Chapter 13: Unsafe Ground: Landslides and Other Mass Movements
Study Plan
Landslide and debris flow atLa Conchita, California, a small seaside community north of Santa Barbara, spring 1995. Many people were evacuated and the houses nearest the slide were completely destroyed. Fortunately, no one was killed or injured.
Credit: R.L. Schuster, USGS
Guide to Reading
Put simply, this chapter is about gravity. To elaborate a bit, the chapter discusses mass movement, which is the movement of rock, regolith, or snow and ice downslope.
Naturally there are details. You learn about different types of mass movement, classified on four factors: type of material (rock, regolith, or snow and ice); velocity of movement (fast, intermediate, or slow); character of the moving mass (chaotic cloud, slurry, or coherent body); and environment of the event (subaerial or submarine). You will study the causes of mass movement, its consequences to Earth and to humans, and the ways to protect ourselves and our structures from its damaging effects.
One difficulty with the subject matter is that it's almost too common; words like landslide are used so casually they can mean almost anything. As you read, concentrate on the exact meaning of even the simple words. For example, a fall implies a vertical drop, whereas a slide means slipping as a coherent mass along an inclined slope, and a flow means turbulent, tumbling motion in which fluid (gas or liquid) is involved. You may still have to remind yourself of subtle variations as you read about mudflows, debris flows, landslides, rock slides, debris slides, snow avalanches, debris avalanches, rock falls, debris falls, and submarine debris flows, and you may even welcome more-sophisticated terms like creep (well, they're not all sophisticated), the cold-climate variation of creep called solifluction, and other new terms such as talus, rock glaciers, slumping (and its components the slump, failure surface, and head scarp), lahars, submarine slumps, olistostromes, and turbidity currents.
To remind you that this is a seriousmatter, the author relates some classic cases of mass movement. Localities to note are:
Yungay, Peru, 1970
Pacific Palisades, California, 1958
Rio de Janeiro, Brazil, 1988
Vaiont Dam, Italy, 1963
Austrian Alps, 1999
Yosemite National Park, California, 1996
Elm, Switzerland, 1881
Madison Canyon, Montana, 1959
Storegga Slide, Scotland and Norway, 30,000-6,000 years ago
Lituya Bay, Alaska, 1958
Gros Ventre River Valley (near Jackson Hole), Wyoming, 1925
Southern California, repeatedly, including 2003
Why do mass movements occur? You'll read about:
- fragmentation and weathering that weaken the surface
- slope stability (stable and unstable slopes, slope failure, downslope forces, and resistance forces)
- angle of repose
- weak surfaces that act as failure surfaces
What triggers a mass movement event? Here you read about shocks and vibrations caused by earthquakes, and special cases that involve quick clay. Whether a slope moves is partially dependent on inherent characteristics of the slope itself. You read about changing slope angles, slope loads, slope support, and changing slope strength (due to weathering, vegetation, and water content). Getting down to the basic reason for all this instability and resultant falling down, you won't be surprised that plate tectonics is the major cause of volcanic eruptions, faulting, and earthquakes, which in turn are primary factors in forming the topography that produces mass movements.
What can we do about all of this? We can't turn off gravity. Ideally no one should build, work, or play in areas prone to mass wasting. But that covers so much territory, and the world is too crowded to allow everyone to avoid the danger zones. Also, any area with significant relief is both a potential danger zone and a scenic place. Many people choose not to stay out of such areas.
So what is the answer? The goal of geology is to assess the risks in potentially dangerous areas so each person can make an informed decision about whether to be there. To do this, geologists analyze risk factors by looking for features like pressure ridges and by considering factors like slope steepness; strength of substrate; degree of water saturation; orientation of bedding, jointing, and foliation relative to slope; vegetation cover, climate, undercutting, and seismicity. They compile all this information into landslide-potential maps which allow the general public to decide what to do with the information.
What can be done to stop mass movement? In one sense that's a silly question, because we all agree you can't turn gravity off. But some factors can be controlled and steps can be taken to reduce the risk that mass wasting will occur. These include revegetation and regrading of slopes, reducing subsurface water, preventing undercutting, using proper construction practices, and even doing controlled blasting of unstable slopes to make mass wasting occur under less dangerous conditions.
In summary, this chapter reminds us that Earth is dynamic, and even when it's not doing something dramatic, like spewing out lava or quaking and shaking, it can do things that upset and even endanger our lives. Life is a gamble. Living in areas of high relief makes life an even bigger gamble. How big a gambler are you? Are you going to take a mountain vacation soon?
Key Terms
Chapter Menu
Other Resources
Instructors now have an easy way to collect students’ online quizzes with the Norton Gradebook without flooding their inboxes with e-mails.
Students can track their online quiz scores by setting up their own Student Gradebook.