Honors students in the Department of Geosciences at the University of Arizona are rewriting the geologic history of the Grand Canyon.
The traditional view of the sedimentary layers exposed in the canyon, as originally described by John Wesley Powell, is that they contain sediment shed from rocks exposed nearby. Honors students have been testing this interpretation by using new techniques of geochronology to determine the provenance of zircon crystals in various sedimentary layers.
The first step in the process was to collect samples from the various layers. These samples were returned to campus, where the students pulverized them and extracted sub-millimeter-size zircon crystals. The final step was to determine ages of the crystals by excavating microscopic pits into the crystals and analyzing the ejected material with a mass spectrometer.
This yielded ages for many different zircon crystals, which could be compared with the ages of rocks in different regions of North America to evaluate provenance.
To the great surprise of the geological community, very few of the crystals match the age of rocks in southwestern North America and instead are a good match with the ages of rocks in the Appalachian Mountains.
This revised view of provenance suggests that the layers in the Grand Canyon formed due to the uplift and erosion of the Appalachian Mountains, which occurred when North America collided with Africa and Europe to form Pangea.
The idea that layering in the Grand Canyon results from the formation of Pangea is a great example of modern geological science — observations made on tiny volumes of crystals provide information about geological processes that operate over continent-scale distances and million-year time scales.