highest peak, Lassen receives the most snow on average. If you go by dates of eruptions (which is not an absolute science at all) Shasta would be due for an eruption in the next 400-600 years and Lassen would expect one in the next 500-700.
But something to consider is how the snow atop Mt. Lassen might be affecting the volcanic activity. If it is, then global warming could increase the probability of Lassen erupting much sooner as the ice melts and stays away. Unfortunately, current data cannot allow us to predict which will erupt first.
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