Geology Reference
In-Depth Information
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What Would You Do
As a member of a planning commission, you are charged with
developing zoning regulations and building codes for an area
with known active faults, steep hills, and deep soils. A num-
ber of contractors, as well as developers and citizens in your
community, are demanding action because they want to begin
several badly needed housing developments. How might
geologic maps and an appreciation of geologic structures
infl uence you in this endeavor? Do you think, considering the
probable economic gains for your community, that the regula-
tions you draft should be rather lenient or very strict? If the
latter, how would you explain why you favored regulations that
would involve additional cost for houses?
54804_10_F18
adjacent areas of sedimentation subside (Figure 10.25). If
continued long enough, the mountains will disappear and
then can be detected only by the plutons and metamorphic
rocks that show their former existence.
◗
Figure 10.24
The Principle of Isostasy An iceberg sinks to an
equilibrium level with 10% of its mass above water level. The larger
iceberg sinks farther below and rises higher above the water surface
than the smaller one. If some of the ice above water level melts, the
icebergs will rise to maintain the same proportions of ice above
and below water level.
Mountains
Continental
crust
to pass through it, the mantle is indeed solid. But when sub-
jected to stress over long periods, it yields by fl owage; thus, at
this time scale, it can be regarded as a viscous fl uid.
What happens when a ship is loaded with cargo and then
later unloaded? Of course, it fi rst sinks lower in the water
and then rises, but it always fi nds its equilibrium position.
Earth's crust responds similarly to loading and unloading,
but much more slowly. For example, if the crust is loaded,
as when widespread glaciers accumulate, the crust sinks
farther into the mantle to maintain equilibrium. The crust
behaves similarly in areas where huge quantities of sediment
accumulate.
If loading by glacial ice or sediment depresses Earth's
crust farther into the mantle, it follows that when vast gla-
ciers melt or where deep erosion takes place, the crust should
rise back up to its equilibrium level. And in fact it does. This
phenomenon, known as
isostatic rebound
, is taking place
in Scandinavia, which was covered by a thick ice sheet un-
til about 10,000 years ago; it is now rebounding at about
1 m per century. In fact, coastal cities in Scandinavia have
rebounded rapidly enough that docks constructed several
centuries ago are now far from shore. Isostatic rebound has
also occurred in eastern Canada where the crust has risen as
much as 100 m in the last 6000 years.
◗
Mantle
Low-density
mountain root
a
Transport
Erosion
Deposition
Subsidence
Uplift
b
Figure 10.25 shows the response of Earth's conti-
nental crust to loading and unloading as mountains form
and evolve. Recall that during an orogeny, emplacement of
plutons, metamorphism, and general thickening of the crust
accompany deformation. However, as the mountains erode,
isostatic rebound takes place and the mountains rise, whereas
c
◗
Figure 10.25
Isostatic Rebound A diagrammatic representation
showing the isostatic response of the crust to erosion (unloading)
and widespread deposition (loading).
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