Friday, July 6, 2012
The Abandoned Lands...A Journey Through the Colorado Plateau: The Seven Stages of Grand Canyon Awareness
This is a response of those seeing the canyon for the first time. Practically everyone has seen pictures of the Grand Canyon, but as we all know, a page or a screen shrinks the canyon to insignificance. There is just that one moment when one walks to the edge of the abyss for the very first time and looks into the depths. It is one moment in a life that cannot be forgotten.
There's a big river down there, the Colorado. Obviously, it carved the canyon. But somewhere around stage #5 or #6 one is going to have to ask which Colorado River? The older upstream river or the younger downstream river?
The walls of the Grand Canyon expose 300 million years of Paleozoic history in 14 different formations representing deposition in shallow seas, tidal flats, river floodplains, and desert dune fields. The layers are about 4,000 feet thick, making up most of the cliffs seen in the canyon.
There are 12,000 feet of additional layers hidden in the depths of the Grand Canyon! They're hidden in most places by the Paleozoic layers. They represent an entire Proterozoic history that dwarfs the Paleozoic story in complexity.
Those are some very old and contorted rocks in the greatest depths of the Canyon. They are metamorphic gneiss, schist and quartzite bodies that have been intruded by granitic dikes. They are rocks that have been buried miles deep in the crust, and yet were eroded to a flat plain before any of the other rocks of the canyon were ever laid down.
Stage #6: What regional and world-wide events caused the rocks of the Grand Canyon to form in the way that they did?
Grand Canyon is part of a much bigger story. The story starts to emerge as one visits other parks in the region, or one works like a detective in an academic setting to see the links between events like the accumulation and break-up of the Pangaea supercontinent, or the formation of a subduction zone offshore of California and the Pacific Northwest. The story is especially complex in the metamorphic rocks of the inner canyon. The exposures are hard to access, and numerous researchers have spent their entire careers teasing out the story of the oldest rocks.
Sometimes the simplest answers are not the correct ones. In the 1870s John Wesley Powell thought the river existed first, and the land rose while the river cut down several tens of millions of years ago, forming the Grand Canyon. The years have passed, and problems with the model cropped up, to the extent that the simple model had to be discarded. But there was no second model to take its place. The Colorado River carved the Grand Canyon. But not the Colorado River we see today. The Colorado at various times has flowed in directions essentially opposite the flow of the river today. It flowed through canyons in the state of Colorado millions of years before it flowed through the Grand Canyon, and the delta of the Colorado at the Gulf of California has only existed for 5 million years. There has been stream piracy, headward erosion, beheaded streams, collapsed highlands, and tectonic uplift that gave us the gift of the Grand Canyon, but the details have yet to be worked out.
Why do I love the Grand Canyon? It amazes us, and satisfies our curiosity at so many levels. Children can understand the link between a flowing river and the carving of a deep canyon. They can feel the thrill of discovering a fossil for the first time, and understand a little of how it came to be. Casual tourists can be understanding something of the story of the formations that make up the canyon walls, and start to appreciate the immensity of geologic time. Students of geology (like myself so many years ago) can learn the basics of geology and find inspiration for future career choices. Huge mysteries remain to be solved by researchers in the field and in the lab. And the canyon is incredibly scenic. In my next post, we explore one of those beautiful places: the lower reaches at Diamond Creek and Peach Springs Canyon.
Here is the explanation of my "abandonment" theme for this series: http://geotripper.blogspot.com/2012/06/abandoned-landsa-journey-through.html