California’s only active volcano, Lassen Peak last gargled violently to life during 1915 when it spewed out five-ton boulders, smothered the surrounding countryside in a 20-ft-deep mud flow and threw a cloud of dust seven miles high. Unlikely to erupt again for many decades, Lassen Peak is the tallest of several peaks within Lassen Volcanic National Park, which fills a 165,000-sq mile caldera (a type of crater) created by the collapsing dome of a gigantic prehistoric volcano called Mount Tehama.

Even though the chances of another eruption may be slim, the sights (and smells) of Earth’s primeval forces, which are abundant throughout the park, provide a lasting reminder that California’s foundations are far from stable. Hwy-89 steers a 35-mile course through the park and the major points of geological interest are indicated by numbered markers corresponding to explanatory descriptions in the free leaflet distributed at the park entrance. Places not to be missed include Bumpass Hell, a seething landscape of hot springs and bubbling mud pits created as pressured gases (responsible for the all-pervasive bad-egg smell) come hissing to the surface through gaps in the lava, and the Devastated Area, where strands of young trees are slowly reclaiming a landscape razed of all vegetation by the mud flows and gases unleashed during the 1915 eruption. To the north of the Devastated Area, you’ll spot the 400-ton boulder named the Hot Rock by locals who were pleased not to be underneath it when it landed here during Lassen’s 1915 eruption; the gigantic rock remained warm to the touch for many years.

Lassen Peak itself can be reached by a 2½-mile zigzag trail, which makes the steep gradient comparatively easy going (beware, though, of altitude sickness). Allow at least five hours to go up, come down, and have time to savor the view of the park’s volcanic phenom­ena from the peak’s 10,475-ft summit.

Detour – the Lassen Backcountry

A route into the northern backcountry of Lassen Volcanic National Park branches off Hwy-44 and concludes at Butte Lake. From the lake, the Cinder Cone Nature Trail makes for a rewarding, if potentially ankle-twisting, trek through loose rock, passing the aptly-named Fantastic Lava Beds and Painted Dunes – multi-colored lava flows created over a 2,000-year period, on the way to the top of 6,907-ft high Cinder Cone.