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Slow mass wasting Slow mass wasting is the slow downhill motion of materials under the influence of gravity. It is the dominant process that moves regolith slowly downhill, where they are picked up by streams and carried off to the edges of continents. The process occurs on even the slightest slopes and is due to repeated expansion and contraction of regolith, soils and sediments. There are two main causes for these expansion and contraction cycles. The first involves clay minerals. When clays become wet, they expand, some by as much as 400%. When they dry up, they shrink again. Similarly when soils are wet and freeze, the soil water expands by some 9% and the entire soil expands. When the ice thaws, the soil shrinks again.
As a soil expands, it will expand outward, in the direction of least confinement which is at right angles to the slope. When it shrinks the shrinkage is straight down because it is being pulled on by gravity. The net effect is a stready downhill motion of the surficial materials, called creep. The role of soil moisture is critical. In arid areas where there is little moisture and therefore limited creep, the materials along the sides of the valleys do not creep downward as much and valley walls remain vertical, forming canyons. The feect of creep can be seen on tilted fencelines and telephone posts, j shaped trees etc. In the Artic, in permafrost areas a special form of flow mass wasting, solifluction moves materials downhill.
Geologically, slow mass wasting is central in moving sediments downhill and, along with streams shapes the landscapes in most places. Rapid Mass Wasting

Rapid earth movements are often devastating. To cite but two examples, a 1970 earthquake in Peru set off landslides that killed 22,000 people. Earlier, in 1963, nearly 400 million cubic yards of rock slid into a reservoir in Vaiont, Italy rupturing a dam and killing over 2,500 people. Although far less geologically important than the processes of slow mass wasting, these rapid movements of unstable slopes, nevertheless represent a major environmental hazard.

The Causes of Rapid Mass Wasting Several factors have been recognized as leading to greater danger. They include:

1. oversteepening: all matterials have a specific angle of repose, a certain slope angle at which they are stable. Exceeding this angle commonly leads to slope failure.

2. overloading. Placing additional weight on a slope, even on one that is below its critical angle may cause the materials to move downslope.

3. type of materials: Weak slopes, composed of loose materials will move more readily than coherent resistant rock.

4. topography: The steeper the slope, the greater the chance of movement.

5. vegetation: Trees and plants may prevent the saturation of soils by transpiring moisture (which would lubricate movement) and further stabilize slopes with their roots.

6. water: Increase in the water content of a material will increase pore pressure, weakening the material while adding weight to the slope, and acting as a lubricant.

7. stucture of the materials: Are they layered, folded or faulted? Are there weaknesses in the bedding planes?

8. triggers: Sudden vibrations, such as sounds or earthquakes, may initiate movement in an otherwise stable slope.

Because many of these catastrophic events are associated with long-term natural factors, they are predictable, and areas with a history of repeated slope failure can be identified and avoided. Increasingly, many slopes have been impacted by human activities such as blasting, lumbering, bulldozing etc., thereby leading to failures in historically stable areas. Sadly, many of these man induced failures could be avoided with the application of proper engineering techniques.

As the water content in the materials increases, it becomes difficult to draw the line between the processes of mass wasting, where materials move under their own weight, and streams, which involve water as a medium of sediment transport.