Major Triggers for Mass Wasting: Water, Slopes, Vegetation Removal & Earthquakes

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  • 0:07 Mass Wasting and Shear…
  • 1:24 Increased Water
  • 2:17 Increased Slope Steepness
  • 3:06 Vegetation Removal
  • 4:03 Earthquakes
  • 4:27 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 is the movement of mass amounts of rock, soil and debris downward due to gravity. Learn about major triggers of mass wasting, such as increased water, increased slope steepness, the removal of vegetation and earthquakes.

Mass Wasting

If you were to wager on a tug-of-war match between Team Gravity and Team Mountain Slope, which team would you put your money on? Well, if you chose gravity, you would be a winner. This is because the pull of gravity never stops. Yet, the resistive forces that hold rock, soil and debris on the slope of a mountain eventually get worn down.

This wearing down, or erosion, is due to factors such as water, the steepness of the slope, the removal of vegetation, and disruptive events such as earthquakes. These factors trigger a process known as mass wasting, and in this lesson, you will learn more about this process and the triggers that put it in motion.

Mass Wasting and Shear Strength

Mass wasting is defined as the large movement of rock, soil and debris downward due to the force of gravity. It happens when the gravitational force acting on a mountain overcomes the resistive forces of the mountain that work to keep rocks and soil in their places. These resistive forces, including the cohesive strength and internal friction between materials, are referred to as the mountain slope's shear strength. Any factor that weakens the shear strength of the mountain slope could lead to mass wasting.

Increased Water

Of all of the factors that play a role in erosion and therefore trigger mass wasting, water may be the most important. Water can both increase the risk of mass wasting and decrease the risk, depending on how much water is present. This can be related to building a sandcastle at the beach. In order to make sand stick together, you need a little bit of water.

The same goes for soil particles on a mountain slope. A small amount of water helps them bond and stick together. However, if you add too much water to the sand, you will have a hard time getting the sandcastle to stand up, and your sandcastle walls will collapse under the influence of gravity. This is also seen with mass wasting on the slope of a mountain. If too much rain falls or soil becomes saturated by increased water, then the soil particles and other materials will wash away down the mountain.

Increased Slope Steepness

As mentioned earlier, mass wasting happens when the gravitational force acting on the mountain overcomes the resistive forces that hold the mountain in place. Sometimes, man gives gravity an edge by cutting a slope too steep during road construction along the side of a mountain. When there is increased slope steepness, gravity has an easier time pulling rock, soil and debris down. This is similar to how you would ski down a sharply sloped black diamond ski run faster than you would down the gently sloped bunny hill.

Man is not the only culprit that can lead to mass wasting due to slope steepness. Nature can also play a role, especially along streams and coastlines where water or waves can erode rock and soil and steepen the slopes.

Vegetation Removal

We also see that vegetation removal can trigger mass wasting. Plants, trees and other forms of vegetation stabilize soil on the surface and protect it from the impact of raindrops, which can detach the bonds holding the soil together. The roots that grow beneath the surface anchor sediment to underlying structures, which further decreases the risk of mass wasting.

You may have noticed the protective function of vegetation when you visited the beach. Older and better-established sand dunes that are formed farther inland often contain vegetation that prevents sand from blowing away and eroding the dunes, whereas newly formed dunes closer to the water's edge are constantly shifting under the influence of water and wind.

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