Saturday, September 28, 2013
Seeing Volcanoes Inside and Out: Exploring California's Volcanoes
The Castle Crags are an especially scenic part of the Klamath Mountains province, a region tucked between the Coast Ranges and the Cascades at the north end of the Great Valley (I've written about them previously, click here to see). Most of the Klamaths are composed of dark colored metamorphic rocks, many of them derived from the mantle, but at Castle Crags the rocks are different. The light colored cliffs are composed of granitic rock (mainly granodiorite, a plutonic rock containing lots of plagioclase feldspar), dating back to around 160 million years ago. The granodiorite was once molten, feeding volcanoes that would have existed several miles above where we were standing. Some of the magma remained in crust, cooling slowly and allowing for the growth of visible crystals. Standing among the granitic towers of Castle Crags is to be standing in the interior of a volcano.
Even as we stood at the viewpoint appreciating the underside of a volcano, we could turn to our right and see a rather astounding view of the outside of a volcano. A very big volcano. Mt. Shasta is the second tallest peak in the Cascades Range, exceeded only by Mt. Rainier, but it is the most voluminous stratovolcano in the Cascades, and perhaps even the world.
Shasta was our next stop of the day. At over 14,000 feet, no roads approach the summit, but in years past a ski area had been constructed at the 8,000 foot level on the south side of the peak. The ski resort was removed long ago because of avalanche danger (a wide swath of fallen trees highlights the hazard), but the paved road remains.
We took the highway out of Mt. Shasta City and headed up the mountain...