Mass in slumps. Rockslides can be extremely dangerous

Mass movement involves the movement of material such as soil, rock, mud or snow, down a slope under the influence of gravity. In this lesson, we will look at what causes mass movement, the types of mass movement and see some real life examples of each.

What is Mass Movement?

Mass movement, often called mass wasting, is the downslope movement of a mass of surface materials, such as soil, rock or mud. This mass movement typically occurs along hillsides and mountains due to the influence of gravity and can happen very slowly or very quickly.Mass movement can occur due to a variety of reasons. The most basic reason is the angle of repose or slope of the hillside.

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If the angle is overly steep, gravity will pull the material downward, causing a mass movement. The angle of repose can also influence how fast the material will move. When you roll a ball down a gentle slope it moves relatively slow; however, if you increase the slope or the angle, the ball will roll down the incline much faster. Earthquakes are also a common cause of mass movement. As the ground shakes, due to the energy released during the earthquake, portions of the hillside or mountain can come loose and move downslope. The lack of vegetation can also contribute to mass movement. Vegetation helps anchor the soil in place, which prevents it from moving.

When vegetation is removed, that anchor is lost and soil can be easily dislodged. An overabundance of water will also make the soil very mobile. Water actually lubricates the soil and contributes additional weight, just like how your clothes are heavier when they get wet. The additional weight helps the material move downslope. Geology also plays an important role. This includes the rock type present, the dip of the rock layers, or the structural nature of the area.

While all of the above can cause mass movement to occur, the ultimate control of mass movement is gravity. Without the help of gravity, there would be no downward movement regardless of the cause.

Types and Examples of Mass Movement

Flows occur when the material, soil, and/or rock, behave more like a liquid or fluid. Flows include mudflows, debris flows or lahars (superheated water that moves down an erupting volcano). Flows occur due to a large amount of water or ice present in the soil or material.

Flows are most often the fastest traveling and can have speeds in excess of 70 miles per hour, depending on the location and steepness of the slope.

A debris flow in the Philippines.
flow

Slumps behave differently than flows. Slumps occur as a wedge or slice of material that moves as one piece along a curved surface. As a result, it often can resemble a spoon scoop.

Slumps typically occur where there is loose material or rock.

Here you can see the typical spoon scoop shape of a slump in Lincoln County, Wyoming.
slump

Landslides typically consist of unconsolidated rock. A great example of this is a rockslide.

The rocks do not behave as a fluid, such as in flows, and they do not move as one unit, as in slumps. Rockslides can be extremely dangerous because they occur very quickly with tremendous force. An avalanche is another example of a landslide. An avalanche occurs when snow or snow and ice move quickly downslope.

This landslide happened in 1995 in Laconchita, California. Notice the steepness of the slope.

Landslide
An avalanche in the Himalayas near Mount Everest.
Mount Everest

Creep is the slowest type of mass movement. It cannot be seen with the naked eye and sometimes takes years to notice.

Creep involves movement of material down gently sloping areas, as opposed to steeper inclines. As the material moves slowly it causes unique features such as gun stalk trees and tipping fences, or telephone poles. The surface of the earth in these areas have a ‘funhouse’ appearance, with a wobbling or melting look.

Here the landscape in England displays the melting or rippling effect that is often the result of soil creep. Compare the gentle slope to the steep slope in the landslide photo.

soil creep

Lesson Summary

Mass movement, often called mass wasting, is the downslope movement of a mass of surface materials, such as soil, rock, or mud. The most basic reason is the angle of repose, or slope of the hillside.

Other causes of mass movements include earthquakes, lack of vegetation, abundance of water, geology, and gravity. There are multiple types of mass movements. Flows occur when the material, soil, and/or rock, behave more like a liquid or fluid. Slumps occur as a wedge or slice of material that moves as one piece along a curved surface. Landslides typically consist of unconsolidated rock. And creep involves movement of material down gently sloping areas, as opposed to steeper inclines.

Mass Movement Types

massmovement
Types Descriptions
Flows occur when soil or rock acts like a liquid
Slumps a slice of material that moves as one piece along a curved surface
Landslides made up of unconsolidated rock; includes rockslides and avalanches
Creeps material that moves slowly down gently sloping areas

Learning Outcomes

After this lesson is done, students should be able to:

  • Explain what mass movement is
  • Recall the types of mass movement
  • Describe the types of mass movement
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