Mass Wasting

3 – Types of Mass Wasting

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Mass wasting is the movement of material down a slope by gravity. Water may be involved in weathering the material, or it may become a part of the material moving, but mass wasting does not include that sediment moving downhill because it is within a moving body of water. When we name or recognize the two types of slow downslope movements we simply assign them a name descriptive of the movement. When we name and classify relatively fast mass wasting types we consider two things: 1) what is moving and 2) how it moves.

Slow Downslope Movements

(Months to Years or More to See Effects)

Creep

The most pervasive type of mass wasting is so slow it can only be seen from its effects. It is more pronounced on steeper slopes, but affects very gentle ones too. The exact cause of motion (besides, of course, “gravity”) is not clear, and may include several different means of movement. Whatever causes it, the effect is obviously more pronounced in the uppermost layers of material and decreases rapidly downward. The obvious consequences of creep are leaning and tilting objects. Trees on steep slopes show it most noticeably because they are tilted downslope each winter (while dormant) and then grow straight upward the next spring. When they are very young the tilt is very pronounced, but as they age (and their roots go deeper) they tilt less and less. The result is a distinct downslope curvature to the lower part of the tree.

Slope near Crede, Mineral County, CO.

More rigid and non-growing things merely tilt down slope. The slope does not have to be very steep.

Creep-affected slope in Morgantown, WV. Image from Google Maps.

Solifluction Solifluction comes from Latin roots that mean “soil flowage”. This type of movement is only a problem in high latitudes and permafrost areas. It generally happens on very low slopes or flat ground. In winter the ground freezes solid to substantial depth. Then in spring and summer it melts, but not all the way through. This leaves very wet soil atop a frozen layer in which the ice fills all the pore spaces and doesn’t allow drainage of the saturated soil above. The cold climate means the soil evaporation rate is very low and so for much of the year the soil and any loose underlying sediment is a soupy mess. Slight changes in pressure on the surface, or slight differences in depth of melting cause the surface to move laterally and squeeze upward and downward into a hummocky surface. The ends of cabins that are heated are known to subside preferentially over time because they keep the underlying soil warmer through the winter. Excavating through the permafrost and pouring footers adds considerably to the cost of construction in far northern areas.

Public domain photo by Maurice Chédel of “garland-like” solifluction in Switzerland. (From Wikipedia)

Solifluction on steeper slopes creates an odd pattern of soil ridges that are convex in the downslope direction.

Fast Downslope Movements

(Seconds to Days to See Effects)

1) What is Moving A) In some instances, particularly where slopes are generally steep, slopes are formed on solid rock. We say that rock “holds up” a high place like a hill or a ridge. In places like this, when solid rocks are the principle material moving we name the mass wasting “rock ” where the motion type fills in the blank – rockfall, for example. Scree slopes are common in the Grand Canyon where rocks from a layer above have wasted onto a lower, gentler part of the slope.

View NE from Desert View, Grand Canyon NP, AZ

B) Other slopes, particularly ones of middling steepness, generally have material other than, or in addition to, rocks on them – soil, loose sediment, vegetation, roads, houses, and so on. If mass wasting occurs on a slope like this, the material is no longer what it once was – soil is not soil, trees are not living trees, houses and roads are not houses and roads. They are all debris after the movement.

Scar

Trees, Soil, Rocks, etc.

Prospect Mt. near Manchester, VT

C) Some slopes are formed on soil or sediment that is very rich in fine grained sediment – clay and silt. These can move in all the ways that coarser materials move and in a couple of additional ones as well. The name of a mass movement involving this type of soil or sediment is usually “mud “.

2) How the Material is Moving There are two components to this aspect of mass wasting – the rate of motion and the mechanics of the motion. Mass wasting at very slow rates may not be obvious until after it has been occurring for a while, and in at least one case the mechanics of the motion are not completely understood. Slow movements generally have a special descriptive name that doesn’t actually specify what has moved. Fast movements are, of course, very obvious, even while they are occurring. These are the ones that are called “rock ______” or “debris ______” or “mud _______”. We’ll begin with the slow movements.

A) The fastest of mass wasting events are falls. In this case the material moving loses ground contact for most or all of its descent. True freefall is included as well as bouncing down the slope. Most of Fg ~= Fd. Obviously this requires a very steep slope. Most falls involve rock (rockfall), but in certain cases steep faces on fine sediment or soil can move in this way, resulting in a debrisfall.

Unlithified sediment or soil can move either as a cohesive mass (left below – each tiny bit of the mass moves in a parallel fashion to all other bits) or as a chaotic jumble of individual particles moving in non-linear paths down the slope (right below). The resultant vectors of all the motions in both cases is downslope, but the individual particles in the right-hand diagram can be moving in any direction at all at any one time. The movements are given different names depending on the type of movement. We will start with the coherent movements.

SLIDES When these large flat boulders eventually come off of Stone Mountain they will slide as cohesive pieces. This will be a rockslide.

Unconsolidated material can also slide as a cohesive unit, though these tend to fall apart as they move and become more chaotic as they proceed. This mass movement of debris in Vermont (from an earlier part of this presentation) almost certainly began as a coherent motion of the soil, loose partly weathered rock, and trees along a basal plane of weakness – the base of the soil horizon. The chaotic jumble of debris at the base is a result of what happened as it progressively piled up at the foot of the scar, where, presumably, the slope became more gentle and the motion was stopped. This was therefore a debris slide. Slopes made entirely of mud can move in this way (mudslides) but usually they move in a different fashion, which we’ll get back to.

SLUMPS In a slide the motion of coherent material is along a surface that already exists long before the motion – an exfoliation fracture, the base of a soil horizon, a bed surface in a sedimentary rock, etc. In a slump the surface of failure is created as the motion occurs. It is, essentially, a kind of fault. The failure is along a surface concave towards the slope that is failing, and as the material moves downward its toe also rotates outward, causing the upper surface to tilt toward the failure surface. There are usually a series of parallel slumps on a slope.

Kaolin mine near Sandersville, GA

Public domain photo by “Chagai” of “a snow avalanche in the Himalaya. (From Wikipedia)

When loose material moves chaotically the movement is called an avalanche.

The word is mostly applied to snow moving in this way, but

loose rock and debris can move this way too, as rock avalanches or debris avalanches.

FLOWS

Mass movements involving largely very fine-grained sediment or volcanic ash can move in another way. When these materials become sufficiently saturated with water that the pore pressure can no longer keep it solid they begin to flow. This is not like water moving and carrying some sediment with it. Instead, the entire mass behaves as a viscous fluid. Think of pea soup flowing down a slope. If there is nothing but mud moving the movement is called mudflow. If there are other materials involved (trees are common) it is a debris flow. If the material is volcanic ash it is a lahar.

Public domain photo by Lyn Topinka of the USGS. (From Wikipedia)

This photo was made in the aftermath of the Mt. St.

Helens eruption in 1980.

The “mud” (ash, actually) has now de-watered and is

a solid mass, but flowed into this canyon as a thick

liquid.

FAST MOVEMENTS ROCKS DEBRIS MUD FALLING: ROCKFALL (debris fall) NONE SLIDING: ROCKSLIDE DEBRIS SLIDE (mudslide) SLUMPING: NONE SLUMP SLUMP AVALANCHING: ROCK AV. DEBRIS AV. NONE FLOWING: NONE DEBRIS FLOW MUDFLOW

SLOW MOVEMENTS

CREEP

(solifluction)

SUMMARY This chart lays out a logical classification of mass wasting types. Movements that are uncommon or geographically restricted (solifluction to permafrost areas) are in lower case and parentheses. Focus on the more common ones.