Mass Wasting - The Changing Earth: Exploring Geology and Evolution - page 288

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Figure 11.14 Mudfl ow, Estes
Park, Colorado Mudfl ows move
swiftly downslope, engulfi ng
everything in their path. Note how this
mudfl ow in Rocky Mountain National
Park has fanned out at the base of
the hill. Also note the small lake
adjacent to the mudfl ow that was
formed after this mudfl ow created a
dam across the stream.

most of the homes in the Turnagain Heights subdivision
(Figure 11.18b).
Solifl uction is the slow downslope movement of water-
saturated surface sediment. Solifluction can occur in any
climate where the ground becomes saturated with water, but
is most common in areas of permafrost.
Permafrost, ground that remains permanently frozen,
covers nearly 20% of the world's land surface (

As might be expected, many problems are associated
with construction in a permafrost environment. A good
example is what happens when an uninsulated building is
constructed directly on permafrost. Heat escapes through
the fl oor, thaws the ground below, and turns it into a soggy,
unstable mush. Because the ground is no longer solid, the
building settles unevenly into the ground and numerous
structural problems result ( Figure 11.19a).

During the warmer season when the upper portion of the
permafrost thaws, water and surface sediment form a soggy
mass that fl ows by solifl uction and produces a characteristic
lobate topography (Figure 11.19b).

◗ Figure 11.20).

Creep, the slowest type of fl ow, is the most widespread and
signifi cant mass wasting process in terms of the total amount of
material moved downslope and the monetary damage it does
annually. Creep involves extremely slow downhill movement of
soil or rock. Although it can occur
anywhere and in any climate, it is
most effective and signifi cant as a
geologic agent in humid regions. In
fact, it is the most common form
of mass wasting in the southeast-
ern United States and the southern
Appalachian Mountains.
Because the rate of move-
ment is essentially imperceptible,
we are frequently unaware of
creep's existence until we notice
its effects: tilted trees and power
poles, broken streets and side-
walks, or cracked retaining walls
or foundations (

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Figure 11.21).
Creep usually involves the whole
hillside and probably occurs, to
some extent, on any weathered or
soil-covered, sloping surface.
Creep is not only difficult to
recognize, but also to control. Al-
though engineers can sometimes
slow or stabilize creep, many times

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Figure 11.15 Debris Flow, Ophir Creek, Nevada A debris fl ow and damaged house in lower
Ophir Creek, western Nevada. Note the many large boulders that are part of the debris fl ow. Debris
fl ows do not contain as much water as mudfl ows and typically are composed of larger particles.