Lassen Peak's Last Eruptions

B.F. Loomis witnessed the spectacular eruptions of Lassen and took numerous photographs to chronicle these events. He climbed to the top of Lassen Peak six times and brought back eye-witness descriptions and amazing pictures to tell the tale of the series of eruptions that occurred during the summer of 1914 and into 1915. Some of these eruptions lasted only moments with puffs of smoke and ash while others lasted up to half an hour and included flying rocks weighing up to 100 pounds, hurled down the slopes. Lance Graham, one of a party of five explorers that climbed the mountain to see its new crater, was hit by one of these catapulted rocks which cut his shoulder and broke his collar bone when he and his party were caught in the eleventh of these eruptions.

The crater’s size was increasing with each eruption and by October of 1914, measured approximately 700 feet long, 350-400 feet wide, and up to 40 feet deep. Several more minor eruptions occurred through the winter, leading up to the "Great Eruptions of 1915." On May 21, 1915, a great flood of water and mud flowed down the east side of the Peak towards Hat Creek and Lost Creek, tearing trees out of the ground as well as rocks and debris. This mud flow, or lahar, was measured at up to 18 feet deep in some places as evidenced by the line of mud and peeled off bark on the trees that remained standing in its path. About half the water and mud flow went down Lost Creek drainage and the other half went down Hat Creek. Many huge rocks, some that were hot, were ejected from the crater and carried down by the torrent of raging water, mud, logs, and debris. One of these hot rocks was deposited on the rise between the Lost and Hat Creek drainages and is a 30-ton boulder that remained hot and sizzling in the water for several days.

The following day, May 22, 1915, a great steam blast known as a "nuee ardente" erupted from Lassen Peak. This was the result of a sudden release of the pressure that had been sealed in by the hardening of the crater lava flow from the eruption a few days earlier. This was Lassen’s "mightiest roar," spewing great hot boulders as large as small cabins with smoke and steam vapors rising to a height of 30,000 feet. As the column of steam and ash peaked, part of it collapsed back towards the crater and over the edge of the volcano as a pyroclastic flow, or a searing hot cloud of gas, which completely leveled the trees in its path. This horizontal steam blast with a high internal velocity, was similar to the "nuee ardente" which destroyed the city of St. Pierre on the island of Martinique as it came down the slope of Mt. Pele in the West Indies. The hot steam shot out of the east side of the mountain in a horizontal blast that had blown down all of the trees spanning about one mile down the head of Hat Creek. The Forest Service estimated that 4-5 million feet of timber was destroyed by the mud flow and the hot steam blast.

A volcano is considered to be active if it has erupted at least one time within recorded history. Until Mount St. Helens erupted in 1980, Lassen peak was the only active volcano in the continental United States. Professor J. S. Diller of the United States Geological Survey, concluded that "Mount Lassen was tiring out" and had "done its worst" in his report dated July 26, 1915. In the fourteen months of its fiery activity, Lassen had 107 distinct eruptions during the 1914-1915 period. Diller proclaimed the great eruption of 1915 was the culmination of "Lassen’s brief career as a revived volcano...the peak is not dead, but dying." Professor Diller’s opinion, gathered from all indications, was that the volcano was in an obvious decline of activity. Sporadic eruptions continued until 1921 and today, though Lassen Peak has been "quiet" since then, evidence of volcanic life still exists in its hydrothermal activity.