Friday, March 14, 2008

Asking questions

An intriguing article in the latest issue of Science, providing a new estimate of the rate at which the Grand Canyon was incised into the Colorado Plateau. The authors dated cave deposits in the walls of the canyon using an improved method of radiometric dating. The caves indicate the position of the water table at the time they were formed. The conclusion: The Colorado River took about 20 million years to cut its way downward through the one mile depth of the canyon.

The river, of course, has stayed pretty much at the same level as the land went up. The land can rise for two reasons: 1) As the regional surface erodes, the crust rises, for the same reason a ship rises in the water as its cargo is removed; and 2) tectonic shifts can force the crust upwards. A comparison of the cutting rate of the Grand Canyon with estimates of the average surface erosion of the region suggests that tectonic uplift contributed to the incising of the canyon.

Altogether, a lovely piece of work that hangs together neatly with the known geological history of the West.

When I used to conduct an introductory earth science course for liberal arts students, I took them to a place on our New England campus (see photo) where the granite crust was manifestly weathering. We knew exactly when this particular outcrop was exposed to the elements by removal of the overbearing deposits when the college sold them off to the state for the construction of a nearby highway -- about 50 years previously. I asked the students to estimate the average rate at which weathering eroded the surface rock. Typically, they came up with rates of about 1 mm per hundred years. ("Oh, about this much per hundred years," they'd say, indicating a tiny gap between thumb and forefinger.) Interestingly, this is consistent with the rates of average surface denudation in the Grand Canyon study -- which suggests that even untutored 19-year-olds can do some pretty nifty science if asked the right questions.

And more. If the granite was implanted, say, a kilometer deep in the crust (as the textbooks suggest) at the time when the New England region was tectonically active, the students could work out when that might have been -- 100 million years ago. A nice order of magnitude calculation by a happy band of scholars, engaging with the deep geological history of the Earth as we stood in a yellow wood on a fine fall day.