The Volcano That Wasn't There, and Two That Were...

Mt. Shasta and Shastina from the north

I hope that your holiday travels were safe and happy. I had a fine time visiting family across the Pacific Northwest, from Seattle to central Oregon, Northern California and home again. I had been looking forward to seeing a bevy of Cascades volcanoes, including Mt. Rainier, Mt. St. Helens, Mt. Hood, the Three Sisters, Mt. McLoughlin and the others. Of course, the Northwest had no intention of revealing its treasures, as clouds and rain dominated our trip from one end to the other. There was plenty of geology, mind you, as we saw all manner of flooding, landslides, and mudflows, but I admit to feeling a tiny bit of disappointment at finding the Cascades hidden from sight.

Mt. Shasta from near Weed (photo by Mrs. Geotripper from a fast moving vehicle!)

It wasn't until the last day of our travels, a 600 mile drive from central Oregon to the Great Valley in California that we saw anything of the high Cascades. We crossed the Siskiyou Mountains in a light snow storm, but by the time we reached the Klamath River in California the skies had almost magically cleared. As we turned around the ridge near Yreka, Mt. Shasta came into glorious view. Shasta is the second highest (14,179 feet; 4,322 meters) and most voluminous of the Cascades volcanoes, and the second most active (after Mt. St. Helens). It last erupted in 1786. The volcano consists of at least four volcanic centers, including Hotlum Cone, the highest peak, and Shastina, a parasitic cone on the west side of the mountain (on the right side of both photos above). Shastina is actually the third highest Cascade volcano at 12,329 feet (3,758 meters) .

Lassen Peak from near Red Bluff

Later, as the sun sank lower in the western sky, we were treated to a rare valley view of snow mantled Lassen Peak (10,463 feet; 3,189 meters). Lassen is a dacite plug dome that came into being during a series of eruptions around 27,000 years ago. It has the distinction of being the most recent volcano in California to erupt. After a series of nearly 200 phreatic explosions from May of 1914 to May of 1915, the volcano produced a dark viscous lava flow that melted much of the snow around the summit that resulted in a major lahar (volcanic mudflow). A few days later there was a large explosive eruption of ash, with a pyroclastic surge that destroyed about four square miles of forest.

Lassen Peak and the remnants of Mt. Tehama

I was even more pleased with the view of the volcano that wasn't there. If you look to the right side of the photo above, you can see a series of ragged peaks, which include Brokeoff Mountain, and Mt. Diller. They are the remnants of a large stratovolcano that would likely have towered over Lassen Peak. The peak, now called Mt. Tehama (or Brokeoff Volcano), grew during a long series of eruptive events between about 590,000 and 385,000 years ago. The eruptions grew silent, and erosion began to attack the extinct volcano. It is interesting to sit in the midst of these peaks (the main park road passes among them) and realize that one is looking out from the deep interior of volcano.

It was nice to see the coating of snow on the mountain peaks of northern California. The snow depths are still less than they need to be, but they are certainly better than the near lack of any snow at all that we saw last year during our holiday travels.

The Mountain That No Climber Can Ever Summit: Mt. Tehama (the Brokeoff Volcano)

The gigantic boulder is a glacial erratic, left behind by sheets of ice that once covered the Lassen region.

After being slightly distracted by ducks and the beautiful fall day in Yosemite, Geotripper is back on track to finish up his exploration of the Cascades volcanoes of Northern California. It was already approaching noon of our last day on the road back at the end of September, and we had six hours of driving ahead, so our list of features we were investigating was becoming short. In fact, one of the features we were interested in just simply wasn't there.

Mt. Tehama, or the Brokeoff Volcano, began erupting around 600,000 years ago just south of the present-day site of Lassen Peak. It was a stratovolcano similar to Mt. Shasta or Mt. Hood, composed mainly of gray andesite with interbedded ash and lava flows. The mountain alternated between explosive eruptions and effusive eruptions and eventually grew to a height in excess of 11,000 feet, hundreds of feet higher than modern Lassen Peak (10,457 feet).

There is a pretty good reason that mountain climbers can never summit Mt. Tehama, though. It's not there anymore. Around 400,000 years ago the magma chamber under the mountain ended volcanic activity. There has been debate about whether the mountain ended with a tantrum or a whimper, but the consensus seems to lie with the latter. The volcano stopped erupting and chemical weathering, river and glacial activity tore it apart. All that remains are a series of lower peaks surrounding the original throat of the volcano including Brokeoff Mountain, Mt. Diller, Eagle Peak and Diamond Peak.

 In the aftermath of the eruptive cycle that ended activity at Tehama, several plug domes erupted and grew on the flanks of the older mountain, including Lassen Peak itself about 28,000 years ago. Hot rock continues to simmer beneath the complex, evidenced by the recent (1914-1917) eruption of Lassen Peak, and the presence of geothermal systems like Bumpass Hell and the Sulphur Works.

From the Bumpass Hell trail, the peaks of Brokeoff Mountain and Mt. Diller seem to provide a near-perfect profile of the long-gone volcano, but the original edifice was much larger, something like 15 miles around. The center of the volcano was in the foreground and the two peaks were just part of the western flank. An aerial photograph of the mountain (from a Canada flight in 2005) offers a different perspective...

I annotated a version of the picture for a previous post on the volcano, and it is reproduced below. I'm mostly satisfied with it, although the height of the peak may be a bit exaggerated. In any case, good luck climbing to the summit!

Up next, the final stop of our trip!