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Mass Wasting: Definition, Types, Causes & Processes

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  • 0:06 Mass Wasting
  • 1:22 Causes of Mass Wasting
  • 3:59 Slumps and Rockslides
  • 5:23 Debris Flow and Earthflow
  • 6:40 Lesson Summary
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Lesson Transcript
Instructor: Rebecca Gillaspy

Dr. Gillaspy has taught health science at University of Phoenix and Ashford University and has a degree from Palmer College of Chiropractic.

Mass wasting or mass movement is the movement of a large mass of rock, soil and debris downward due to the pull of gravity. Learn about the process of mass wasting, the factors that must be present to trigger it and the different types.

Mass Wasting

If a rock slides off of a mountain and no one is around to hear it, does it make a sound? Well, I don't know about the noise this activity would create, but I do know that mountains erode and that rocks and debris can slide and fall down mountain slopes in massive amounts. In this lesson, you will learn about a process called mass wasting and the factors that cause this movement of material.

Mass wasting, which is sometimes called mass movement or slope movement, is defined as the large movement of rock, soil and debris downward due to the force of gravity. In other words, the earth's outer crust is being 'wasted' away on a 'massive' scale and falling to lower elevations.

Mass wasting is a type of erosion, and it is capable of making big changes to the side of a mountain. These changes can happen suddenly, as in one minute the rock is there and the next it is gone, or it can happen more slowly over time. You might think of this process as a landslide, and this term is sometimes used interchangeably with mass wasting. However, the term landslide is a bit limiting and does not allow for a description of the many different triggers and types of erosion that can happen on this large of a scale.

Causes of Mass Wasting

Now, we mentioned that mass wasting is mainly due to gravity. So, we see that mountains have an ongoing tug-of-war with gravity. Gravity is constantly trying to pull rock and debris down the slope of a mountain. At the same time, the resistive forces of the mountain, including the cohesive strength and internal friction between the materials, referred to as the mountain's shear strength, constantly pulls back against gravity.

The shear strength works to maintain the slope's stability and keep the materials in place. This is a lot like a mountain climber gripping onto the side of a mountain and resisting gravity. The climber uses his grip strength to resist gravity, like the mountain uses its shear strength.

With this understanding, we see that the causes of mass wasting occur when gravitational force overcomes the resistive forces of the mountain. And, since gravitational pull is always constant, then we see that mass wasting occurs when something changes the mountain's ability to resist gravity.

For instance, an increased slope steepness increases mass wasting simply because the gravitational force acting on a steep slope is greater than the force acting on a gentle slope. Increasing the steepness of a slope is one way man can increase mass wasting. For example, if a road crew cuts away a slope to make room for a new road but makes the angle of the slope too steep, the slope will be prone to mass wasting, and you will want to cross your fingers when you drive past this steep slope so no rocks or debris fall on your car!

Increased water is another factor that plays an important role in mass wasting. Water can wash away small particles that help keep the mountainside intact. This is similar to what happens when a wave comes ashore and washes away a sandcastle. The abundant water breaks apart the small sand particles and destroys the structural stability of the castle you spent the afternoon building.

If an area has decreased vegetation, it will be more prone to mass wasting. Vegetation stabilizes soil particles on the surface and anchors soil under the surface through its root system. This is much like comparing two sand dunes on a beach. If one sand dune has grasses growing on it, it will resist the erosion of water and wind better than a sand dune without vegetation.

Another factor that plays a role in mass wasting is earthquakes. The violent shaking that occurs in a region where an earthquake takes place has the ability to break off sections of mountains or hills, causing them to slide down the slope.

Slumps and Rockslides

Because there are different factors that cause mass wasting, not all wasting happens in the same way. In some cases, we may see a large portion of the slope fail and slide or roll downhill. In other cases, large collections of debris may flow, like a river, down the side of a mountain mixed with water or air.

One of the types of mass wasting that is an example of the slope failing is a slump. This is the sliding of coherent rock material along a curved surface. It's almost as if the section of the mountain or hillside slumps its shoulders and shrinks in height. This is easy to image happening if you consider that a slump often results due to an undercutting of the mountain's base. For example, water might erode the base of a cliff. Without the base, the outer sections of the mountain slumps down as a unit or multiple units.

A rockslide is another example of mass wasting of a slope. A rockslide is the sliding of rock material down a mountain. It is similar to a slump, but a rockslide does not move along a curved surface like a slump. Therefore, in a rockslide, we see rocks sliding down a pre-existing surface, such as an underlying layer of rock. In a way, it is as if the rocks are moving down a slide on a playground. As a result, we will see a collection of fallen rocks at the base of a rockslide.

Debris Flow and Earthflow

Other types of mass wasting involve the downward movement of unconsolidated materials. These are collectively referred to as flows because the materials flow, like a fluid, down a slope, much like cement flows out of a cement truck.

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