A Sight Fearfully Grand: The Eruptions of Lassen Peak, 1914–1917

The eruption column of May 22, 1915, seen from Red Bluff, 40 miles west. It rose more than 30,000 feet. Photograph by R.E. Stinson; courtesy of the National Park Service.

The volcano

Lassen Peak, in Lassen Volcanic National Park, is the southernmost active volcano in the Cascade Range and the largest of more than 30 lava domes erupted in the area over the past 300,000 years — steep piles of lava too stiff to flow away.

• It formed about 27,000 years ago, probably within a few years, rising more than 2,000 feet above its surroundings with a volume of half a cubic mile — one of the largest lava domes on Earth.
• At first it would have looked like the nearby Chaos Crags, 1,100 years old, with steep sides of broken rock. From 25,000 to 18,000 years ago glaciers reshaped it; one carved the bowl on its northeast flank and reached 7 miles from the dome.

Its 1914–17 eruptions were the last in the Cascades before Mount St. Helens in 1980. A century later, USGS scientists working with the Park Service have pieced together what really happened — correcting early accounts that missed key evidence in the deposits and misread the photographs.

Initial rumblings, 1914

On May 30, 1914, Lassen Peak woke from a 27,000-year sleep with a steam explosion — magma rising toward the surface had heated shallow groundwater until it flashed to steam. More than 180 more steam blasts followed over 11 months, enlarging the summit crater to about 1,000 feet long.

The new summit crater on June 20, 1914, then about 250 feet long. Photograph by B.F. Loomis.

A steam explosion on June 14, 1914, seen from west of Manzanita Lake, about 6 miles away. Such blasts lasted minutes to hours and sent plumes thousands of feet high. Photograph by B.F. Loomis.

May 14–20, 1915: dome, avalanche, lahar

• On the evening of May 14, 1915, glowing blocks of lava could be seen bouncing down the peak from Manton, 20 miles west. By morning a dome of dacite lava (63–68% silica) filled the crater.
• Late on May 19, an explosion shattered the dome. Hot blocks fell on snow more than 30 feet deep — the winter of 1914–15 was the first recorded El Niño year in the western United States — launching a half-mile-wide avalanche of snow and rock 4 miles down the flank and over Emigrant Pass into Hat Creek.
• As the hot rock melted the snow, the avalanche became a lahar (volcanic mudflow) that ran 7 miles down Lost Creek. Its water flooded lower Hat Creek Valley early on May 20, damaging ranch houses near Old Station — the few people there escaped with minor injuries — and ran more than 30 miles to the Pit River, killing many fish.
• That night new lava filled the crater and spilled down the west and northeast flanks in two lobes.

Deposits from the May 1915 eruptions. USGS.

The climactic eruption, May 22, 1915

After two quiet days, at about 4:00 p.m. on May 22, Lassen Peak exploded.

• The blast carved the larger of the two craters at the summit today.
• Within 30 minutes an ash column rose more than 30,000 feet, visible from Eureka, 150 miles west.
• Part of the column collapsed onto the northeast slope as a pyroclastic flow — a racing mix of hot ash, pumice, rock and gas — that devastated 3 square miles. Melting snow turned it into a fast lahar that rushed 15 miles down Lost Creek nearly to Old Station and flooded Hat Creek again.
• It swept away the northeast lava lobe but left the west lobe, sent small mudflows down every flank, laid pumice and ash 25 miles to the northeast, and dropped fine ash as far as Elko, Nevada, 280 miles east.

The northeast flank before 1914; at noon on May 22, 1915, after the May 19–20 avalanche and lahar uprooted 100-foot trees; and in June 1915, after the May 22 blast. The "hot rock," a piece of the lava dome, can still be seen on the Devastated Area Interpretive Trail. Photographs by B.F. Loomis.

Lassen Peak from the west shortly after May 22, 1915. Hot pumice falling on snow set off seven small mudflows. Photograph by B.F. Loomis.

The photographer

The eruptions were among the first after photography became widely available to amateurs, and the San Francisco Chronicle and other papers made them famous. Benjamin Franklin Loomis (1857–1935), a local businessman and supporter of the park, photographed them with an 8×10-inch view camera and glass plates he coated himself just before each exposure. He left the most complete record of the events, much of it in his 1926 book Pictorial History of the Lassen Volcano. Seeing the peak erupt on June 14, 1914, he wrote: "The sight was fearfully grand."

Afterward

• For years, spring snowmelt seeping into the hot volcano set off steam explosions; strong blasts in May 1917 made the northern summit crater. Steam vents lingered into the 1950s.
• The 1915 eruptions totaled only about 0.007 cubic mile — tiny beside Mount St. Helens' 0.24 — and their thin deposits are fast disappearing under plants and erosion. The Devastated Area changed little for 50 years; young trees are now reclaiming it.
• Lassen sleeps, but steam vents, hot springs and mudpots remain elsewhere in the park, and future eruptions are almost certain. The USGS California Volcano Observatory monitors earthquakes, ground deformation and gas to catch magma rising early.

Sources

Based on Michael A. Clynne, Robert L. Christiansen, Peter H. Stauffer, James W. Hendley II and Heather Bleick, "A sight "fearfully grand": eruptions of Lassen Peak, California, 1914 to 1917," U.S. Geological Survey Fact Sheet 2014–3119 (2014), with the Lassen Association, the National Park Service and the U.S. Forest Service; a work of the United States government in the public domain. The map and the historical photographs of 1914–1915 are taken from the fact sheet's PDF.