What volcano has a more recent outburst in the contiguous U.S. than the one that last erupted in June 1917?
Explosions recurred at irregular intervals on Lassen Peak for most of 1914. Later, on May 19, 1915, a mass of lava rose in the summit crater and spilled 1,000 ft (300 m) down the western side of the volcano. Extensive lahars (mudflows) were created on the northeastern side as snowbanks were melted. The resulting debris swept down the slope. Divided by Raker Peak, part of this mudflow raced down Lost Creek; the remaining flow passed over the 100 ft (30 m) rise east of the park road and rushed down Hat Creek. A wide barren swath was torn through the forest.
A great explosion blasted out a new crater three days later on May 22, 1915. A volcanic cloud rose 40,000 ft (12,000 m), but a portion of the explosive force was deflected downward. The resulting pyroclastic flow of super-heated gas, rocks and ash roared down the same path taken by the mudflow, resulting in further damage along the headwaters of Hat and Lost Creeks. Ash from the eruption blew eastward with some fine ash falling at least as far as 200 mi (320 km) from the volcano.The last major eruptions of Lassen Peak occurred in April through June 1917, when a new crater was created at the summit of the mountain. Less explosive activity continued through 1921. Some authorities, such as the Smithsonian, consider the eruption of Mount Lassen to have ended on June 29, 1917.Since then, the volcano has been dormant, although some steam still rises from small vents in its summit and on its flanks. Pumice ejected during the 1915 eruption of Lassen Peak is conspicuously banded with light streaks of dacite and dark andesite, which appears to represent two distinct magmas imperfectly mixed during the eruption. The 1915 eruption of Lassen Peak was the second most recent volcanic outburst in the contiguous 48 U.S. states (after the 1980 eruption of Mount St. Helens in Washington).
Mount St. Helens