Geology of the Lassen Country
The Bumpass Hell Trail is one of the most popular destinations in the Lassen Country. The trailhead is at the large parking area 0.25 mile (0.4 km) east of Lake Helen. The trail traverses the dacite of Bumpass Mountain for the majority of the hike. Also, note the well-preserved glacial polish and striae in the dacite alongside the first few hundred yards of the trail. They were created by the glacier that started from the Lake Helen cirque. At the viewpoint where the trail turns east, about 0.5 mile (0.8 km) from the trailhead, it crosses briefly into andesite of Mount Diller. Here a National Park Service trailside interpretive display shows a model of Brokeoff Volcano before erosion.
Trailhead: Bumpass Hell trailhead parking lot
Distance: 3.0 miles (4.8 km) round trip
Key Geologic Features: Hot springs, clay-altered bedrock
At 0.9 mile (1.4 km) the trail comes to an overlook that provides a great view of Bumpass Hell. In late spring and early summer, when the trail usually opens, steam can be seen rising off the hot pools. Rising steam is always present but in the summer it is not as visible. Note the discolored water and rocks turned to clay by the acidic hydrothermal waters.
From the overlook the trail descends 160 feet (49 m) in 0.6 mile (1 km) through dacite of Bumpass Mountain explosion breccia into the Bumpass Hell basin. During the descent, the stench of hydrogen sulfide, characterized by its rotten egg smell, will be apparent.
The National Park Service has constructed boardwalks that safely lead hikers through the hydrothermal area. What appears to be an area safe to walk on is often underlain by scalding hot water beneath a thin crust that cannot support humans. Therefore, for your safety and to prevent damage to the delicate features in the hydrothermal area, PLEASE stay on the boardwalks.
Much of the volcanic bedrock underlying the 0.05 square mile (0.13 square km) hydrothermal area is strongly altered to opal (water-bearing silica) and kaolinite, a white clay mineral. The orange and yellow surface coatings are sulfate minerals. Many steam vents are lined with native sulfur. Pyrite can be seen as black scum floating on the surface of the pools, vent and discharge channels, and dispersions in the black or gray mud pots.