Avalanche
Under different circumstances, granular matter can exhibit properties reminiscent of the solid, liquid and gaseous state. A mass of sand, or other granular material in equilibrium, resembles a solid. However, the slope at the edge of a mass of granular material becomes to great, Avalanches develop in the material and the pile begins to flow like a fluid. The unusual behavior of granular matter, such as the avalanche instability that develops within it, are a wide open area of research. Photo Courtesy of Sidney Nagel, U. Chicago.