Mass Wasting

Consider a chunk of rock currently attached to a jagged outcrop high on a mountain. It will move to the sea as a result of three processes: weathering, mass wasting, and erosion.

On warm days, water from melting snow trickles into a crack which has begun to form between this chunk and the rest of the mountain. Frigid nights make this water freeze again, and its expansion will widen and extend the crack. This and other mechanical, biological, and chemical processes (such as the growth of roots, and the dissolution of the more soluble components of rock) break apart bedrock into transportable fragments. This is called weathering.

Once the crack extends through it and the chunk has been completely separated from the rest of the mountain, it will fall and join the pile of rocks, called talus, beneath it that broke off the mountain previously. This pile of rocks is called a talus pile. This movement is an example of mass wasting, known as a rockfall. As the rocks in the talus pile slip and slide, adjusting to the weight of the overlying rocks, the base of the talus pile extends outward and eventually all the rocks making up the pile will move down slope a little bit to replace those below that also moved downslope. This type of mass movement is known as rock creep, and a talus pile that is experiencing rock creep is called a rock glacier.

In the valley at the bottom of this mountain, there may be a river or a glacier removing material from the base of the talus slope and transporting it away. Removal and transport by a flowing medium (rivers, glaciers, wind) is termed erosion.

These processes occur in many other situations. A river erodes by cutting a valley through layers of rock, transporting that material using flowing water. This erosion would result in deep canyons with vertical walls if the erosion by the river were the only factor. But very high, vertical walls leave huge masses of rock unsupported except by the cohesive strength of the material of which they are made. At some point, the stresses produced by gravity will exceed the strength of the rock and an avalanche (another type of mass movement) will result. This will move some of the material down the slope into the river where erosion will carry it away.

Erosion and mass wasting work together by transporting material away. Erosion produces and steepens slopes, which are then reduced by mass wasting.