Mass movements (also called mass-wasting) is the down-slope movement of Regolith (loose uncemented mixture of soil and rock particles that covers the Earth's surface) by the force of gravity without the aid of a transporting medium such as water, ice, or wind. Still, as we shall see, water plays a key role. Mass movements are part of a continuum of erosional processes between weathering and stream transport. Mass movement causes regolith and rock to move down-slope where sooner or later the loose particles will be picked up by another transporting agent and eventually moved to a site of deposition such as an ocean basin or lake bed. Mass movement processes are occurring continuously on all slopes; some act very slowly, others occur very suddenly, often with disastrous results. In this discussion, we hope to answer the following questions:
We start with a discussion of the forces acting at the surface that cause mass movements. Gravity Gravity is the main force responsible for mass movements.
On a slope, the force of gravity can be resolved into two components: a component acting perpendicular to the slope, and a component acting parallel to the slope.
The Role of Water Although water is not always directly involved as the transporting medium in mass movement processes, it does play an important role. Addition of water from rainfall or snow melt adds weight to the slope. Water can seep into the soil or rock and replace the air in the pore space or fractures. Since water is heavier than air, this increases the weight of the soil. If the material becomes saturated with water, vibrations could cause liquifaction to occur, just like often happens during earthquakes. Water can reduce the friction along a sliding surface. Water has the ability to change the angle of repose (the slope angle which is the stable angle for the slope). Think about building a sand castle on the beach. If the sand is totally dry, it is impossible to build a pile of sand with a steep face like a castle wall. If the sand is somewhat wet, however, one can build a vertical wall. If the sand is too wet, then it flows like a fluid and cannot remain in position as a wall.
Another material that shows similar swelling and compaction as a result of addition or removal of water is peat. Peat is organic-rich material accumulated in the bottoms of swamps as decaying vegetable matter.
Weak Materials and Structures Rocks often contain planar structures that become slippage surfaces if weight is added or support is removed.
Mass Movement Processes The down-slope movement of material, whether it be bedrock, regolith, or a mixture of these, is commonly referred to as a landslide. All of these processes generally grade into one another, so classification of mass movement processes is somewhat difficult. We will use a common classification of mass movements, which
divides the processes into two broad categories and further subdivides these categories.
Sediment Flows Sediment flows occur when sufficient force is applied to rocks and regolith that they begin to flow down slope. A sediment flow is a mixture of rock, regolith with some water. They can be broken into two types depending on the amount of water present.
Each of these classes of sediment flows can be further subdivided on the basis of the velocity at which flowage occurs.
Mass-Movements in Cold Climates Mass movements in cold climates is governed by the fact that water is frozen as ice during long periods of the year. Ice, although it is solid, does have the ability to flow, and freezing and thawing cycles can also contribute to movement.
Subaqueous Mass movement Mass movements also occur on slopes in the ocean basins. Most slope failure can occur due to over-accumulation of sediment on slope or in a submarine canyon, or could occur as a result of a shock like an earthquake. 3 types can occur - (1) Submarine slumps – Coherent blocks break and slip, similar to slumps on land.
(2) Submarine debris flows – Moving material breaks apart and flows, similar to debris flows on land. (3) Sediment moves as a turbulent cloud, called a turbidity current. (See figure 16.10 in your text). Gigantic submarine slope failures are widespread on the ocean floor, particularly around islands like Hawaii and off the east and gulf coasts of North America. They are much larger than land-based slope failures and are an important process sculpting
adjacent land. When they occur, they create catastrophic tsunamis. (See figures 16.11 in your text). Triggering Events A mass movement can occur any time a slope becomes unstable. Sometimes, as in the case of creep or solifluction, the slope is unstable all of the time and the process is continuous. But other times, triggering events can occur that cause a sudden instability to occur. Here we discuss major triggering events, but it should be noted that it if a slope is very close to instability, only a minor event may be necessary to cause a failure and disaster. This may be something as simple as an ant removing the single grain of sand that holds the slope in place.
Which factor influences the mass movement of particles?Gravity is the main force responsible for mass movements. Gravity is a force that acts everywhere on the Earth's surface, pulling everything in a direction toward the center of the Earth.
What is mass movement influenced by?Mass movement, also known as mass wasting, is a type of erosion that moves materials downslope as a result of the force of gravity. The slope of the land, seismic activity, vegetation, water, and the geology of the area are also factors that affect mass movement, but the number one factor is gravity.
What are the 4 factors that influence mass movements?landslides. Mass wasting is the downhill movement of Earth materials under the pull of gravity. Mass wasting is influenced by slope, material strength, water content, and amount of vegetation. Mass wasting can be triggered by storms, earthquakes, eruptions, and human activity.
What are two factors that influence mass movement?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.
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