On May 22, 1915, an explosive eruption at Lassen Peak, the southernmost active volcano in the Cascade Range, devastated nearby areas and rained volcanic ash as far away as 200 miles 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 Mt. St. Helens.

Lassen Peak in Lassen Volcanic National Park, northern California, is the largest of a group of more than 30 lava domes erupted in that area during the past 300,000 years. These steep-sided accumulations of volcanic rock were created by the pushing up of lava too viscous to flow away from its source. Eruptions 27,000 years ago formed Lassen Peak, probably within a period of only a few years. Standing more than 2,000 feet above its surroundings and having a volume of half a cubic mile, it is one of the largest lava domes on Earth.

Plug dome volcanoes are formed by nonexplosive outpourings of viscous lava that piles up around a vent, but can be preceded or followed by explosive eruptions. Domes commonly occur within a crater or along the flanks of larger composite volcanoes. Growth occurs largely from within by expansion of lava that is too thick to flow. As it grows, the outer surface cools and hardens, then shatters, spilling lose fragments down its sides. At 10,457 ft, Lassen Peak is one of the largest plug dome volcanoes on earth. A smaller dome formed inside Lassen’s crater. During its last eruption, a large explosion shattered the dome causing hot blocks of lava to fall from the peak, creating the Devastated Area.

Credits and Sources:

“A Sight “Fearfully Grand”—Eruptions of Lassen Peak, California, 1914 to 1917,”http://pubs.usgs.gov/fs/2014/3119/downloads/fs2014-3119.pdf, Accessed on June 29, 2015.