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Guide to Reading
Put simply, this chapter is about gravity. To elaborate a bit, the chapter discusses mass movement, which is simply the movement of rock, regolith, or snow and ice downslope.
Naturally there are details. You learn there are 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 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 it’s not all repetition 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 terminology 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 this is serious science, 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
- Armero, Colombia, 1985
- Vaiont Dam, Italy, 1963
- Austrian Alps, 1999
- Yosemite National Park, California, 1996
- Hawaii, submarine coastal areas
- Elm, Switzerland, 1881
- Madison Canyon, Montana (near Yellowstone), 1959
- Storegga Slide, North Sea coast
- Lituya Bay, Alaska, 1958
- Gros Ventre River Valley (near Jackson Hole), Wyoming, 1925
- La Conchita, southern California, 1994-95
- Olympus Mons, Mars
- Portuguese Bend Slump of the Palos Verdes area, California, 1956-85
- Los Angeles, a mobile society (tongue-in-cheek)
Why does anything fall down? Gravity brings things down, but how do things get up in the first place? More scientifically worded, what causes the relief of the area? Possible causes are:
- convergence at plate boundaries and either resultant subduction or continental collision
- faulting in rift areas
- pile up of extrusive rocks at volcanic vents
- buildup of sediments
- human activity that builds things up
- human activity that tears things down
- weathering and erosion that tear things down
- lowering of sea level
Why do just some things fall down? Why isn’t everything that’s up in the process of falling down? 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 shocking events and vibrations (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 very basic reason for all this instability and resultant falling down, what else could any basic reason in this book be but plate tectonics. You’re given a short reminder that plate tectonics is the basic cause of volcanic eruptions, faulting, and earthquakes, which in turn are basic causes of 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 a potential danger zone and also a scenic place. Many people choose not to stay out of such areas. So what is the answer? The goal of geology is to ascertain just what the risks are in potentially dangerous areas so each person can make an informed decision about whether to be there. To do this, they assess risk factors by looking for features like pressure ridges and by analyzing factors like slope steepness; strength of substrate; degree of water saturation; dip of bedding, jointing, and foliation relative to slope; vegetation cover, climate, undercutting, and seismicity. They compile all this information into landslide-potential maps which are made available to government agencies and to the general public. It’s up to them 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 there are some factors that can be controlled and steps that 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 when you want it to happen, not when it chooses to happen.
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? |
