Identify and describe types of mass movement.

In this section, you will learn the term mass wasting. You will also be exposed to the various types of mass wasting, their causes and their effects.

Slides and Falls

Figure 1. Pieces of rock regularly fall to the base of cliffs to form talus slopes.

Rocks that fall to the base of a cliff make a talus slope (figure 1).
We call the initial drop a “fall” where a rock travels through the air.
If the talus slope becomes “over-steepened” then the mass will slide downhill.
What makes for a downslope movements?=> These are governed by gravity, mechanical weathering, and the presence of interstitial water.

Landslides and avalanches are the most dramatic, sudden, and dangerous examples of earth materials moved by gravity. Landslides are sudden falls of rock, whereas avalanches are sudden falls of snow.

• Here is a video of a snow avalanche.

Landslides and Falls

Although many types of mass movements are included in the general term “landslide,” the more restrictive use of the term refers only to mass movements, where there is a distinct zone of weakness that separates the slide material from more stable underlying material. The two major types of slides are rotational slides and translational slides. Rotational slide: This is a slide in which the surface of rupture is curved concavely upward and the slide movement is roughly rotational about an axis that is parallel to the ground surface and transverse across the slide (figure 2a). Translational slide: In this type of slide, the landslide mass moves along a roughly planar surface with little rotation or backward tilting (figure 2b). A block slide is a translational slide in which the moving mass consists of a single unit or a few closely related units that move downslope as a relatively coherent mass (figure 2c).

Figure 2. These schematics illustrate the major types of landslide movement.

When large amounts of rock suddenly break loose from a cliff or mountainside, they move quickly and with tremendous force (figure 3). Air trapped under the falling rocks acts as a cushion that keeps the rock from slowing down. Landslides and avalanches can move as fast as 200 to 300 km/hour.

Figure 3. (a) Landslides are called rock slides by geologists. (b) A snow avalanche moves quickly down slope, burying everything in its path.

Landslides are exceptionally destructive. Homes may be destroyed as hillsides collapse. Landslides can even bury entire villages. Landslides may create lakes when the rocky material dams a stream. If a landslide flows into a lake or bay, they can trigger a tsunami (figure 4).

Figure 4. The 1958 landslide into Lituya Bay, Alaska, created a 524m tsunami that knocked down trees at elevations higher than the Empire State Building (light gray).

Landslides often occur on steep slopes in dry or semi-arid climates. The California coastline, with its steep cliffs and years of drought punctuated by seasons of abundant rainfall, is prone to landslides. At-risk communities have developed landslide warning systems. Around San Francisco Bay, the National Weather Service and the U.S. Geological Survey use rain gauges to monitor soil moisture. If soil becomes saturated, the weather service issues a warning. Earthquakes, which may occur on California’s abundant faults, can also trigger landslides.

• Here is a video of rapid downslope movement of material.

Lahars and Mudflow

Added water creates natural hazards produced by gravity (figure 5). On hillsides with soils rich in clay, little rain, and not much vegetation to hold the soil in place, a time of high precipitation will create a mudflow. Mudflows follow river channels, washing out bridges, trees, and homes that are in their path.

• This video shows a debris flow.

A lahar is mudflow that flows down a composite volcano (figure 6). Ash mixes with snow and ice melted by the eruption to produce hot, fast-moving flows. The lahar caused by the eruption of Nevado del Ruiz in Columbia in 1985 killed more than 23,000 people.

Figure 6. A lahar is a mudflow that forms from volcanic ash and debris.

The 1970 Ancash earthquake, magnitude 7.7 (so, pretty strong!), causes a landslide that involves nearly 100 million cubic feet of material and buries the town of Yungay, Peru.
Sadly, more than 47,000 people were killed.  At the town of Yungay, the only survivors were observers on highpoint (a cemetery!).

Slump and Creep

Less dramatic types of downslope movement move earth materials slowly down a hillside. Slump moves materials as a large block along a curved surface (figure 7). Slumps often happen when a slope is undercut, with no support for the overlying materials, or when too much weight is added to an unstable slope.

Figure 7. Slump material moves as a whole unit, leaving behind a crescent shaped scar.

Figure 8. Trees with curved trunks are often signs that the hillside is slowly creeping downhill.

Creep is the imperceptibly slow, steady, downward movement of slope-forming soil or rock. Movement is caused by shear stress sufficient to produce permanent deformation, but too small to produce shear failure. There are generally three types of creep:

1. Seasonal, where movement is within the depth of soil affected by seasonal changes in soil moisture and soil temperature
2. Continuous, where shear stress continuously exceeds the strength of the material
3. Progressive, where slopes are reaching the point of failure as other types of mass movements. Creep is indicated by curved tree trunks, bent fences or retaining walls, tilted poles or fences, and small soil ripples or ridges

Curves in tree trunks indicate creep because the base of the tree is moving downslope while the top is trying to grow straight up (figure 8). Tilted telephone or power company poles are also signs of creep.