The talented science writer Joel Achenbach has a piece on the Washington Post website about new research which suggests the Grand Canyon as an erosional feature is much older (~17 Ma) than we previously thought (~6 Ma). Achenbach's article is based on a study published in today's Science by Victor Polyak, Carol Hill, and Yemane Asmerom, of the University of New Mexico. These researchers* calculated the ages of cave mammillaries (rounded speleothems that form in caves near the water table) with U/Pb isotopic dating to infer when the water table had dropped in the past, implying a deep canyon.

* With all the talk in the geoblogosphere about death-defying geological research, it should be noted these folks were rappelling hundreds of feet down the canyon's cliffs to get to some of these caves...

Anyhow, dates from the calcite deposits suggest that the water table dropped more slowly (and hence the erosion rate must have been slower) in the western canyon than in the eastern section. The western part of the canyon yields inferred erosion rates of 55 to 123 m/Ma, while the eastern canyon's caves yield inferred rates of 166 to 411 m/Ma (about 3 times as fast). The authors interpret this to mean the Grand Canyon formed in two pieces: one started slowly propagating from the west, then another formed from the east (relatively rapidly working its way westward), and the two broke through and met in the middle, yielding the Canyon we know and love.

The photo above is a view from one of the sampled caves. [Photo is by Art Palmer, taken from the "Achenblog" site (Joel Achenbach's blog).]

Check out more details here. Or see the NYT's treatment here.

Main reference:
Victor Polyak, Carol Hill, and Yemane Asmerom, 7 March 2008. "Age and Evolution of the Grand Canyon Revealed by U-Pb Dating of Water Table-Type Speleothems." Science, Vol. 319. no. 5868, p. 1377 - 1380. DOI: 10.1126/science.1151248

Commentary in the same issue of Science:
Tim Atkinson and Mike Leeder. 7 March 2008. "Canyon Cutting on a Grand Time Scale." Science, Vol. 319, no. 5868, p.1343-1344. DOI: 10.1126/science.1155286

Labels: caves, grand canyon