# Factors that Influence Mass Wasting

Once rock material has been broken down into smaller, unstable pieces by weathering, the material has the potential to move down slope because of gravity. This is called mass wasting (also called mass movement or a landslide).  Before looking into the various types of landslides, the factors that influence them must be examined. Ultimately, this comes down to the dynamics of two factors: gravity and steepness of slope. The steeper the slope, the greater potential for gravity to pull objects down. The shallower the slope, the less effect gravity has on pulling objects down.

There are several factors that influence mass wasting, but ultimately it is a battle between friction and gravity. If the friction on a rock is stronger than gravity for a particular slope, the rock material will likely stay. But if gravity is stronger, the slope will fail. The steeper the slope, the stronger the friction or rock strength must be to resist down-slope motion. The steepest angle a slope can be before the ground will slide is about 35 degrees. This is called the angle of repose. Many times we will cut through a slope to make room for a road or other forms of development. So to help prevent the slope from sliding along these cut areas, retaining walls must be build. More on this later.

## Composition of Slope Material

Another factor that determines mass wasting is the slope's material. Mass wasting is more prone on slopes that contain clay and shale. Clay particles absorb water and prevent water from percolating through the ground. A layer of clay on a slope can prevent water from filtering through the slope. Instead, the water stays near the surface and saturates the ground. This can cause the surface layers to lose friction and slide.

## Weight and Friction of Slope

A third factor that influences whether a slope will fail is the load or weight of that slope. Adding weight to a weakened slope can obviously cause it to slide easier, especially on steep slopes. This added weight tends to occur by building on top of weak slopes, increasing the steepness of the slope, or over-saturating the slope. As already noted, as long as the friction along the slope is stronger than gravity, the ground is unlikely to slide. But if that friction is weakened, slope failure becomes more likely. There are several other ways friction can be reduced along a slope: wildfires, removal of vegetation, or adding too much water. Gravity is probably the ultimate driving force of mass wasting. The force of gravity pulls all things on the planet toward the center of the Earth. Without gravity, mass wasting would not occur. But unlike many of the other factors, humans have no influence on or control over gravity.

## Regional Climate Conditions

A region's climate can also determine the likelihood of a landslide. In western Oregon, for example, mass wasting is common in the winter from heavy precipitation and in spring when snow melt, water saturation, and runoff is greatest. The type of climate will also help determine the type of mass wasting. Humid climates tend to have flows or slides, where water-saturated slopes fail and fall. Drier climates tend to have rocks that fall, especially in early spring. Canyons and places prone to wildfires tend to have debris flows.

## Water Content within Slopes

The amount of water in the soil is a major factor in the stability of a slope. When you build a sand castle, water is needed to build the walls and towers. That is because water has surface tension and is attracted to itself. This allows you to build towers greater than the angle of repose. So a little water can actually prevent slopes from sliding. But too much water lubricates the individual grains of sediment, decreasing friction between each grain, so the possibility of mass wasting increases. The increase of water within the soils can come from over watering, pipe or swimming pool leaks, or prolonged stormy weather. In Oregon and many other mountainous regions, spring runoff of snow melt increases the water content within the soil.

## Gravity

Finally, gravity is the driving force of mass wasting. The force of gravity pulls all things on the planet toward the center of the Earth. But unlike many of the other factors, humans have no influence or control on gravity. For more information on what causes landslides in Oregon, click here.