On May 22, 1915, an explosive eruption at Lassen Peak, California, the southernmost active volcano in the Cascade Range, devastated nearby areas and rained volcanic ash as far away as 300 km (about 200 mi) to the east. This explosion was the most powerful in a 1914-17 series of eruptions that were the last to occur in the Cascades before the 1980 eruption of Mount St. Helens, Washington.
The 1914–17 eruption is the most recent eruption in the Twin Lakes sequence. It comprises a complex eruptive sequence consisting of adacite dome and lava flow, dacite pyroclastic flow and fall deposit, and phreatic eruption, debris-flow (lahar)and flood deposits.
The eruptive sequence began on May 30, 1914 with a phreatic explosion at the summit of Lassen Peak. During the following year, 180 steam explosions had blasted out a 300-m-wide (nearly 1000 ft) crater at the summit. By mid-May of 1915, the eruption changed in character; a lava dome appeared in the summit crater and subsequently flowed about 100 m (328 ft) over the west and probably over the east crater walls. On May 22, an explosive eruption produced a pyroclastic flow that devastated an area as far as 6 km (3.7 mi) northeast of the summit (known today as the Devastated Area at Lassen Volcanic National Park). The eruption also generated lahars that traveled more than 20 km (12.4 mi) down Lost Creek and floods that went down Hat Creek. A vertical eruption column resulting from the pyroclastic eruption rose to an altitude of more than 9.5 km (6 mi) above the vent and deposited a lobe of pumiceous tephra that can be traced as far as 30 km (18.6 mi) to the east-northeast. The fall of fine ash was reported as far away as Winnemucca, Nevada, more than 325 km (200 mi) east of Lassen Peak. Intermittent eruptions of variable intensity continued until about the middle of 1917.