In late June a friend and I had the distinct pleasure of getting to hike across the Grand Canyon from the North Rim (via North Kaibab Trail) to the South Rim (via Bright Angel Trail). Hiking across the canyon is quite an amazing experience that takes you from aspen/ponderosa forests on the North Rim, juniper forests, and then desert scrubland towards the base of the canyon. Here is just one example of all the different flora you get to see on the hike:
The plants and animals are not the only thing that’s interesting about this place but there are so many fascinating stories to be told from the rocks themselves. The Grand Canyon contains an extensive record of North America’s natural history going back 1.7 billion years. The canyon itself is an erosional feature that formed from the Colorado River incising down into the landscape.
Rivers carving out huge canyons or gorges like this are typically a result of one of two scenarios: A.) there is a drop in the river’s base level (the lowest point to which a river flows) or B.) the land itself is uplifted causing an increase in the river’s gradient thus an increase in it’s erosional power. In the case of The Grand Canyon it is evident that a large section of the Earth’s crust was uplifted to form what is now the Colorado Plateau. What caused this uplift? The best working hypothesis we currently have is the uplift was caused by a tectonic event we call the Laramide Orogeny where the shallow subduction of oceanic crust off the coast of North America caused mountain building and deformation throughout the Southwest roughly 70 million years ago. I have illustrated a possible sequence of events:
I think it’s interesting how such a huge block of mostly sedimentary rock stayed relatively intact during all of this motion. I would assume it would have broken apart into smaller pieces along different points of weakness or areas of higher stress from below. How The Colorado Plateau staid mostly intact during the uplift still seems to be a mystery in the scientific community.
The uplift explains why the canyons of the colorado plateau are so deep but it doesn’t account for the width of the canyon. The Grand Canyon is deep due to the erosional force of the river but extremely wide due to the erosional force of gravity. After the river carved downward it destabilized the rocks exposed along the canyon walls causing them to mass waste, break apart, fall down into the river, and widen out the canyon over the course of millions of years.
While this may explain how the canyon itself formed it doesn’t answer how the rocks formed that make up the stratification of the canyon. In order to answer that question we need to go much further back in time. In my next blog post we will explore the actual rock units that make up the canyon and find out what stories they tell.