Close view of a lahar deposit that originated as a large prehistoric landslide from a volcano in Chile. The lahar deposit contains many dark angular clasts (rock fragments) that were derived from basalt lava flows on the volcano--they were shattered, broken, and split apart during the landslide and its transformation to a lahar. The rounded light-colored material is rock from the volcano that had been hydrothermally altered to silt and clay-sized particles; the silt and clay is also thoroughly mixed in the lahar deposit.
Unlike the 1980 landslide of Mount St. Helens, most of which was deposited in a river valley within 25 km of the volcano, volcano landslides may continue to travel downstream as a lahar. In such cases, the only evidence of a landslide likely to be preserved in the geologic record is a lahar deposit many tens of kilometers downstream.
A distinguishing characteristic of many lahar deposits that began as a volcano landslide is the presence of a clay-rich matrix (at least 3 percent clay-sized particles, often more than 3-5 percent). Scientists often refer to these clay-rich lahar deposits as muddy lahars or cohesive lahars. The silt and clay comes from the hydrothermally altered rocks from within volcano. The repeated intrusion of magma into the cone and release of volcanic gases turns a volcano's groundwater system into a hot, acidic brew that over time changes hard rocks into weak, clay-rich and water-saturated material.