15.3 Unstable Ground: Mass Movement

Identify different types of mass movements and describe their behavior and causes.

We have already explored many natural hazards in this book, including tornadoes, hurricanes, volcanoes, and earthquakes. Another significant hazard is posed by soil and rock sliding down a slope by the force of gravity. Landslides and other similar events are collectively called mass movement (or mass wasting) events. Mass movement is the movement of rock, soil, snow, or ice downslope by gravity.

mass movement

(or mass wasting) Downslope movement of rock, soil, snow, or ice caused by gravity.

Why Mass Movement Occurs

A slope is stable when it is unlikely to fail and unstable when it has failed (experienced mass movement) in the past or is likely to do so soon. Several factors, summarized in Table 15.1, create weak layers within rocks, regolith, or snow and potentially create an unstable slope.

Table : TABLE 15.1 AT A GLANCE: Factors That Make Slopes Unstable

Geologic faults

Jointing in rocks

Foliation planes in metamorphic rock

Layers of saturated clay or sand

Layers of ice between snow layers

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Conversely, several factors increase slope stability. Friction and electrical charges hold soil particles together. Soil moisture and plant roots also anchor regolith and keep it from moving. Vegetated regolith that is moist, but not saturated, is far more stable than dry and unvegetated regolith. The surface tension of water and the roots of plants both help bind particles together.

A material’s angle of repose (see Section 14.1) is determined by the equilibrium between gravity pulling particles downward and friction holding particles in place. Larger particles have steeper angles of repose than smaller particles. Put another way, the stability of a slope depends on the relationship between resistance force, which keeps the material in place, and downslope force (or gravitational force), which induces the material to slip downhill (Figure 15.20).

Figure 15.20

Slope stability. (A) The angle of repose is the steepest slope a material can maintain while remaining stable. It is a result of the relationship between gravitation and friction among particles, and it varies depending on the shape, size, and wetness of those particles. (B) If the resistance force (Fr) is equal to or greater than the downslope force (Fd), the material will not move. If the resistance force is less than the downslope force, the material will move. In this example, the downslope force is increased by steepening of the slope, as might occur where a hillside is cut by a new road.

Mass movements can be caused by any factor that increases the downslope force or decreases the resistance force. The main factors that can change these forces and result in slope failure are summarized in Table 15.2.

Table : TABLE 15.2 Factors That Cause Mass Movements

FACTOR

EFFECT ON DOWNSLOPE AND RESISTANCE FORCES

Earthquakes

Ground shaking can increase downslope force. Ground shaking can also separate particles of regolith and decrease the friction that holds them together.

Rivers and roadcuts

Steeper slopes are subject to greater downslope force. Rivers often undercut their banks and make them steeper. Roadcuts may make slopes steeper and more likely to fail.

Ground saturation

As soils become saturated, they become heavier, and the downslope force increases. Saturation moves soil particles farther apart, reducing friction between them. Mass movement commonly follows storms that bring heavy, soaking rains. Broken water pipes on steep slopes can also lead to mass movement.

Weathering

Increased weathering weakens the integrity and strength of rocks and regolith.

Removal of vegetation

Removal of vegetation and its anchoring roots weakens the ground and decreases the resistance force.

Types of Mass Movements

There are many different types of mass movements, and they range in duration from hundreds of years to a few seconds. We will explore mass movement types, starting with the slowest and most gradual and working up to the fastest and most dangerous, in the sequence shown in Table 15.3.

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Soil Creep

Soil creep is the imperceptible downslope movement of soil and regolith as their volume changes in seasonal expansion-contraction cycles. Clay particles in soil expand when wetted or warmed and then, as the season changes, contract when dried or cooled. In addition, wet soils that freeze in winter can expand by about 10%. When thawed in spring, they contract by the same amount. As they expand, soils move outward perpendicularly away from the sloped surface. As they contract, they settle downward vertically. These movements result in soil creep (Figure 15.21).

Figure 15.21

Soil creep. (A) Soils work their way downslope as they expand and contract with the seasons. (B) Soil creep is fastest near the land surface and slows down nearer bedrock.

soil creep

The imperceptible downslope movement of soil and regolith as their volume changes in seasonal expansion-contraction cycles.

These small movements add up to significant downslope migration of soils after many years. Soil creep is like the tides: It is too slow to be seen in motion, but its effects are easy to see in a landscape (Figure 15.22).

Figure 15.22

Soil creep and its effects. Structures and vegetation on hillsides sometimes sag downslope because of soil creep.
(Bruce Gervais)

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A special type of soil creep, called solifluction, occurs in northern and alpine tundra (see Section 8.4). Solifluction is a type of soil creep in which freeze-thaw, expansion-contraction cycles cause the soil to flow slowly downslope in overlapping sheets, as shown in Figure 15.23.

