Thursday, August 30, 2012

Small Rockfall at Grand Canyon Witnessed by Grand Canyon Sememster Students

This semester at Northern Arizona University, I will teach an Honors class (HON 343) as part of a program called Grand Canyon Semester. Thirteen students from around the US have arrived on campus and will undertake an intensive 16-week course focused on science and issues relating to the Grand Canyon region.

During their Orientation Week, they camped at the South Rim for four nights. I led them on a hike down the South Kaibab Trail to introduce them to some aspects of the geology of the canyon. On Thursday, August 23, storm clouds began to develop as they listened to a talk at Hopi Point. A few claps of thunder could be heard away in the distance. Then, one was heard that just kept rumbling and rumbling and never stopped. The students quickly looked into the canyon and saw a large billow of red dust and rock floating up from the south side of Shiva Temple. A rockfall was underway! This image was captured by NPS ranger Jacob Fillion.

The rockfall appears to have initiated in the upper portions of the Redwall Limestone, deposited during the Mississippian time period about 340 Ma (million years ago). Although the amount of material likely let loose in this fall would dwarf anything human, it is quite small compared to the overall dimensions of the canyon. Notice that evidence for other recent rockfalls is absent in this view and in most views of the Grand Canyon. It is likely that the canyon is not currently experiencing active widening or deepening in modern times and that it may undergo periods where it just sits there with nothing much happening. It is similar in many ways to the biological concept of punctuated equilibrium, whereby species undergo long periods of stasis interspersed with shorter periods of rapid change. Grand Canyon may also undergo a sort of punctuated equilibrium with regard to its formation. The agents that produce the punctuations in this case would be climate change, runoff amount in the river and uplift of the rocks. These are what initiate canyon cutting and widening.

In any event, the fact that our students got to experience a brief moment of canyon widening bodes well for their semester here in northern Arizona. We wish them well on their journey of discovery!

Photo courtesy of Jacob Fillion, NPS
The plume of dust and rock captured in this image is nearly 1,000 feet high from bottom to top. Compared to the overall dimensions of the canyon, this is likely a rather small event with a "flake" of rock only being dislodged. Yet anything directly below the fall having experienced a rather traumatic and violent crushing by the material that was let loose. The canyon challenges us to see our world on many different scales with respect to time and space, large and small. We are moved by its immense size and the lessons in earth history it provides to us.

2 comments:

Michael Chesley Johnson said...

I remember being at Arches NP several years ago and asking a Ranger what the likelihood was of any of the arches giving way. He said, "Not very likely, since things happen slowly over geologic time - but when it happens, it HAPPENS!"

Gaelyn said...

What a great experience to observe erosion happening.