Not only is Smith Rock Park the origin of all the animals, it also houses some of the most interesting rock features in the High Desert. I was taught that these rocks were upthrust from the plateau during the great upheaval of the Cascade Mountains. However, time has passed, and the rocks have told a different story.
It seems that the Smith Rock park formations are actually remnants of the Crooked River Caldera. Sometime around 29 million years ago, the Smith Rock area sat atop the same deep hot spot which underlies Yellowstone Park today. Here is a short presentation on the Crooked River Caldera. The ryolites of Smith Rock park are the remains of the northern edge of the caldera’s subsidence crater, which is about 25 miles in diameter.
In relation to the mantle hot spot, the North American plate is moving southwest. However, as the North American plate crashed into the Pacific plate, portions of Oregon were shoved north. Periodically, the crust over the hot spot would subside in a pyroclastic flow of molten rock and ash, and a new caldera was born. Over eons, the plate would scoot away from the hot spot and the subsidence basin would cool. Here is a description of the process as it is understood today.
The following image from Wikipedia shows more recent remnant hot spots in Idaho. The numbers represent the approximate age of the various calderas in millions of years. The Crooked River Caldera is much older than the ones depicted in the image.
Another feature of the Eastern Oregon landscape are the lava flows. In most areas of the high desert basaltic lava covers the crustal sedimentary layer to a depth of two to three miles. This lava came from vent cracks that flowed for millions of years. Here is a description of the history of these basaltic flows.
This process is better understood as many modern volcanoes. An example of a similar type eruption is happening as I write this in Fagradalsfjall, Iceland. This eruption started as a single vent that looked like it was going to evolve into a conic mountain. Over time, more vents have opened and it now looks like it is trending to a ridge vent, like those that spewed the basalt into the high desert. It’s not over yet.
The locals have setup a couple of live cameras to let you can watch things as they happen in real time.
The difference in the rocks between the bonded ash (tuff) in the Crooked River caldera, and the basalt found southeast of there, meant the ancient peoples used different techniques in their rock engravings. Tuff is much softer than basalt, so the engravings are carved more deeply into the rock. I’d like to say that the subjects of the images are different, but I can’t really document that. In general, I’ve seen more female images in tuff and more animal and male images in basalt, but I haven’t seen that many images in tuff. I’ve been told that pictographs are more common on basalt and petroglyphs more common on tuff.
This brings me to my planning for this year’s desert trips. From the aerial imagery I’ve noticed a series of parallel gashes in the earth, which look to be the remnants of long dead volcanic vents. I will use my first trip of the year to explore them.
If they are vents, then there should be hardened lava blobs (bombs) in the area. I’m also hopeful of finding a few more petroglyphs to photograph. Regardless, just getting back out will be reward enough. I’ll let you know what I find.
Oh, one more thing. My more careful readers will note that the above theory of the behavior of the Yellowstone hot spot implies that, contrary to the doom and gloom predictions of a past due Yellowstone supervolcano eruption, the Yellowstone fumaroles and geysers are more likely to die from cooling as the crust drags them beyond the heat source. Only then, far into the future, will another caldera be born. But hey, maybe Nick and Carrie Gordon got that part wrong.