Air hockey for giants

Natural History, July, 2005 by Dave Forest

The Heart Mountain rockslide in northwestern Wyoming is the world's largest rockslide and one of geology's biggest puzzles. What could cause a mass of rock nearly 450 square miles in area, and weighing trillions of tons, to slide along a stretch of nearly flat ground and end up, in a matter of minutes, as a sheet of rubble covering about 1,300 square miles? Two geologists, Edward C. Beutner of Franklin and Marshall College in Lancaster, Pennsylvania, and Gregory P. Gerbi of the Massachusetts Institute of Technology, have a theory: the slide was a gas.

Beutner and Gerbi examined an unusual layer of rock known as a microbreccia, lying along the fault on which the Heart Mountain rocks broke free and slid. That layer holds minute grains of carbonate rock and volcanic glass. The grains, the investigators point out, are much like the ones that often form in gas clouds released by volcanic eruptions. Beutner and Gerbi say the microbreccia layer is made of fragments carried in a gaseous cushion that "greased" the surface along which the rocks slid 48 million years ago.

The slide was probably triggered by a nearby volcanic eruption. Then the heat and pressure generated by the friction of the moving mass caused the limestone below it to break down, releasing a cloud of carbon dioxide gas that floated the rubble. Relieved of the ordinary friction of rock on rock, the slide could have traveled extraordinary distances at a hundred miles an hour. (GSA Bulletin 117:724-35, 2005)