Geology - Why does it look like it does?

GRAND CANYON Explorer	home
Geology

Why does it look like it does?

The reason that it looks the way does is due to the sequence in which the events that help to create it happened. We already know that there was once a very tall chain of mountains in the area that occupied the Grand Canyon. These mountains were, over many millions of years, eventually eroded away to form a level plain. Fluctuations in climate then caused the oceans to move in over successive periods and each time a new rock layer was deposited. The rock layers were deposited one on top of the other and sometimes there were long periods in between in which some of the upper layers were eroded away, sometimes completely.

And now the Colorado River comes into play. The ancestral "Colorado River" came into being when the Rocky Mountains to the east of the Grand Canyon were formed, at sometime around 60-70 million years ago, as the primary western drainage for these mountains. Over millions of years the course of this ancestral river changed its course a number of times as the terrain around it was altered. The course of the ancestral Colorado River probably started in Colorado and at one point it entered the region of Marble Canyon, but that is about all that can be agreed upon at this point.

Some geologists believe that very young rock layers to the west of the Grand Canyon, dated at only 5 and 10 million years old, and through which the Colorado now flows, indicate that the river could not have been flowing there prior to that time. The river had to cut through these layers after they were deposited. The search for another exit for the Colorado River from the Grand Canyon has been a hotly debated issue. Some geologists believe that it flowed out of Marble Canyon where the Little Colorado now enters, others believe that it exited near present day Diamond Creek and still others believe that it exited through massive caves in the Redwall Limestone. The most likely exit at this point seems to be up through Kanab Creek which would have had the ancestral river flowing back up into Utah and then across Nevada and California to the Pacific.

At around 17 million years ago, while the river was flowing across this ancient landscape, the land mass know as todays Colorado Plateau began to uplift. The uplift was caused by pressures deep with the Earth and may have been caused by additional conflict between the North American Plates and the Pacific Plates. This process continued until around 5 million years ago which interestingly enough is the date of the sedimentary layers just west of the plateau. At its greatest hieght the Colorado Plateau was once about three miles above sea level. The rise of the plateau probably prevented the seas from submerging it again and instead the topmost layers were eroded away and carried into the sea. The most favorable currently accepted theory is that the Colorado River continued to cut through the Colorado Plateau while the land rose around it.

At some point around 5 million years ago something happened to cause the Colorado to change its course and exit via its present route down to the Gulf of California. The most likely cause for the change in its course was probably due to it being captured by another river, which was draining the western portion of the Colorado Plateau. This other river eroded northward along the San Andreas fault, then eastward and eventually entered the Grand Canyon and joined with the Colorado near present day Kanab Creek. The Colorado would then have abruptly changed its course and flowed out this newly formed exit.

Much of the eastern Grand Canyon was already formed by the time the river changed its course. Side canyons had formed along fault lines in the rock and these were eroded away and the rock within them carried down to the Colorado. The Colorado River took all of the rock that was put into it and carried it off to the Pacific Ocean. Over many more millions of years the erosion along the course of the Colorado continued to widen the Canyon to present the vistas that you see today. Before the Glen Canyon Dam was built the Colorado River used to carry three cubic miles of sediment into the Pacific Ocean every hundred years.