Geology Reference
In-Depth Information
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Bainbridge Island,
Puget Sound,
Washington
Fig. 1.5 Comparison of aerial
photograph with “bare-Earth”
lidar image of an active fault.
Dense forest obscures geological
structure and geomorphology in an
aerial photograph of the southern
tip of Bainbridge Island in
Washington's Puget Sound (top).
Removal of the forest canopy using
lidar data (bottom) yields a
shaded-relief, bare-Earth model
whose topography reflects the
climatic and tectonic history of the
site. Glacial grooves and ridges
reveal the former ice flow direction
to the south, whereas the abrupt
linear escarpment in the north
shows the surface expression of the
Toe Jam Hill Fault, a fault that was
not previously known to break the
surface. This fault is a north-dipping
backthrust of the Seattle Fault, and
the lidar imagery provided the
foundation for a trenching
campaign that revealed the
earthquake history of this hazard-
producing fault zone (Nelson et al .,
2003). Images courtesy of Samuel
Johnson and the USGS.
p
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1 km
New View of
Topography
L
I
D
A
R
T o e Jam Hill
fault
N
i
m
a
g
e
glacially
sculpted
topography
raised marine
platforms
believed that mountain uplift led to cooling and
helped to precipitate the Ice Ages. In 1990,
Molnar and England challenged this entrenched
idea and suggested nearly the opposite: as cli-
mate changed to more glacial conditions in late
Cenozoic times, enhanced rates of erosion within
mountain belts caused increased rates of valley
incision, which in turn incited isostatic uplift of
the residual peaks (Molnar and England, 1990).
Although isostatic uplift can produce ranges
with higher summits, it does not implicate tec-
tonic uplift as the cause of climate change.
So, how do we tell whether the climate caused
uplift of the summits or whether surface uplift of
the ranges caused changes in climate? Potential
resolutions to this quandary require many ingre-
dients, including the nature and magnitude of
changes in mean elevation of mountains, in rates
of erosion, in climate, and in elevations of sum-
mits and valley bottoms. Moreover, we would
like to know when and how rapidly changes
took place. If you think carefully about any one
of these ingredients, you quickly realize the
reason why tectonic geomorphological studies
are often interdisciplinary in nature. Consider,
for example, the concept of mean elevation of a
range. Using digital topographic data, the current
mean elevation is straightforward to calculate.
But how do you determine mean elevations in
the past? As described in subsequent chapters,
approaches to estimating former altitudes range
from paleobotanical to isotopic studies. Or,
consider the effects of changes in climate. During
Ice Age times, was there more precipitation or
less? Did the expansion of glaciers lead to
enhanced rates of erosion? There is intense inter-
est in, and considerable argument about, whether
glaciers are effective agents of erosion in com-
parison to rivers. The controversy has spawned a
flood of recent research into the physical and
 
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