The Cascade Mountains form the northern portion of the Cascade-Sierra province. The Cascades were created from thousands of small, short-lived volcanoes along the Cascadia subduction zone that over millions of years built a foundation of lava and volcanic debris on which the mountains rise. This range includes thirteen major volcanic centers and are part of the Pacific Ocean's Ring of Fire. The two most recent eruptions are the Mount Lassen eruptions from 1914 to 1921, and the Mount St. Helens eruption in 1980.