Physical Geology lecture outline - Mass Wasting

Definition of mass wasting: movement of material down slope under the influence of gravity.

Colluvium: material deposited by mass wasting. Often poorly sorted and only crudely bedded (if at all) with angular clasts. Clasts are local. Has some similar characteristics to glacial till, but far traveled erratics, striations, and context usually allow a distinction to be made.

In this photo, from just outside of Cody Wyoming, colluvium has been deposited on alluvium. The red dashed lines traces the contact. The alluvium is composed of well rounded and sorted, dark volcanic clasts from the upstream Absaroka volcanics that have been stacked into an imbricate pattern by river flow. Some larger colluvium blocks of lighter colored limestone that fell into the channel during deposition are preserved in the river alluvium. Above is the angular debris that came from the cliff slopes above and formed colluvium. Note the massive, poorly sorted character of the material and the angular clasts. It was probably deposited by a combination of debris flows and rock falls.

Talus: debris from rock falls and debris flows at the base of slopes.

The adjacent image is of a mountain side in Spitsbergen. There is over 2000 feet of relief here. Note the conical masses of sediment at the base of the cliff. These are talus cones, and they form by a combination of mud flows and rock falls from the overlying cliffs. Each cone originates from an incision into the mountain side known as a chute. If one looks carefully, a distinct channel with levees exists on the middle and largest talus cone. This is a more recent mudflow channel. Scars of older mudflow channels are also evident. While it looks steeper here, the talus slope has around a 30 degree slope.

What are critical factors that influence whether mass wasting occurs or not, whether a slope is stable or not?

Figure caption: This is a schematic diagram of a potential slide block produced by the erosion of the slope toe by a river, and it helps elucidate the mechanics of slip. The contact between the yellow sandstone and the underlying granite would be a possible slip plane. The driving force is gravity (the green arrow). It can be resolved into its normal and slip components. As the inclination of the slip plane increases the gravitational force will remain the same, but the resolved normal force diminishes in size, while the shear force grows. Friction can be described as the ratio of normal to shear force that produces movement. Also important is the pore pressure which acts in opposition to the normal force, thereby making it easier to slip. This is why surfaces inclined at only a few degrees can slip - they are weakened by high pore pressures. Some type of question on these mechanics and this diagram will be on the test!

What are major classification factors for types of mass wasting?

Some major types of mass wasting:

Some examples:

For what geologic features is mass wasting important?

This photo of a cliff face at Scotts Bluff National Monument shows the scar of a large rock fall that occurred in 1997. It is evident as light area that covers the vegetated slope. The lines in the cliff face represent joint surfaces which are very important in this mass wasting phenomena.

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