What is Mass Movement?

Mass movement or mass wasting refers to the slow or catastrophic (rapid) movement of soils, sediments and rocks down hills, mountains, or other slopes on land surfaces.  The materials may be dry, saturated with water, or in between. Usually, the term mass movement is restricted to processes on land rather than submarine flows and slides on the ocean and sea floors.  We'll discuss submarine flows and turbidites in Chapter 17.

Mass movement results when materials lose their resistance to the effects of gravity.  For example, weathering may cause part of a rock on the side of a hill to split.  Once split, gravity may cause the loose portion of the rock to slide down the hillside.  Previously, we discussed how lahars, a type of mass movement associated with active volcanoes, may form and kill people and wildlife. Obviously, mass movements (especially catastrophic mass movements) can threaten lives and damage property.  In the US, mass movement kills about 25 to 50 people/year, and does about $1 to $2 billion worth of annual damage to property.
 

What Factors Cause Mass Movement?

Whether a material on a slope will slowly or catastrophically migrate down slope depends on the following four factors:
 

1. Whether the materials are unconsolidated (loose sediments and soils) or consolidated (such as, unfractured rocks).

Obviously, cliffs of solid unfractured granite are much more stable than a pile of loose sand.  Overall, steeper slopes on loose material produce a greater chance of mass movement.   At some critical angle, called the angle of repose, part or all of the materials on a slope will give way to gravity and mass movement will occur.  On dry, loose material, such as sand or gravel, the angle of repose is typically 30-35^o (Figure 11.1, p. 233).  Similar angles of repose are seen in piles of sand at construction sites and piles of crop grains on farms.  If the slopes become any steeper, the material tends to slide or flow under the force of gravity.  As shown in Figure 11.1 (p. 233), the angle of repose varies with particle size, the amount of water, and the angularity of the particles. For example, stacked angular particles are less likely to roll than stacks of well-rounded particles.  That is, it's possible to stack five books on top of each other, but not five golf balls.

Angle of Repose: With round grains, platy grains, and with wet round grains.
 
 
 
 
 
 
 
 
 

2. The amount of water in the material, which depends on the amount of precipitation, the porosity of the materials, and whether the slope is below or well above the water table (note: for our purposes, the water table is the boundary between unsaturated sediments and deeper water-saturated sediments, which have their pores entirely filled with water).

Water behaves like "glue" and allows wet sand to be molded into the vertical walls of sand castles (Figure 11.4, p. 234).  Surface tension is the attractive force between water molecules, which holds the water and sediment particles together in sand castles (Figure 11.3, p. 234).  The tensional forces between water molecules on the surface of water also allow a paper clip to float on water (Figure 11.2, p. 234).

Rocks and hard (consolidated) sediments can form nearly vertical stable slopes.  As shown in Figure 14.19 (p. 319), loess (wind-blown silt) forms very stable nearly vertical slopes.  Unlike loose gravel, small amounts of water cause the finer-grained silts to adhere fairly well to each other so that nearly vertical slopes may be stable.

While the surface tension of the water in damp sand and finer materials helps particles to adhere to each other, too much water keeps the sediment particles apart and allows them to freely flow (Figure 11.3, p. 234).  That is, too much water acts as a lubricant and allows particles to flow downslope.  Water may seep into fractures and bedding planes in loose rocks, soils and sediments and cause the materials to slip or slide down slope (Figure 11.5, p. 235).
 

3. The steepness and instability of the slopes, which promote the sliding, rolling or falling of soils, sediments and rocks.

Fractured rocks, such as shales, weather more easily than dense, massive granites.  The stability of a slope depends on the weathering and the amount of fracturing in the sediments, soils and rocks on the slope.
 

4. The amount of vegetation.

The roots of vegetation often hold soils and sediments in place and prevent mass movement.  Leaves and tree limbs also act as "umbrellas" and prevent intense rain splash from eroding underlying soils. The removal of vegetation by drought or fire (Figure 11.5, p. 235) allows wind and water erosion to promote mass movement.
 

Types of Mass Movement

Mass movements may be classified into several different types according to the following properties:

1. The types of materials in the mass movements (rocks or unconsolidated materials [sediments]).

2. The speed of the movement (slow [few mm per year] or catastrophically fast [km per hour]).

3. The nature of the movement (sliding, rolling and/or flowing).
 

Table 11.2 (p. 239) shows a classification for mass movements based on these three properties: composition, time and type of movement.
 

Rock Mass Movements

Rock mass movements are classified as rockfalls, rockslides, or rock avalanches.  If one or several rocks slide or fall at high speeds (5 km/hour or higher), they're called rockfalls (Figure 11.8, p. 239).  Rockfalls generally occur when one or several blocks become detached from cliffs or steep mountainsides because of chemical and physical weathering.

If the moving materials consist mostly of rock and if they have moderate speeds (about 1-4.9 km/hour) and mostly slide or fall, they may be classified as rockslides (Figure 11.9, p. 240).  In rockslides, the numerous rocks slide as debris down the sides of mountains or other steep slopes.

If the rocks are somewhat smaller and rounder, they may actually tumble and flow.  Rock avalanches have relatively smaller boulders and other rock debris that flow at high speeds down slopes (Figure 11.10, p. 241).  The presence of water or air in the rock avalanches causes the particles to tumble freely and flow.
 

Unconsolidated Mass Movements

In some cases, mass movements involving soils and sediments may be much slower and less catastrophic than mass movements with rocks (Table 11.2, p. 239). Slow is defined as movements of 1 cm/year or less.  A moderate velocity is 1-4.9 km/hr and fast movement is defined as 5 km/hour or more.

