Chapter 5.3 Mass Movements

Earth’s land is made of slope and as the force of gravity works on the slope materials move downslope. This movement is called mass movement Weathering and mass movement produce most landforms.

Triggers of Mass Movements Gravity is the force behind mass movements but other factors that trigger mass movements are saturation of surface materials with water, over steepening of slope, removal of vegetation, and earthquakes.

Water Heavy rains and rapid snow melts can cause mass movement by saturating surface materials. This causes mudflows.

Oversteepened Slopes For slopes to maintain materials they need to be between 25 to 40 degrees. If the slope goes over that a mass movement becomes more likely. This can be caused by a stream undercutting a valley wall or waves pounding against the base of a cliff. Also people can cause oversteeepening by excavating during construction.

Removal of Vegetation Plants make slopes more stable because their root systems hold the soil together. When plants are removed by fire or logging and farming this increases the likelihood of mass movements.

Earthquakes An earthquake can move large amounts of rock and unconsolidated material.

Types of Mass Movements Mass movements are classified by the kind of material that moves, how it moves, and the speed of movement. 5 basic types of mass movements are rockfalls, slides, slumps, flows, and creep.

Rockfalls This occurs when rocks or rock fragments fall freely through the air. This is common on steep slopes.

Slides This is a block of material moving suddenly along a flat, inclined surface. Rockslides include bedrock. Rockslides are fast moving, over 200 km per hour.

Slumps This is the downward movement of a block of material along a curved surface. This material doesn’t travel very far or fast. Slumps leave a crescent-shaped cliff just above the slump.

Flows These are mass movements of material containing a large amount of water, which move downslope as a thick fluid. Mudflows – move quickly. Caused usually by a heavy downpour or rapid snowmelt. Earthflows are slower only a millimeter to several meters a day. They may happen for years.

Creep This is the slowest type of mass movement, only travels a few cm a year. Freezing and thawing contributes to creep. This causes vertical structures to tilt downward.