Mud Flow (World of Earth Science)
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