A dormant plug dome, Lassen Peak is the focus of Lassen Volcanic National Park. The peak began erupting in May 1914, sending pumice, rock, and snow thundering down the mountain, and gas and hot ash billowing into the atmosphere. Lassen's most spectacular outburst was in 1915 when it blew a cloud of ash almost 6 miles high. The resulting mudflow destroyed vegetation for miles in some directions;
the evidence is still visible today, especially in Devastated Area. The volcano finally came to rest in 1921.