Mount Lassen was once a vent on the flank of the ancient Mount Tehama, believed to have been 11,000 feet in elevation with a base 11 miles wide. It has been thousands of years since the collapse and erosion of Tehama, but remnants of its existence still remain. Bumpass Hell is believed to be the location of Mount Tehama's central vent.

Lassen is the southernmost volcano in the Cascades Range and Pacific Ring of Fire. Lassen is a plug dome volcano which means it was formed by an extremely rigid and viscous lava flow that solidified as it cooled, causing a "plug" in the volcanic vent.

The hike to Lassen Peak is about 5 miles round trip with an elevation gain of 2,000 feet. Due to minimal shade, it's best to start early in the day with plenty of water and something warm to wear if it happens to be windy at the top.