Some landslides proceed at a snail's pace downslope, causing property damage. Others, however, gain a catastrophic momentum and often claim lives. What accounts for the difference? According to a recent report in the journal Science, it may well have something to do with tiny variations in the porosity of the soil--that is, how loosely the soil is packed.

In order to address this question, R. M. Iverson of the U.S. Geological Survey and his colleagues made a mock mountainside from a tilted concrete platform covered with soil. They peppered it with sensors to track changes in porosity and soil movement as the "mountain" was watered. The results were dramatic. Adding water to loosely packed soil (high porosity) caused the pores to shrink, which augmented pore water pressure and decreased the friction between soil grains. A high-speed flow ensued. Yet soils only one tenth denser (less porous) experienced pore dilation upon watering, which reduced water pore pressure. Frictional strength thus increased, leading to a creeping flow. "We conclude that pore pressure diffusion can seldom keep pace with soil deformation," the team writes, "and that relatively small variations in porosity can influence landslide behavior profoundly."