These illustrations of a large landslide show a single mass of rock debris breaking loose high on the flank of a volcano and sliding into a wide valley. Note the initial development of a headwall scarp at the top of the landslide (top image), the widening path of the landslide as it enters the valley, and the horseshoe-shaped crater carved from the cone (bottom image).

Many landslides actually involve several large blocks of debris that collapse and merge together on the volcano or as they spread into low-lying areas. Such large landslides typically travel much farther from a volcano than shown in these two illustrations. If the rock debris contains a high water content or consists of a significant amount of clay particles (>3-5 percent), a landslide typically transforms directly into a lahar and continues flowing for many tens of kilometers.