Wednesday, June 18, 2014

How Did the Grand Canyon Happen? It's a Mystery, a Delightful Mystery!

And that's something that I really love about science and geology: the acknowledgement that there are things we don't know...yet. No matter how ingenious our technology becomes, no matter how much data we collect, there will always be new things to discover, and each week research reveals something new about our Universe that no one has ever known before. And that's the fun of exploring a wonder like the Grand Canyon. We don't completely understand why it's there!

There are many things we can say about how the Grand Canyon formed. For instance, the Colorado River carved the Grand Canyon. That's an answer that satisfies many, because the river is big, drains a large region, and lies at the bottom of the present-day canyon. It carries millions of tons of silt and sand downstream for temporary storage in Lake Mead before it ends up in the Gulf of California. The canyon is getting a tiny bit deeper and wider each and every day. That much is clear.
The first problem is a big one: how did the Colorado River get over the Kaibab Plateau (above)? The Kaibab tops out at over 8,000 feet, but the lands east from which the Colorado flows are 3,000 feet lower. The easy answer is that the river was there and the land rose around it while the river cut down, keeping pace with the uplift. That is an appropriate hypothesis, but unfortunately can't be true because the plateau is 70 million years old while the Colorado River cannot have exited the plateau at the present Grand Wash Cliffs locale prior to 16 million years ago. The main lesson of several different possible explanations is that there has not always been a single through-going Colorado River. There have been several river systems that were later integrated into a single river.
So which of those Colorado Rivers was it that carved the Grand Canyon? Was it the Colorado River we see today that flows (or once flowed) into the Gulf of California? If that's the case, the Grand Canyon can be no more than 4-6 million years old, because that's when the Gulf of California opened up. Was it the "Colorado River" that carved these entrenched meanders near Peach Springs (photo below)? If it was, then we are talking about a river that carved a 4,000 foot deep canyon more than 50 million years ago. A river that flowed northeast, almost opposite of what the Colorado River does today.
Or was it this river (below)? Looking at the shape of the tributary streams, it looks like this river is flowing northeast...but it's not. That's Marble Canyon, occupied by the Colorado River just downstream of Lees Ferry, and just upstream of the Grand Canyon itself. The river flows southwest, but the topography is strongly suggestive of a river that once flowed in exactly the opposite direction.
How in the world can a river completely change direction? It seems to fly in the face of physics and the laws of gravity. But there are ways that it can happen and we don't have to break the law (of nature). One of them is called stream piracy, and it is happening even today on the Colorado Plateau, at another familiar national park, Bryce Canyon.

The gradient of a river is the relative slope of the river, the altitude it drops over a given distance along the riverbed. Higher gradient streams flow faster, and therefore tend to have greater erosive capacity. Since the highest gradients are often at the headwaters of a stream, erosion often acts to eat away at the edge of the drainage or plateau where the river originates. This upstream lengthening of the river is called headward erosion.

Check out the Google Earth image below of Bryce Canyon National Park. The famous hoodoos, or spires, form at the edge of the plateau and are light orange in color. The erosional amphitheaters represent the headwaters of the Paria River and other tributaries to the Colorado River. The amphitheaters completely surround a second north-flowing drainage, the East Fork of the Sevier River. The headwaters are carving away at the plateau edge, getting closer and closer to the channel of the Sevier River. In the near geologic future, the water that flows north towards Salt Lake City will instead flow southwest towards Las Vegas. Early settlers in the region took advantage of the impending piracy; they dug a canal that diverts some of the Sevier River into the Paria River drainage to support agricultural fields downstream.
The picture below highlights the edge of the plateau and the two drainages. Ultimately it is tectonic activity (raising or sinking of the crust) that determines the response of river channels. And there have been many tectonic changes in and around the Colorado Plateau where the Grand Canyon was eroded.
How are we to make sense of these discrepant pieces of data? In essence, it is the discrepancies that lead us to better understand how the Grand Canyon might have happened. While we don't have a full story to explain the development of the canyon, there are certain things we do know (this information is summarized nicely in Wayne Ranney's excellent book Carving Grand Canyon: Evidence, Theories and Mystery, published by the Grand Canyon Association).
From 80 to 30 million years, river in the Colorado Plateau region flowed northeast into large inland lakes. The Claron Formation, which forms the hoodoos of Bryce Canyon, was one of these lake systems. The highlands from which the rivers flowed originated in the compressional tectonics of the Laramide Orogeny.

From 30 to 16 million years there was a dark period for which there is little evidence revealing the type of river activity. According to Ranney, the drainages became ponded, dry, rerouted, reversed, or "confused".

From 16 to 6 million years, basin and range style faulting severely disrupted the landscape, creating deep sedimentary basins that were isolated from one another. The sinking of these basins may have formed the high gradients that initiated deep canyon cutting, but until 6 million years ago, no Colorado River flowed of the Colorado Plateau at any particular point with any relationship to today's course.

Between 6 and 4 million years ago, the opening of the Gulf of California caused the integration of different river drainages into the unified Colorado River we know and love today. Much of the cutting of the Grand Canyon probably took place during this time period, aided in large part by extremely high river discharges fed by melting glaciers in the Rocky Mountains during the last ice age. 
What we can say is that the Grand Canyon and the Colorado River exist today, and the canyon continues to be carved and excavated by the river. We are lucky to have evolved just in time to see the canyon, because in a geologically short period of time, the entire plateau can be expected to erode away, removing nearly two billion years of geological evidence. It's true that other canyons will form, but it's hard to imagine any place as grand as this one. Get out and see it while you can!

Post a Comment