The term mud flow, although not part of the classification system used by most landslide specialists, is a form of mass movement or mass wasting widely used in a manner that is synonymous with wet to very wet, rapid to extremely rapid earth flow. Mud itself is defined by most geologists as an unlithified mixture of silt, clay , and water ; therefore, a mud flow is a flow consisting primarily of silt, clay, water, and other minor constituents such as sand , cobbles, boulders, trees, and other objects. A flow in which mud is a minor constituent relative to sand-size or coarser particles is by definition a debris flow or, if large pieces of bedrock are involved, a rock flow.
Because volcanic ash deposits commonly weather into clayey materials, mud flows are common on and around volcanoes as well as areas covered by deposits of fine-grained volcanic ash known as loess. A mudflow on a volcano can also be referred to as a fine-grained or muddy lahar .
Like debris flows, mud or earth flows can begin by mobilization from a landslide, incorporation of muddy sediments into flooding, or rapid melting of snow and ice during a volcanic eruption. Regardless of their mode of origin, mud or earth flows can be dangerous and destructive because of their great density (typically more than 50% solid material) and velocity. The density of debris and mudflows also allows them to transport unusually large boulders compared to floods consisting primarily of water. A typical debris or mudflow consisting of 60% solids and 40% water would have a density of about 125 lb/ft3 (2,000 kg/m3), or twice that of water. Thus, the buoyant force exerted on a boulder by a mud or debris flow would be about twice that exerted on the same boulder by water.
See also Floods