Figure 15.23

Solifluction in the Tian Shan Mountains, Kyrgyzstan.
(© Marli Miller)

solifluction

A type of soil creep in which freeze-thaw cycles cause the soil to flow slowly downslope in overlapping sheets.

Another type of imperceptible soil movement is caused by the movements of livestock, particularly cattle, on the slopes of hills. This phenomenon is explored in Picture This.

Slumps

A slump is a type of mass movement in which regolith detaches and slides downslope along a spoon-shaped plane, called a failure surface, and comes to rest more or less as a unit. Slumps are often called rotational slides because they follow the concave failure surface over which regolith moves. The topmost point of detachment of the slump, and the resulting cliff, is called the head scarp. The base of the slump is called the toe, as shown in Figure 15.24.

Figure 15.24

Slumps. (A) Slumps can be triggered by earthquakes or heavy rains. Their downhill rate of movement ranges from millimeters per day or less to several meters per minute. They range in size from a few meters to a few kilometers across. (B) This slump occurred in Stone Canyon near Los Angeles. Heavy spring rains were blamed for the slump.
(Tom McHugh/Science Source)

Video

Landslide hazards

http://qrs.ly/8i49ed1

slump

A type of mass movement in which regolith detaches and slides downslope along a spoon-shaped failure surface and comes to rest more or less as a unit.

Flows and Landslides

Flows and landslides are common occurrences in all mountainous regions. Landslide is a general term for the rapid movement of rock or debris down a steep slope. Flows are always mixed with large amounts of water, and overall, they move less quickly than landslides.

landslide

Rapid movement of rock or debris down a steep slope.

There are several different kinds of flows. Their surface areas vary from a few square meters to tens of square kilometers. Their rate of movement depends on the type of flow, the water content of the moving material, and the steepness of the slope. On gentle slopes, they may move at the rate of a slow walk, but on steeper slopes, it may be impossible to outrun them.

Mudflows are fast-moving flows composed mostly of mud. In debris flows, a fast-flowing slurry of mud is mixed with large objects, such as rocks and vegetation. Volcanic eruptions often produce debris flows called lahars (see Section 14.2). One of the most deadly debris flows in history, however, occurred in northern Venezuela not because of a volcanic eruption, but because of heavy rains (Figure 15.25).

Figure 15.25

Caraballeda debris flow. (A) In December 1999, after heavy rains, a debris flow devastated Caraballeda, in the state of Vargas, Venezuela. Up to 30,000 people died, and 75,000 were displaced. The damage totaled up to $3 billion. (B) Boulders the size of small houses were transported in the debris flow.
(A. U.S. Geological Survey, photo by Matthew C. Larsen; B. U.S. Geological Survey, photo by Matthew C. Larsen)

mudflow

A fast-moving flow composed mostly of mud.

debris flow

A fast-flowing slurry of mud mixed with large objects, such as rocks and vegetation.

Picture This

(© Laura Alice Watt)

Cattle Terraces

These parallel ridges, called cattle terraces (or livestock terraces), are near Hollister, California. The terraces are trails that cattle have created as they walk along the hillside. There is nothing special about the soils where cattle terracing occurs. The terraces are formed because the slope is too steep for the cattle to walk straight up or down, so they must instead follow a line of equal elevation, or a contour. Cattle terraces are common on slopes of intermediate steepness where cattle are grazed in large numbers. If a slope is too gentle, cattle will not follow the contours, and terraces will not develop. Likewise, if a slope is too steep, cattle will not graze on it for fear of falling. Cattle terraces are a form of mass movement similar to soil creep or solifluction.

Consider This

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All landslides occur in mountainous terrain where slopes are steep (but not vertical). They can move at rates of several hundred kilometers per hour and come to rest within minutes after the initial movement begins.

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A rock slide is a landslide that consists predominantly of rocks and broken rock fragments, and a debris slide (or debris avalanche) consists of regolith and other material, such as soil and trees. Heavy rains and earthquakes often trigger the slope failure that gives rise to landslides. At the end of this chapter, the Geographic Perspectives discusses the hazards of landslides and the efforts of scientists to predict slope failure and save lives. Figure 15.26 details the techniques scientists are using to monitor ground movement on unstable slopes.

Figure 15.26

SCIENTIFIC INQUIRY: Can scientists predict dangerous landslides? The key to developing an early warning system for deadly landslides is monitoring ground movement long before the slope fails. Heavy rains are more likely to trigger a mass movement event on slopes that have been moving before the rains. Long-term monitoring stations allow scientists to develop a history of slope movement for a given location. Scientists issue evacuation warnings if the slope shows signs of moving, although accurate predictions of the timing of slope failure are not possible.
(Clockwise from top left: © 2014 Nicole Feidl and UNAVCO; © TUM, Forschungsgruppe alpEWAS; © TUM, Forschungsgruppe alpEWAS)

rock slide

A landslide that consists of rocks and broken rock fragments.

debris slide

A landslide that consists of a mixture of rocks, soil, and vegetation.

