Small landslides typically flow about twice the distance they’ve fallen downslope before they run out of energy. But large slides (such as the 30-million-cubic-meter flow that occurred in Mesa County, Colorado, and ran more than 4.5 kilometers in 2014, shown) can sometimes travel more than 20 times farther than they fall—and sometimes even, like a fluid, slosh up and over hills. Now, scientists may have figured out why. In the past, teams have suggested a variety of reasons for unusually long runouts, including having the material floating on a cushion of air (à la an air hockey puck) or gliding atop a friction-reducing layer of water or ice. But those scenarios don’t explain how unexpectedly far-reaching flows occur on the moon or other dry, largely airless bodies in our solar system, researchers say. In the mid-1990s, one team came up with a computer model that reproduced far-flowing slides by simulating the interactions between individual particles in the flow, but computers at the time didn’t have the computing power to investigate what was going on in the flow as a whole. Now, a team that resurrected the 2-decade-old model has found that pressure waves set up within exceptionally large flows help boost their overall runout. When the volume of a landslide exceeds about 1 million cubic meters, particles bouncing against each other create variations in pressure within the flow, with bits in some zones experiencing higher pressure than normal and others feeling below average pressure and thus sliding more readily. Because all particles will at some time or another feel lower-than-average pressure, the flow as a whole travels farther than expected, researchers report online today and in a forthcoming Journal of Geophysical Research: Earth Surface. The new findings don’t particularly help disaster planners, however: The larger the flow, the more the pressure waves boost the slide’s runout. So, to accurately estimate the distance a particular landslide would travel, they’d need to know how much material would be involved before the flow even started.
*Correction: 5 April, 5:06 p.m.: The article has been updated to correct our original misidentification of the date of the landslide pictured.