City of Rocks Earthcaching
The closely spaced horizontal layers of Almo Pluton granite can bare a striking resemblance to sedimentary rock, although the process of their formation is remarkably different.
Unlike jointing, which is generally believed to be the result of cooling, and/or extensional tectonic stresses, the process of exfoliation is believed to be the result of relief of stress as the regolith (or loose heterogeneous layer of material that covers solid rock) is removed, allowing for expansional pressures, forcing the underlying layers upward, resulting in parallel joints or cracks in the underlying bedrock which gives these features their characteristic "onion skin" appearance.
In terms of the time it took for this process to occur, it is thought that this may started occurring as early as the Miocene era. The evidence for this is the number of small pockets of rhyolite tuff which can be found throughout the Reserve, particularly in Emigrant Canyon a short distance South of Twin Sisters. The softer rhyolite exerted pressure on the underlying Almo Pluton, but through the process of erosion, exposed the outcrop in these areas. The resulting release of pressure resulted in cracks and joints as the bedrock expanded. Once exposed, weathering expanded these cracks, resulting in exfoliations which can be seen today. This process would be developing over the last 10 million years.
Logging Requirements:
1. Standing at the posted coordinates, do the layers of exfoliations extend to top of the rock formation? Why or why not do you think this is so.
2. Does the rock making up the exfoliations look different from the rock on the main formation? Which one do you think will erode faster?
3. Describe what this area may look like over the next 10 million years as this dynamic process continues.
References:
Etched In Stone: The Geology of City of Rocks National Reserve and Castle Rocks State Park, Idaho; Keven R. Pogue
Geologic Interpretive Trail, City of Rocks National Reserve; A self guided journey to discovering secrets in the rocks of City of Rocks, Prepared by Idaho Department of Parks and Recreation and the National Park Service.