Atmospheric tides speed the flow of the Slumgullion landslide. Credit: USGS

The intermittent stop and go of a Rocky Mountain landslide is controlled by the diurnal ebb and flow of air pressure, a study suggests.

The four-kilometre-long Slumgullion landslide in the San Juan Mountains of southwestern Colorado, so named for its yellowish loamy soil, which reminded early settlers of slumgullion stew, began about 700 years ago. At its centre, soil and weathered rocks creep downhill at an average speed of half a millimetre per hour.

The movement of the landslide accelerates during the spring snowmelt period. And, puzzlingly, it is generally faster during the night than in the day.

William Schulz of the United States Geological Survey in Denver, Colorado, and colleagues set out to investigate the landslide's mysterious movements. Over a period of nine months they monitored the landslide at two spots, taking hourly measurements of slide speed, the water pressure inside pores in the soil, and weather conditions. They discovered that the movements of the rubble were in sync with 'atmospheric tides' — daily highs and lows in atmospheric pressure that are excited by changing exposure to the Sun's rays and by the Moon's gravitational pull.

During nightly hours of low atmospheric pressure, the landslide moves fastest, the team found. When air pressure increases during the day, the flow slows down or stops. Their work is published in Nature Geoscience1.

The scientists suspect that changes in air pressure alter the frictional forces that hold the landslide in place. During periods of relatively low atmospheric pressure, air and water particles contained in sediment pores in the soil tend to move upwards to areas of lower pressure, easing frictional forces, they believe, and allowing easier sliding. The researchers say that their mathematical model of the basic physical forces driving landslide movement shows that their proposed mechanism is plausible.

Creeping menace

Land- and mudslides, often triggered by excessive rainfall or by earthquakes, pose a deadly natural hazard in many parts of the world. Earlier this month, more than 160 people were killed in mudslides in the Cordillera mountains of the Philippines, in the wake of Typhoon Parma.

"We do know that heavy rainfall is the most common trigger mechanism," says Falk Lindenmaier, a landslide expert at the University of Karlsruhe in Germany. "But this must not lead us to ignore other processes at play."

Studies in the European Alps have shown that small changes in environmental conditions do sometimes suffice to destabilize slopes2. But not all mechanisms that trigger landslides are well understood. One reason for this, Lindenmaier says, is that scientists normally get to study slides only after they have begun or finished moving. The trigger mechanism for the landslide that occurred in 2007 near Doren in Austria is unknown, for example.

"That's why the new findings are so interesting," says Lindenmaier. "They don't directly affect safety considerations in the Slumgullion area. But they do help us better understand, and possibly predict, the behaviour of landslides."

Schulz and his team go one step further. They suspect that atmospheric tides could be involved in other phenomena that involve sliding surfaces, including earthquakes, volcanic eruptions and glacier movement.