1915: Lassen’s most recent eruption

Loomis photo taken May 22 Lassen Peak is the southernmost active volcano in the Cascade Range, and prior to the eruption of Mount St. Helens in 1980, the 1914-1917 eruption of Lassen Peak was the most recent in the Cascades. Although the eruptions went on for 3 years, the activity culminated during a week of May, 1915, and produced the Devastated Area on the northeastern flank of Lassen Peak. This eruption was one of the first to be extensively photographed. Thanks largely to Benjamin Franklin Loomis, a local businessman and amateur photographer, there is a rich photographic legacy of the 1914-1917 eruptions. One of his photographs, displayed at the right, has proven particularly invaluable in deciphering the events of May, 1915.
Lassen Peak is the largest of a group of more than 30 lava domes in the Lassen domefield. Such domes are near-vent accumulations of highly viscous lava. About 27,000 years ago, Lassen Peak formed over a short time, probably no more than a few years. Its 2,000-foot height and nearly 2-cubic kilometer volume make it one of the largest lava domes in the world. When it formed, Lassen Peak looked much like the Chaos Crags domes do today-- covered with angular rock talus and with oversteepened flanks. During the last ice age, 18,000 to 25,000 years ago, its shape was significantly modified by glacial erosion. A tiny reminder of one of these glaciers survives on Lassen’s eastern flank. The depression on the northeastern flank of Lassen Peak is a glacial cirque, where ice accumulated to form a glacier 10 kilometers long.


Initial Rumblings

On May 30, 1914, Lassen Peak awoke from its long repose when it was shaken by a steam-blast explosion. Volcanic steam-blasts do not erupt magma directly but, rather, are explosively expanding steam. formed by the heating of shallow ground water. The steam rises under pressure through cracks and, upon reaching the surface, decompresses and vents explosively. Over the year from May 30, 1914 to May, 1915, more than 180 steam explosions were recorded, and many more went unrecorded. They blasted a crater in the summit area of Lassen Peak that gradually enlarged to about 1,000 ft across. On the evening of May 14, 1915, incandescent blocks of lava were seen bounding down the flanks of Lassen Peak from as far away as Manton, 30 kilometers to the west. The next day a black craggy mass could be seen poking over the crater rim. It was a growing dome of dacite lava that had welled up into the crater, filled it, and was spilling over the edge. That lava was the first indication of an important change in the character of the activity; from steam-driven to magma-driven. Remnants of this lava dome are preserved near the summit of Lassen Peak.


Eruption of May 19-20, 1915

The winter of 1914-1915 (an El Nino year) had been particularly wet and 30 ft of snow, nearly twice the normal amount covered the northeast flank of Lassen Peak. After growing for 5 or 6 days, late in the evening of May 19, a large steam explosion blasted a new crater through the lava dome. No new magma was ejected in this explosion, but blocks of the incandescent lava dome fell on the summit area and snow-covered upper flanks of Lassen Peak, generating an avalanche of snow and lava blocks that roared 5 kilometers down an 0.8 kilometer-wide path straight over a low divide at Emigrant Summit into Hat Creek. Fragmentation of the avalanching hot lava blocks melted snow to generate mudflows that quickly followed the avalanche. The bulk of the mudflows were deflected by Emigrant Summit into Lost Creek, flowing 10 km down Lost Creek before coming to rest. The avalanche and mudflows released water to create a flood in Hat Creek Valley in the early morning hours of May 20. On the same night of May 19-20, new dacite lava, which was more fluid that the dome lava, emerged from the now uncovered vent, spilled over low spots on the crater rim, and flowed 1,000 feet down the steep west and northeast flanks of the volcano. The northeast lobe of this lava flow can be seen in upper photograph on the previous page, taken on the morning of May 22.


Climactic Eruption of May 22, 1915

After two quiet days, the volcano exploded again. Late in the afternoon of May 22, an ash laden column of volcanic gas blasted out of the new crater at the summit. The column reached 30,000 ft and was visible from as far away as Eureka, on the California coast. A heavy fall of pumice from the eruption column onto the northeastern slope of Lassen Peak generated a hurricane velocity flow of hot pumice, volcanic ash and gases that swept over still snow-covered parts of the northeast flank. Incorporation and rapid melting of snow transformed the hot ash flow into a highly fluid mudflow that rushed down Lost Creek and lower Hat Creek nearly 15 kilometers to Old Station. This eruption swept away the northeast lobe of the lava flow erupted two days earlier on the steep cirque of Lassen Peak (compare photographs on previous page). Continued eruption and fallout deposited a lobe of pumice and ash traceable for 40 kilometers to the northeast and produced smaller mudflows on all flanks of Lassen Peak.


Waning Activity and Prospects for the Future

Each spring for the next few years after 1915, percolating snowmelt triggered steam explosions signaling that the rocks beneath the volcano remained hot. Particularly vigorous explosions in May 1917, blasted an additional crater west of the summit of Lassen Peak. Steam vents could be found around the crater areas into the 1950s but gradually waned and are difficult to locate today. Lassen Peak sleeps again, but the geothermal features and young volcanoes of Lassen Volcanic National Park attest to its continuing volcanic legacy. No one can say when, but it is highly likely that the Lassen area will erupt again.


Summary of the 1914-1917 Eruption of Lassen Peak

May 30, 1914 to mid May 1915: steam explosions blast a crater at the summit of Lassen Peak

Mid-May 1915: a small lava dome fills the new crater; late in the evening of May 19 a large steam explosion disrupts the dome, generating an avalanche and mudflows; the reopened vent erupts a lava flow

Afternoon of May 22: Pumice fall from a high vertical eruption column creates a hot ash flow and mudflows; continued fallout of hot pumice on snow generates additional mudflows, and a fallout lobe to the NE

May 1917: renewed steam explosions blast a new crater; vigorous steaming continues into the 1920s


Photo captions:

Comparison of the effects of the May 19 and 22, 1915 eruptions of Lassen Peak. Both pictures were taken from the same location by B.F. Loomis. The upper picture, taken on the morning of May 22, shows the path of the avalanche from the May 19 eruption. The large rock in the foreground is a piece of the lava dome that filled the crater during the week before May 19 and was carried to its present location by the avalanche. Today, you can see it on the Devastated Area Interpretive Trail. Note the tree-covered slopes at the left and right margins of the avalanche and the black tongue of new lava erupted from the reopened vent.

The lower picture was taken in June, 1915. Trees that were standing on the morning of May 22 were blown down or snapped off by the May 22 eruption, and their trunks left lying on the ground, pointing away from Lassen Peak. The lava flow below the summit is gone, and additional mudflows smoothed the surface of the May 19 avalanche deposit in the middle ground.


Michael A. Clynne, Robert L. Christiansen, Tracey Felger, Peter Stauffer and James Hendley.

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This page was last updated on 5/10/99