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Landscapes and Mass Wasting (chapter 13)

Terms in this set (13)

Landforms created by the buildup of sediments are called
depositional landforms.
Landforms sculpted by erosion of the landscape are called
erosional landforms.
Because some types of rock and sediment are more easily eroded than other types, landscapes often show
the effects of differential erosion ,in which resistant materials stand up as ridges and mountains, while softer materials form valleys.
Mass Wasting
Mass wasting is the downslope movement of materials under the influence of gravity. The movement can be fast or slow, and the material can move as a single coherent unit or as a chaotic jumble.
Factors
A slope is stable (that is, the material stays in place) when the frictional resistance to movement is greater than the gravitational force pulling the material down slope. Anything that reduces the resistance or increases the downslope component of the force (by adding more load, for example) will make the slope unstable. A trigger such as an earthquake can make an unstable slope fail, resulting in mass wasting movement.
Angle of repose
the angle of repose of an unconsolidated material such as sand is the steepness of the slope at which the gravitational force and the frictional resistance are precisely balanced. If the slope becomes steeper, it will fail. For most materials, the angle of repose is between 25 and 40 degrees. Large, angular particles generally have steeper angles of repose than small, rounded particles.
Role of water
When ground becomes saturated with water (as after a rainstorm), the added weight of the water can make a slope likely to fail simply because of the extra load on the system. But water can also affect the frictional resistance to movement. A certain amount of water in a soil or sediment can bind the grains together through surface tension and make the slope more stable. But if more water is
added, the film of water between the grains becomes thick enough to lubricate the grains, which reduces friction and makes the material more likely to flow. As a result, water-saturated slopes often fail.
Types of mass wasting-Creep
Creep is the slow movement of soil or other material down a slope due to freeze-thaw or wet-dry cycles. With each freeze cycle the soil expands outward and then contracts moves slightly downslope as it thaws. Curved tree trunks and tilted posts and fences are common indicators of soil creep.
Slump
Slump is the (usually) slow movement of a block of material along a curved surface at its base. It is often called "rotational" slump because the block tilts backwards as it slides.
Slides and Flows
Slides and flows are rapid movements of materials downslope.
In a slide (such as a rock slide)
the material remains mostly coherent and moves along a defined surface.
In a flow (such as a debris flow or mud flow)
the material is saturated with water and flows as a viscous fluid.
A lahar is a
type of flow that occurs on volcanic landscapes when water-saturated ash flows down steep volcanic
slopes.
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