Seeing Volcanoes from the Inside Out: A Visit to Castle Crags State Park in California

We recently completed a rather awesome trip to study the volcanoes of California, most particularly those that are found in the northern part of the state. Nearly all of our sites were the result of recent volcanic activity like Mt. Shasta, Lassen Peak, and Medicine Lake Highland, but our first visit was unique: we were exploring a volcano from the inside out!

It's not easy to visit the underside of a volcano when you think about it. The magma chamber that feeds the volcano may be four or five miles underground, and the temperatures and pressures are far, far beyond the abilities of human technology to conceivably hope to visit any time soon (there is that Unobtainium that was used in Avatar and The Core, but folks might be surprised to find it doesn't exist). About the only thing we can do is be patient. Really, really patient. First, we need to let the hot magma cool down slowly, a process that may take tens of thousands of years. Then we need to wait for a mountain-building event to cause the crust to be uplifted several miles, and then wait for the forces of erosion to remove all of the overlying rock. That's a bit longer, at least several tens of millions of years. But once it's done, the magma chamber will be sitting there, exposed for all to see. Luckily for us, this exact sequence of events took place in the Klamath Mountains of Northern California, just a few miles away from Mt. Shasta. It's a state park called Castle Crags.

The first view of Castle Crags is dramatic. Driving north on Interstate 5, one is treated to miles of forested hills, but at Castella the hills give way to sheer cliffs and spires of granitic rock. The granitic rock exposed in the Crags is the visible evidence of a magma chamber that once fed volcanoes in the along the western margin of North America in the age of dinosaurs 165 million years ago.

The sharp spires and rounded domes of the Crags are the result of having a great weight removed. Having formed at depths of four miles or more, the rocks expanded as erosion removed the heavy overlying rocks. But rocks can't expand like marshmallows; they fracture, much like the crust of baking loaves of bread. Vertical cracks are joints. Closely spaced joints promote the formation of the spires and towers of granitic rock. Fractures parallel to the surface are called exfoliation sheets. Exfoliation tends to remove to remove corners and edges, resulting in the formation of domes (Half Dome in Yosemite is a half-good example).

The Castle Crags were also glaciated, but with top elevations of less than 7,000 feet, the glaciers were small, and had less to do with the overall shape of the mountains than jointing and exfoliation. A few small lakes and moraines are found on the north side of the peaks.

We didn't have the time to hike among the trails that reach the base of the crags, but we stopped along Castle Creek to see what's been carried down the mountain by the glaciers and rivers that tear away at the granite. We could also see the more ancient metamorphic rocks that had been intruded by the granitic rock.

Seen from above (the picture below was taken on a lucky day flying home from Canada a few years back), the Castle Crags can be seen as isolated mass of granitic rock. In our geology textbooks, we find that a batholith is a single intrusion exposed over an area of 100 square kilometers (about 40 square miles). The term can also refer to a vast agglomeration of many dozens of adjacent plutons, as is the case in the Sierra Nevada. There are several of these composite batholiths in the western United States, including the Sierra Nevada, the Idaho, and the Southern California batholiths. The Castle Crags and other small isolated plutons are referred to as stocks. The surrounding rocks are the more easily eroded metamorphic rocks of the Eastern Klamath Terrane (the Trinity Complex).

We finished our subterranean exploration of the ancient volcano and headed back to the Earth's surface to find a more modern version of a volcano: Mt. Shasta. More on that incredible mountain in the next post.

The Other California: Taking Stock of the Castle Crags

Driving on Interstate 5 north of Redding is a sometimes terrifying affair. The highway follows the Sacramento River in a winding canyon with plenty of twists and turns. The terror isn't necessarily the road itself as much as it is the giant trucks and recreational vehicles which are being driven as if they were still on a straight freeway in the Central Valley. They don't exactly stick to their lanes. The other hazard comes from following geologists on their way north to see Mt. Shasta: at a particular loop on the highway near Dunsmuir, they are very likely to slam on the brakes as the Castle Crags come into view...

