Shakespeare's character Macbeth could have been speaking about Slumgullion when he mentioned tomorrow creeping in a petty pace from day to day. This relatively loose, nearly 4-kilometer-long tongue of soil and rocks is indeed creeping: It's moving down a slope in southwestern Colorado at about a centimeter every 24 hours. But several times a day, according to new research, a surprising force pushes it at a faster rate.
Earthquakes and torrential rains typically start landslides. Most break away suddenly, releasing thousands or even millions of cubic meters of material and crushing anything in their path. But others, like Slumgullion--which is technically known as an earthflow--move at a sluggish but inexorable pace.
To study the nature of that movement, a team led by geologist William Schulz of the U.S. Geological Survey in Denver, Colorado, set up shop atop Slumgullion about 6 years ago. They noticed that the earthflow seemed to move at different rates at different times of day. So the researchers installed motion sensors at several locations that could detect less than a millimeter's worth of displacement. They also monitored the barometric pressure of the air and the hydraulic pressure of the groundwater within the landslide to determine if any changes correlated with movement.
It turns out that Slumgullion doesn't slide continuously, the team reports online this week in Nature Geoscience. Rather, the earthflow stops and starts several times a day. And for short periods--usually 20 seconds or so--it moves at up to 400 millimeters an hour, a breakneck speed for such a mass of rubble.
Reviewing the barometric and hydraulic pressure data, the team found that Slumgullion's accelerations always tracked daily drops in barometric pressure, known as atmospheric tides. The discovery was "very surprising to us for several reasons," Schulz says. For one thing, the best-known triggers for landslides are earth tremors and increased groundwater pressure from torrential rains--factors that don't affect Slumgullion. For another, he says, the changes in atmospheric pressure are incredibly small--amounting to only about 0.5%.
The team's hypothesis is that the drop in air pressure, caused each day by the warming effect of sunlight, gently siphons some of the groundwater upward through the rocks, thereby slightly reducing the friction between the landslide material and the base soil. That allows the whole mass to move a little more easily.
Slumgullion is one of hundreds of such slow-moving landslides around the globe, and Schulz says they all could be nudged by drops in atmospheric pressure as well. Such drops might even trigger earthquakes where stress has pushed the situation "close to the failure level," says seismologist Leonardo Seeber of the Lamont-Doherty Earth Observatory in Palisades, New York, who was not affiliated with the research.
It's a significant study, because it represents "the first time that such an effect has been noted," says geologist and landslide blogger David Petley of the University of Durham in the United Kingdom. The findings could be particularly useful in the study of landslides caused by typhoons and other severe storms, he adds, because they bring on very low-pressure conditions, "far more so than these atmospheric tides."