Dacite of Mount Helen near Lassen Peak

Dacite in a road cut just below the Lassen Peak parking lot.

While at Lassen Volcanic National Park last month, we pulled over on our way back over the summit of Highway 89 to examine a roadcut that had grabbed my attention earlier in the day. A pullout overlooking Lake Helen on the west side of the road, not far south of the Lassen Peak parking lot, is a great place to stop; see this nearly identical Google Street View view of the same roadcut.

Closer view of the dacite.

Turns out that this is the dacite of Mount Helen, containing what are described here as mafic, phenocryst-poor inclusions. The inclusions are locally quite large and form as much as 20 percent of the rock. It is these large inclusions that grabbed my eye on our first pass through the area. The inclusions or xenoliths were probably plucked from the walls of the dacitic magma chamber or from the volcanic throat through which the dacite erupted.

An even closer view.

The dacite of Mount Helen is middle Pleistocene in age (K-Ar date of 249±12 ka, Clynne and Muffler, 2010) and part of what is called the Bumpass sequence of the Lassen volcanic center, the youngest of several volcanic centers of the area. The several volcanic centers are shown in Figure 4 here, and described more extensively here.

Portion of the Geologic map of Lassen Volcanic National Park and vicinity, California, Sheet 1, courtesy USGS.

The dacite of Mount Helen is indicated as "dh" on the geologic map. Photos were taken just downhill and west of the peak of Mount Helen.

Selected Reference:

Clynne, M.A., and Muffler, L.J.P., 2010, Geologic map of Lassen Volcanic National Park and vicinity, California: U.S. Geological Survey Scientific Investigations Map 2899, scale 1:50,000.