© William Schulz/USGS
The slow-moving Slumgullion landslide in southwestern Colorado.
A river of rock and soil nearly 2.5 miles long and 1,000 feet wide, the Slumgullion landslide winds like an earthy freight train down the hills of southwestern Colorado. But this incredible force of nature is swayed by the tiniest push.

According to a new study, the daily ups and downs in air pressure -- equivalent to the weight of about half a glass of water -- are enough to get the behemoth rolling.

Just like the ocean, the atmosphere has tides of air that swish over the planet, controlled by the sun's heat. Around the hottest part of the day, air pressure is diminished -- 'low tide' -- and it gradually goes up as things cool off.

William Schulz of the United Stated Geological Survey in Denver compared detailed records of the Slumgullion landslide's movement against pressure readings taken in the area.

They fit hand-in-glove: each time pressure lowered during the warmest part of the day, the Earth slid a little bit faster.

Someone watching the slide would be rather bored with its snail's pace of just a few millimeters an hour. That's the paradox of landslides; the bigger ones are usually slower and less dangerous. But what the team is learning from Slumgullion could apply to smaller, violent slides that claim thousands of lives around the world each year.

"There should be a greater, not less effect on smaller, shallow landslides," Schulz said.

That's because as atmospheric pressure releases over a slide, it draws air and water up through the soil. Like a breeze blowing over a sandy beach, the subterranean air and water movement causes friction, pulling soil and rock along with it.

But the effect only reaches down about 30 feet beneath the surface. Slumgullion is thought to extend deeper than 100 feet in places.

"For something that's precariously balanced and ready to go, atmospheric tides could trigger a slide," Schulz said of smaller, catastrophic slides.

He was quick to add that rainfall, earthquakes and other, much stronger disturbances will still trigger the majority of dangerous landslides. And no one has yet shown that tides affect them at all.

"That's the next step," Scott Burns of Portland State University said.

"Now, what the rest of us (researchers) need to do is say 'is there any relationship between atmospheric tides and catastrophic slides?'" he said. "We just don't know right now."