Mass WastingChapter 5.3

Mass wasting is the movement of rocks and soil downhill due to the force of gravity.

Mass wasting can take many different forms.

Examples of mass wasting include avalanches, rockslides, mudflows, and many more.

Gravity is the main cause of all mass wasting.

The likelihood of a mass wasting event can be influenced by the following four factors.

Water Saturation – A little bit of water can actually stabilize the soil, but when soils become saturated they turn into mud.

Oversteepening of Slopes – There are both natural and man-made sources for oversteepening of slopes.

The steeper the slope, the more mass wasting.

Removal of Vegetation – Plants have roots that hold the soil in place. When vegetation is removed, mass wasting increases.

This area in California experienced a wildfire, reducing the vegetation in 2003.

Intense rainfall two months later produced a debris flow in the canyon below, killing 13 people.

Earthquakes – Earthquakes can accelerate the rate of mass wasting. Seismic waves produced by earthquakes can dislodge loose rocks and cause unsaturated soils to become saturated.

During the 1964 Earthquake in Alaska, many places in Southcentral Alaska experienced mass wasting in the form of liquefaction.

Geologists classify mass wasting events based on the kind of material that moves, how it moves, and the speed at which it moves.

Rockfalls – Occurs when large rocks free fall through the air. Occurs on very steep slopes. These are the fastest type of mass wasting.

Here is a picture of a rockfall in Yosemite National Park

Rockslides – Occurs when large rocks slide along a flat inclined surface. These are the next fastest type of mass wasting, reaching speeds of 200 km per hour (100 MPH).

Mudflows – Mudflows are the next fastest type of mass wasting. Mudflows contain a mixture of soil, rock, and water. They have the consistency of wet concrete.

A particular type of mudflow created by a volcano melting glaciers on its summit is called a lahar.

Mudflows are some of the most dangerous types of mass wasting events.

In 1988, a dangerous type of mudflow called a lahar, was triggered by a volcanic eruption in Columbia that killed 25,000 people.

Earthflows – earthflows are relatively slow. They move about 1 mm to several meters per day. They vary in size from a few meters long and 1 meter deep, to a kilometer long and 10 meters deep. They occur most often on wet hillsides and they can last for several years.

Slump – Slumps are similar to earthflows. They move relatively slow. Slumps do not travel very far, and they move along a curved surface and leave behind a crescent-shaped cliff when they occur.

Creep – Creep is the slowest type of mass wasting. Creep only travels a few millimeters or a few centimeters per year.

Creep is generally caused by contraction and expansion of soils as a result of the regular freeze-thaw cycles due to the seasons.