Rockfalls and rockslides. Rockfalls occur when pieces of rock break loose from a steep rock face or cliff. These result from the rock face being undercut by rivers or wave action. Frost wedging may also eventually loosen large blocks, causing them to fall. The accumulation of rock debris at the base of a steep slope is called
talus.
Rockslides usually follow a zone of weakness, such as a bedding plane or foliation plane. Separation of the rock is more likely along these planes because of their reduced shear strength. Water also tends to be channeled along these planes, which increases slippage. Collisions down the slope generally break the rock mass into rubble that eventually comes to rest. If steep slopes are involved, a fast‐moving rock avalanche may result. The rockslide or rock avalanche loses energy and speed as it moves across more level terrain.
Debris flows. Debris flows are defined as mass‐wasting events in which turbulence occurs throughout the mass. Varieties of these are called earthflows, mudflows, and debris avalanches.
When earth material moves down a hillside as a fluidlike mass, it is called an earthflow. These flows typically occur in humid areas on steep slopes with thick, clay‐rich soil that becomes saturated with water during storms. The earthflow usually leaves a steep scarp behind where it separated from the hillside. Earthflows can be fast (a few hours) or slow (a few months). Velocities range from 1 millimeter per day to meters per day. Intermittent activity can continue for years as the earthflow continues to settle and stabilize. Earthflows typically have rounded, hilly fronts. A common trigger for an earthflow is the undercutting of the slope by erosion from wave action or rivers or by construction projects.
A variety of earthflow called solifluction is the flow of watersaturated earth material over an impermeable surface such as permafrost. It occurs frequently in bitterly cold regions such as in Alaska or Canada. Springtime temperatures thaw only the first few feet of the frozen ground (the active layer), which becomes saturated quickly and slowly flows over the ever‐frozen permafrost below. Solifluction can occur on even the gentlest of slopes. Not forceful enough to break apart the surface vegetation, the migrating material drags it along like a wrinkled green rug. The soil finally settles on level ground at the base.
A mudflow is a liquidy mass of soil, rock debris, and water that moves quickly down a well‐defined channel. Generally viscous and muddy colored, it can be powerful enough to move large automobiles and buildings. Mudflows occur most often in mountainous semiarid environments with sparse vegetation and are triggered by heavy rainfall that saturates the loose soil and sediment. They are also the natural result of volcanic ash build‐ups on flanks of volcanoes and of forest fires that have exposed the soil to rapid erosion. A mudflow originating on a volcanic slope is called a lahar.
The deadliest variety of debris flow is the debris avalanche, a rapidly churning mass of rock debris, soil, water, and air that races down very steep slopes. It has been theorized that trapped air may increase the speed of an avalanche by acting as a cushion between the debris and the underlying surface.
Creep. A slow, gradual movement of soil or regolith downhill over time is called creep. Velocities are typically less than a centimeter per year. Freezing and thawing contribute to soil creep by progressively moving soil particles down the hill. Creep is manifested at the surface by such things as tilted utility poles that become more out of alignment every year. Vegetation helps reduce the rate of soil creep.
Slump. Earth material that has moved as a unit along a curved surface is called slump. A slumped mass of sediment is typically clay rich. Slump usually results when the geometrical stability of a slope is compromised by the undercutting of its base, such as by wave action, a meandering river, or construction.