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.
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.
100th anniversary of the May 22, 1915 explosive eruption
The 100th anniversary of the May 22, 1915 explosive eruption of Lassen Peak was celebrated in the summer of 2015.
The eruption forever altered an already dynamic landscape and led to the creation of a national park which serves as a place of discovery for curious visitors and a living laboratory for a variety of scientists. To find out more about Lassen and the 1915 eruption, visit the links listed below.
- A Sight "Fearfully Grand": Eruptions of Lassen Peak, California, 1914 to 1917—Four-page informational fact sheet detailing the events surrounding the 1915 eruption with historic photographs by Benjamin Franklin Loomis who witnessed the eruption.
- USGS Lassen Volcanic Center Website—USGS-hosted website with information about Lassen's geologic history and potential volcanic hazards. Website also includes current volcano monitoring information, publications, and images.
- Volcano Hazards Assessment for the Lassen Region, Northern California—Detailed assessment of the volcanic hazards associated with possible future activity from the Lassen Volcanic Center region, published in 2012.
- Geologic Map of Lassen Volcanic National Park and Vicinity, California—Comprehensive geologic map of the area within and surrounding Lassen Volcanic National Park, published in 2010.
- Database for the Geologic Map of Lassen Volcanic National Park and Vicinity, California—Geographic Information Systems database containing all the geologic information used to publish the 2010 geologic map plus 48 annotated photographs.
- Lassen Volcanic National Park Website—National Park Service website with information relating to visiting and staying in the park with current conditions.
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.
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.
100th anniversary of the May 22, 1915 explosive eruption
The 100th anniversary of the May 22, 1915 explosive eruption of Lassen Peak was celebrated in the summer of 2015.
The eruption forever altered an already dynamic landscape and led to the creation of a national park which serves as a place of discovery for curious visitors and a living laboratory for a variety of scientists. To find out more about Lassen and the 1915 eruption, visit the links listed below.
- A Sight "Fearfully Grand": Eruptions of Lassen Peak, California, 1914 to 1917—Four-page informational fact sheet detailing the events surrounding the 1915 eruption with historic photographs by Benjamin Franklin Loomis who witnessed the eruption.
- USGS Lassen Volcanic Center Website—USGS-hosted website with information about Lassen's geologic history and potential volcanic hazards. Website also includes current volcano monitoring information, publications, and images.
- Volcano Hazards Assessment for the Lassen Region, Northern California—Detailed assessment of the volcanic hazards associated with possible future activity from the Lassen Volcanic Center region, published in 2012.
- Geologic Map of Lassen Volcanic National Park and Vicinity, California—Comprehensive geologic map of the area within and surrounding Lassen Volcanic National Park, published in 2010.
- Database for the Geologic Map of Lassen Volcanic National Park and Vicinity, California—Geographic Information Systems database containing all the geologic information used to publish the 2010 geologic map plus 48 annotated photographs.
- Lassen Volcanic National Park Website—National Park Service website with information relating to visiting and staying in the park with current conditions.