K-AR AGES OF PLEISTOCENE LAVA DAMS IN THE GRAND-CANYON IN ARIZONA

At least 13 times during the Pleistocene Epoch lava flowed into the in ner gorge of the Grand Canyon and formed lava dams, as high as 600 m, that temporarily blocked the flow of the Colorado River, K-Ar ages on these lava dams indicate that the seven youngest formed within a short period of time between about 0.6 and 0.4 mega-annum (Ma). The physiog raphy of the lava dam remnants within the canyon shows that each dam w as destroyed by erosion, the Colorado River rapidly reaching its pre-e xisting grade level, before the next dam was emplaced by new eruptions . The total time for emplacement and destruction for an individual lav a dam was probably as little as 0.01-0.02 million years. The K-Ar ages of the two oldest dams, the Lava Butte dam (0.577 +/- 0.054 Ma) and t he Prospect dam (0.684 +/- 0.051 Ma) are somewhat younger than the phy siography of their remnants suggest.