Lassen Peak, also known as Mount Lassen, is the southernmost active volcano in the Cascade Range. It is part of the Cascade Volcanic Arc which is an arc that stretches from southwestern British Columbia to northern California. Located in the Shasta Cascade region of Northern California, Lassen rises 2,000 ft (600 m) above the surrounding terrain and has a volume of 0.5 cu mi (2 km³), 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 1,000 ft (300 m) higher than Lassen Peak.

On May 22, 1915, a powerful explosive eruption at Lassen Peak devastated nearby areas, and volcanic ash rained down as far as 200 mi (300 km) to the east. This explosion was the most powerful in a series of eruptions from 1914 through 1917. Lassen Peak and Mount St. Helens – which had a massive eruption in 1980 – are the only two volcanoes in the continental United States to erupt during the 20th century. Lassen Volcanic National Park was created in Shasta County, Calif., to preserve the devastated areas as they were, for future observation and study, and to preserve the nearby volcanic features.