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Guide
to Reading 
Put simply, this chapter is about gravity. To elaborate
a bit, the chapter discusses mass movement (or mass wasting), 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 by 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, and debris falls,
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 slumping
(and its components the slump, glide horizon, and head scarp), and
lahars.
To remind you this is serious science, the author
relates some of the classic cases of mass movement. Localities to
note are these:
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
Elm, Switzerland, 1881
Madison Canyon, Montana (near Yellowstone), 1959
Lituya Bay, Alaska, 1958
Gros Ventre River Valley (near Jackson Hole), Wyoming, 1925
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 glide horizons
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, geologists
determine how long rock surfaces have been stable by use of cosmogenic
dating. 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 that 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?
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