Located in the Shasta Cascade region of Northern California, Lassen rises 2,000 feet above the surrounding terrain and has a volume of half a cubic mile, making it one of the largest lava domes on Earth. It was created on the destroyed northeastern flank of now gone Mount Tehama, a stratovolcano that was at least a thousand feet higher than Lassen.

Lassen Peak has the distinction of being the only other volcano in the Cascades besides Mount St. Helens to have erupted during the 20th Century. On May 22, 1915, an explosive eruption at Lassen Peak 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 Mount St. Helens in Washington.

Lassen Volcanic National Park was created in Shasta County, California to preserve the devastated area and nearby volcanic wonders.