A 1915 ash column from a California volcano rose 30,000 feet and drifted 200 miles to Nevada

On May 22, 1915, an explosive eruption at Lassen Peak in Northern California sent a vertical column of ash 30,000 feet into the sky, and prevailing winds carried its fallout 200 miles east to Winnemucca, Nevada. The blast came roughly a year into a sequence that began with a single steam explosion in 1914 and closed out, in 1917, the last eruption the Cascade Range would see before Mount St. Helens in 1980.

Lassen Volcanic Center still carries a Very High threat ranking from the U.S. Geological Survey today, the same tier assigned to Mount Rainier and Mount St. Helens, largely because 1915 sits well within the geologic blink of an eye that hazard planners use to judge risk. Snowmelt triggered by the eruption’s heat sent mudflows racing 20 to 30 miles down mountain drainages even as the ash cloud drifted toward Nevada, a combination that still shapes how officials think about the volcano’s next episode.

The steam blasts that built toward a lava dome

The 1915 explosion did not come out of nowhere. Eruptions of Lassen Peak, 1914 to 1917 began on May 30, 1914, with a single phreatic blast, and over the following year 180 steam explosions blew out a crater roughly 300 meters wide at the summit. By mid-May 1915, a lava dome had pushed up through that crater and flowed about 100 meters over its walls, a shift from steam-driven blasts to molten rock breaching the surface that USGS treats as the eruption’s turning point toward its climax.

The vent’s record for May 22 itself is stark: a vertical column of ash exploded from the vent reaching altitudes of 30,000 feet, and ash from the top of that column drifted downwind 200 miles to the east, as far as Winnemucca, Nevada, according to USGS’s hazard summary for the site. Pyroclastic flows raced down Lassen’s flanks the same day, devastating ground as far as 6 kilometers northeast of the summit, an area still called the Devastated Area more than a century later.

A hot rock and a lava show visible 20 miles away

USGS’s own account of the eruption’s final stretch reads almost like a field diary. On May 14, 1915, eight days before the big blast, incandescent blocks of lava could be seen bouncing down the flanks of Lassen Peak from as far away as Manton, 20 miles to the west, the agency’s centennial retrospective says. Days later, ejecta landed close enough to the vent that one boulder — later nicknamed the Loomis Hot Rock after Benjamin Franklin Loomis, the photographer who documented it — stayed too hot to touch for several days after the eruption.

Melting snow did its own damage on both eruption days. Lahars raced more than 20 kilometers down Lost Creek and backed floodwater into Hat Creek downstream, part of a mudflow reach USGS’s broader hazard summary puts at 20 to 30 miles from the volcano.

The 1914-17 sequence was not one clean event but a mix of nearly every hazard a volcano can produce in a single stretch. USGS’s science page for the eruption describes it as “a complex eruptive sequence consisting of a dacite dome and lava flow, dacite pyroclastic flow and fall deposit, and phreatic eruption, debris-flow, and flood deposits.” The damage was visible and lasting enough that it helped push Congress to establish Lassen Volcanic National Park around the mountain, preserving the Devastated Area as a permanent record of what the eruption did to the landscape.

Hundreds of eruptions packed into 825,000 years

Lassen’s 1915 outburst was not an isolated event; it was the latest entry in a very long ledger. The Lassen Volcanic Center has experienced hundreds of eruptions scattered over about 500 square kilometers during the last 825,000 years, with more than 50 effusive eruptions in just the last 100,000 years, a pace that traces back to the Juan de Fuca oceanic plate sliding beneath the North American continent at the southern edge of the Cascade Range. The area then sat relatively quiet for roughly 25,000 years before three eruptions marked its return to activity: Chaos Crags roughly 1,100 years ago, Cinder Cone around 1666, and the Lassen Peak sequence of 1914 to 1917 itself.

That subduction, described in USGS’s geologic history of the Lassen segment, is the same mechanism driving volcanism along the length of the Cascades, which is why Lassen shares its Very High threat tier with peaks hundreds of miles to the north rather than standing alone as a California curiosity.

A threat rating that has not softened with a quiet century

None of that history is settled science filed away in an archive. USGS’s National Volcano Early Warning System still lists Lassen Volcanic Center as Very High threat potential, a ranking driven partly by a legacy the 1915 eruption left behind: a vigorous geothermal system, numerous hot springs, steam vents, and boiling mud pots that remain active across the park today, evidence that the heat driving the 1915 dome never fully went away.

The agency’s own read on the stakes has not eased with a hundred quiet years. USGS’s California Volcano Observatory put it plainly in its centennial account of the eruption: “one hundred years ago few people lived in the Lassen area; today a similar eruption would have a far more devastating effect on people’s lives and the economy of northern California.”

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.

More from Morning Overview