This is part of my continuing series on the "Other California", an exploration of those wonderful parts of our state that don't always show up on the postcards. Today we are wrapping up a journey through the Klamath Mountains. It has not been an exhaustive survey as it is one of the corners of the state that I have yet to fully explore. I want to reiterate my invitation: be a geotripper geoblogger! Have you been to Shasta Caverns? Backpacking in the Trinity Alps? Explored any gold mines near Weaverville or Shasta City? Write a short narrative, or if you don't trust your writing skills, just send some nice pictures, and I will find something to say.

The Castle Crags are certainly a shock when first seen from Interstate 5. The light-colored cliffs rise 3,000-4,000 feet above the river canyon, and stand in stark contrast to the lower heavily forested ridges that make up most of the surrounding area. The peaks and domes remind some people of the Sierra Nevada, and the comparison is apt; the Crags are composed of granitic rock, and as noted previously, the Klamaths are a northern extension of the Sierra Nevada. Their geologic history is similar, with one big difference: the Sierra range is composed mainly of granite intrusions (plutons), but in the Klamath Mountains, the intrusions are smaller and isolated from each other.

A batholith is a single intrusion exposed over an area of 100 square kilometers (40 square miles), although the term can also refer to a vast agglomeration of many dozens of adjacent plutons, as is the case in the Sierra Nevada. There are several of these composite batholiths in the western United States, including the Sierra Nevada, the Idaho, and the Southern California batholiths. The Castle Crags and other small isolated plutons are referred to as stocks. The limited areal extent of the Castle Crags pluton is apparent in the photo below. The surrounding rocks are the more easily eroded metamorphic rocks of the Eastern Klamath Terrane (the Trinity Complex).

The rocks of the Castle Crags formed about 163 million years ago when the Pacific Plate sank beneath the edge of the North American continent in an extensive subduction zone (the same kind of subduction that produces the Cascades volcanoes in the present day). Water released from the descending plate acted like a catalyst leading to the melting of rock deep in earth's interior, and the resulting magma bodies rose until they lay just a few miles beneath the surface. The rock cooled slowly, over tens of thousands of years, forming granodiorite (a coarse-grained granitic rock with significant amounts of plagioclase feldspar). At times, magma reached the surface producing volcanic eruptions, but the volcanoes at Castle Crags have long been worn away. In other words, standing on the granitic rock of the peaks here, one is actually perched under a long-gone volcano.

The sharp spires and rounded domes of the Crags are the result of having a great weight removed. Having formed at depths of three miles or more, the rocks expanded as erosion removed the heavy overlying rocks. But rocks can't expand like marshmallows; they fracture, much like the crust of baking loaves of bread. Vertical cracks are joints. Closely spaced joints promote the formation of the spires and towers of granitic rock. Fractures parallel to the surface are called exfoliation sheets. Exfoliation tends to remove to remove corners and edges, resulting in the formation of domes (Half Dome in Yosemite is a half-good example).

The Castle Crags were also glaciated, but with top elevations of less than 7,000 feet, the glaciers were small, and had less to do with the overall shape of the mountains than jointing and exfoliation. A few small lakes and moraines are found on the north side of the peaks.

Castle Crags State Park honors the Castle Crags, but does not actually encompass them. The park boundaries include the heavily forested southern and eastern flanks of the crags, and part of the Sacramento River, but the granitic cliffs and domes are protected as the Castle Crags Wilderness Area, administered by Shasta-Trinity National Forest. The state park offers a nice campground, with several trailheads that provide access to parts of the wilderness, as well as 8 miles of the Pacific Crest Trail. A park road leads to a spectacular viewpoint that takes in the Crags and nearby Mt. Shasta.

Vennum, Walter, 1980, Petrology of the Castle Crags pluton, Klamath Mountains, California: Summary, GSA Bulletin; v. 91; no. 5; p. 255-258.

Vennum, Walter, 1994, Castle Crags, California Geology, March/April, pages 31-38.