STURZSTROMS IN THE SMOKIES:LONG RUNOUT LANDSLIDES AT GATLINBURG AND COBBLY KNOB, TENNESSEE

Landslides are mass wasting phenomena that occur frequently wherever conditions are favorable on Earth or other planets. Average landslides are often costly and sometimes deadly natural phenomena, and their mechanics are generally understood. There are, however, unusual “long-runout” landslides, called sturzstroms, that involve enormous volumes of material and anomalously long runout distances, traveling an order of magnitude greater distances horizontally than their vertical drop. Although the triggers for these landslides may be the same as average landslides—exceptionally large precipitation events, earthquakes, oversteepening of slopes—they defy explanation through conventional landslide mechanics. Two prehistoric “long-runout” landslides have occurred in the Great Smoky Mountains National Park, TN, near Gatlinburg, TN, and Cobbly Knob, TN, located 20 km (12 mi) E of Gatlinburg. The vertical drops of these landslides are 610 to 730 m (2000 to 2400 ft), while their horizontal runout distances range from 5 to 7 km (3 to 4 miles), almost certainly qualifying both as sturzstroms. Their sources are coarse-grained Great Smoky Group Thunderhead Sandstone exposures at high elevations, but today boulders of these rock units rest far into the outcrop belts of fine-grained Snowbird Group Pigeon Siltstone. The mechanics of sturzstrom behavior are not well understood and no consensus exists, although several hypotheses have been proposed: cushioning by water or air, dispersive grain flow, fluidization by sound waves, and self-lubrication by frictional melting. The mechanism of dispersive grain flow is the strongest among the current hypotheses proposed to explain sturzstrom behavior, although several mechanical processes together likely contribute to the origin of sturzstroms, including those identified in the Great Smoky Mountains National Park.