If slow movement is involved, the sediments or soils may behave as a viscous liquid and creep down slopes.   Evidence of creep includes: tilting power poles, grave stones, fences, and trees (Figure 11.11, p. 241).   By reading dates off of leaning gravestones, the average rate of creep in millimeters/year may be calculated.

Solifluction (Figure 11.18, p. 245) is a special type of mass movement involving unconsolidated materials in cold climates.  During spring thaws, the upper layers of the sediment melt and become saturated with water, but the lower layers remain hard and frozen.  Because the water in the overlying soils and sediments cannot seep into the frozen underlying soils and sediments, the water and the surface materials may flow downslope. In most cases, solifluction is a slow process.

The faster movement of sediments and soils (unconsolidated materials) down a slope includes earthflows (Figure 11.12, p. 242), mudflows (Figure 11.14, p. 243), debris flows (Figure 11.13, p. 242), slumps (Figure 11.16, p. 244), debris slides (Figure 11.17, p. 245), and debris avalanches (Figure 11.15, p. 243).   As shown in Table 11.2 and Figures 11.12-11.18, mass movements involving soils and sediments may be classified by whether they are: 1) wet or dry, 2) slow or fast, and 3) coarse- or fine-grained.

As shown in Figure 11.14 (p. 243), mudflows tend to contain much more water than other mass movements with unconsolidated materials. As mentioned earlier, lahars are special types of mudflows that are associated with active volcanoes. The eruption of Nevado del Ruiz in Columbia in 1985 melted ice and produced lahars that killed nearly 25,000 people.

Debris avalanches may also contain a substantial amount of water (Figure 11.15, p. 243).  Debris flows tend to have coarser materials than earthflows (Figures 11.12-11.13, p. 242).  Slumps include rigid sediments that are bounded by normal faults and tend to move as blocky units rather than breaking up in a chaotic turbulent mass (Figure 11.16, p. 244).  In debris slides, much of the materials slide as slabs that are often held together by the roots of vegetation (Figure 11.17, p. 245).
 

Causes of Mass Movement

Mass movement may be triggered by catastrophic events, such as floods, earthquakes, or heat from volcanoes (lahars).  Other mass movement events, such as rockfalls, may result from physical and chemical weathering processes that slowly work to loosen rocks from cliffs, hills or mountain sides.

Like erosion, human activities may promote or hinder mass movement by affecting slope inclinations, the amount vegetation on the slopes, the quantity of water in the slope materials, and the amount and the type of materials that are present on the slopes.  For example, the construction of roads along hillsides may increase the steepness of slopes and promote mass movement.  In the drawing on p. 237, an already unstable slope becomes even less stable by cutting a vertical notch into the hillside and attempting to hold back the upslope materials with a thin concrete wall.  Notice that the slope is underlain by slippery shale layers that parallel the direction of the slope.  Under wet conditions, water will lubricate the shale and cause the materials on the slope to slide (p. 237).  Looking for unstable slopes and considering the possibility of mass movements are important when buying a home in Kentucky!

Increased irrigation of crops may remove water from a nearby slope.  The removal of lubricating water may increase the stability of soils and sediments on the slopes.  On the other hand, less water could also produce dry conditions that may kill off vegetation that is needed to maintain slope stability.

The clear-cutting of forests may also promote mass movement.  Deforestation removes tree roots that are important in stabilizing slopes. Logging operations also require roads that may undermine the stability of slopes.  Clear-cutting and road construction were responsible for initiating debris slides that killed five people in Oregon in 1996.

Preventing Mass Movements or Hindering their Damage

Several activities may prevent or decrease the probabilities of mass movement.  These preventive measures either reduce the forces that tend to promote mass movements and/or strengthen the slope materials so that they're less likely to fail.  Preventative measures include decreasing slopes (e.g., terrace farming), planting vegetation on slopes, reinforcing failing slopes with suitable walls (unlike the weak example on p. 237), and diverting excess water that may initiate mass movements on slopes.  Individuals should also avoid building homes and businesses on slopes, especially where the rock layers parallel the slopes.  Catastrophic mass movements are very possible in such situations.

Which House is Least Likely to be Destroyed by Mass Movement?
 
 
 
 
 
 
 
 
 
 
 

Again, the roots of vegetation are very effective in stabilizing slopes by holding soils in place and by removing water from soils that could cause mass movement.  Vegetation also intercepts the fall of raindrops, which hinders rainsplash erosion and slope failures during heavy rainstorms.

Disasters from Mass Movements

Your textbook lists several examples of mass movement catastrophes that killed people and damaged property.  As shown in Figure 11.7 (p. 238), the Great Alaskan Earthquake of 1964 caused a landslide that killed three people and destroyed several homes.  Earthquakes in Tadzhikistan in 1989 produced mudflows that created a lot of damage (Figure 11.14, p. 243).

A debris slide in Utah in 1983 blocked a railroad and highway (Figure 11.17, p. 245).  The slide also dammed a river, which flooded the nearby town of Thistle, Utah.  A similar situation occurred in Jackson Hole, Wyoming in 1925 (Figure 11.19, p. 246; Figure 11.20, p. 247).  At Jackson Hole, the Gros Ventre River cut through a sandstone, which supported rocks on the slope of a hill, as groundwater lubricated the shale underlying the hill slope (Figure 11.20, p. 247).  This caused the hillside to slide catastrophically into the river (Figure 11.20, p. 247).  Subsequently, the river was temporarily dammed.  However, the dam was soon broke creating a second catastrophe (Figure 11.21, p. 247).

Another catastrophe related to mass movement occurred in Italy in 1963.  Part of a slope gave way and slid into a nearby dam reservoir (Figure 11.22, p. 248).  The movement of the debris slide into the reservoir created a giant wave that spilled over the dam and rushed downstream to kill 3,000 people.