Question 15.11

Where are the highest sea cliffs in the world?

The north coast of Moloka‘i, in the Hawaiian Islands, has sea cliffs that are the highest in the world.

Landslides also occur on the seafloor where slopes are steep. Much of the coastline of the Hawaiian Islands, for example, is dominated by enormous head scarps formed by undersea debris slides that occurred before the islands were settled by people (Figure 15.27).

Figure 15.27

Undersea debris slides in the Hawaiian Islands. (A) The north coast of Moloka‘i, shown here, has been shaped by large debris slides. These sea cliffs are the highest in the world. (B) The debris slides that shaped sea cliffs continued down the steep submarine slopes surrounding the Hawaiian Islands. The brown areas in this map show the extent of these submarine debris slides, most of which are larger than the islands themselves. The 2,000 m (6,560 ft) depth contour is shown.
(A. © Richard J. Anderson)

Avalanches

National Geographic photographer and adventurer Jimmy Chin did what few people have ever done: He rode a snow avalanche 300 m (984 ft) down a mountain and survived. While Chin was on a photo shoot in the Grand Tetons of Wyoming, a “large, wet slab” cracked loose around him as he began a downhill ski run. As the churning cloud of snow consumed him, he struggled to stay on top. He knew that when it came to rest, the snow would compact and entomb him like cement and he would suffocate within minutes. He managed to work his way to the surface before the avalanche came to a rest. He was bruised and shaken, but alive.

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Broadly defined, an avalanche is a turbulent cloud of rock debris or snow that is mixed with air and races quickly down a steep slope. Snow avalanches are composed mostly of snow. If the avalanche consists of rock, broken trees, and other material, it is called a debris avalanche. Avalanches are mixed with air and are therefore extremely turbulent. They gain such momentum that they are capable of knocking down trees and buildings in their path. Like slumps, they are triggered when a slip at a failure surface releases snow or debris. Snow avalanches often happen in the same area repeatedly, forming avalanche chutes through which snow and debris avalanches regularly move (Figure 15.28).

Figure 15.28

An avalanche chute. Avalanche chutes form where the topography of mountainous areas guides snow avalanches. This snow avalanche is moving down an avalanche chute in the Pamirs, a mountainous region in Tajikistan.
(© Medford Taylor/National Geographic/Getty Images)

avalanche

A turbulent cloud of rock debris or snow that is mixed with air and races quickly down a steep slope.

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Rockfall

Rockfall occurs when rocks tumble off a vertical or nearly vertical cliff face (Figure 15.29). As rocks fall, they are broken apart into smaller fragments and dislodge other rocks as well. Rockfall is particularly common along roadcuts. Near rocky cliffs, the asphalt surface of roads is often pitted by earlier rockfall events.

Figure 15.29

Rockfall in Yosemite Valley. (A) This map shows the locations of rockfall events in Yosemite Valley over 150 years. The season is coded by color, and the amount of material that fell is indicated by the size of the colored circle. (B) This photo captured a major rockfall event as it occurred on October 11, 2010. (C) Large angular boulders dot Yosemite Valley’s floor. Many of these boulders, which originated from prehistoric rockfall events, traveled far into the valley before they came to rest.
(A. Greg Stock, Yosemite National Park, National Park Service; B. © Tom Evans, www.elcapreport.com; C. Steven M. Bumgardner, Yosemite National Park, National Park Service)

rockfall

A type of mass movement in which rocks tumble off a vertical or nearly vertical cliff face.

Rockfall caused by frost wedging on steep mountain faces often creates piles of rock. The pieces of angular broken rock that accumulate at the base of a steep slope or vertical cliff are called talus (or scree). Repeated rockfall events in the same location can carve notches into the bedrock called rockfall chutes. Talus accumulates at the base of rockfall chutes in cone-shaped piles, or talus cones, as shown in Figure 15.30.

Figure 15.30

Talus. Talus accumulates to form a talus cone at the base of Kearsarge Pinnacles in the Sierra Nevada, in California. The talus has settled at its angle of repose. When two or more talus cones converge, as shown here, a talus apron is formed.
(Bruce Gervais)

talus

(or scree) Pieces of angular broken rock that accumulate at the base of a steep slope or vertical cliff.

The agents of weathering and mass movement all act simultaneously and continuously to reduce the vertical relief that Earth’s internal geothermal energy has built up. Figure 15.31 reviews these processes in the context of the Grand Canyon.

Figure 15.31

GEO-GRAPHIC: Carving the Grand Canyon.
(© Gary Crabbe/age fotostock)

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