After 1915, steam explosions continued for several years, indicating extremely hot rock beneath Lassen Peak's surface. In May 1917, an especially strong steam explosion formed the northern crater on Lassen Peak's summit, with eruptions lasting two days and producing an ash cloud that extended into the sky. June saw 21 additional explosions reported, further transforming the crater and creating a new vent on Lassen Peak's northwestern summit. In June 1919, steam eruptions occurred, and similar activity was observed on April 8 and April 9 in 1920, followed by steam eruptions lasting 10-12 hours in October of the same year. During February 1921, white steam erupted from eastern fissures on the volcano. In total, about 400 eruptions were observed between 1914 and 1921, which were the last eruptions in the Cascades before the 1980 eruption of Mount St. Helens, which was the only other volcanic eruption in the contiguous United States during the 20th century.