Physical Geology lecture outline - Mass Wasting

Lecture outline:


Introduction

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

Mass wasting comes in a great variety of forms largely dependent on the character of the material moving, and the mechanism(s) of movement.

Colluvium: mass wasting deposits. This material is offten poorly sorted, only crudely bedded (if at all), and can have angular clasts. Clasts are local (from adjacent slopes), because of the relatively short transport distances involved with mass wasting. Colluvium has some similar characteristics to glacial till, but far traveled erratics, striations, and context that characterize glacial material 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 between the overlying colluvium and underlying alluvium. The alluvium is composed of well rounded and sorted, dark volcanic clasts from the upstream Absaroka volcanic rocks. Note how the pebbles lean on each other in a pattern known as imbrication. 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 fell, rolled, slid and flowed from the cliff slopes above and formed colluvium. Note the massive, poorly sorted character of the material and the angular clasts. This colluviumwas probably deposited by a combination of debris flows and rock falls.
Talus: loose debris from rock falls and debris flows at the base of mountain 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.

Looking up a talus slope of pinkish quartzite boulders at Devils State Park in Baraboo, Wisconsin. The cliff above is the source. Note the lichen growing on the talus blocks - this indicates that this talus pile has been stable for a good while, and is not presently significantly that active.

 


Slope stability factors

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

To the left is a schematic diagram of a potential slide block (in bright yellow) produced by the erosion of the slope toe by a river. It can help elucidate the mechanics of slip. The contact between the yellow sandstone and the underlying granite would be a possible slip plane (it could be a bedding plane or a fracture plane). The driving force is gravity (the green arrow). It can be resolved into its normal and shear components (the orange and black arrows respectively). Basically the normal plus the shear vector equal the gravitational vector. 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 (shown as a blue vector) 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 can be weakened by high pore pressures. This is also why mass wasting can be triggered by wet periods where the water has time to soak into the slope. Some type of question on these mechanics and this diagram will be on the test!


Classification of mass wasting features

What are major classification factors for the different types of mass wasting?

Multiple classifications are used by different groups considering mass wasting for different reasons or in different places. Below are some of the common examples of types of mass wasting:


Case histories


Mass wasting's contributions to landscape

For what geologic features does mass wasting play an important role?

Image of active mass wasting produced by wave undercutting along Alaska's north coast, leading to sea cliff formation and very active cliff retreat. Warmer conditions and more open water have allowed waves to increase in size and has increased the rate of mass wasting and shoreline retreat here. Note the permafrost ice exposed in the sea cliff. Image from USGS site on coastal erosion: http://energy.usgs.gov/alaska/ak_coastalerosion_images.html.

The Cliffs of Mohr along the western coast of Ireland on a typical gray and rainy day. A close look at the lower center of the photo of the cliff just above the beach shows a section of the rock face that has recently mass wasted. During large storms as the waves reach the base of the cliff this material will be ground up and carried away, and the cliff face will be further undermined in a repeated process that on a geologic time scale causes the cliff to retreat.

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. Perhaps contrary to intuition, mass wasting along one section can destabilize other sections. In a visit to Scott's Bluff in 2016 the trail down from the top was closed because of a mass wasting event on this slope. Note how the slope is fairly straight. The mass wasting is being heavily controlled by fractures in these rocks.


Harmon D. Maher Jr. reserves copyrights to the materials in this site. Material may be used for non-profit educational purposes as long as proper attribution is given. For permission for any other use please contact author. Thank you.

Return to Physical Geology index page.

Return to my home page index.