Geology Reference
In-Depth Information
These predictions are the output of a
numerical model that represents a great simpli-
fication of both tectonic and geomorphic
processes. In fact, the physics of many of these
processes are still poorly understood. Nonetheless,
it is satisfying to think that the thoughtful, yet
apparently contradictory, landscape develop-
ment models put forth during the past 100 years
are indeed reconcilable and that simple numeri-
cal models can help enhance our understanding
of tectonically active landscapes.
known as laser scanning (Carter
et al.
, 2007),
uses concentrated pulses of light emitted from an
airborne instrument to penetrate through open-
ings in a forest canopy. By measuring the return
time and direction of the light, the vector to the
ground can be calculated from the “last returns”
(those that were not reflected by above-ground
vegetation). Via an integration of all of the last
returns and with precise knowledge of the posi-
tion of the aircraft, a
bare-Earth
topographic
image of the Earth's surface can be created. The
resolution of the image depends on how high
the aircraft is flying and the nature of the light
beam, but commonly DEMs are created with 1-m
spatial resolution and topographic uncertainties
of a few centimeters. The stunning success of
lidar in revealing previously unknown fault
scarps beneath the canopy of dense northwest-
ern US forests (Fig. 1.5) made converts of skep-
tics and launched widespread efforts to acquire
lidar data over diverse tectonic and geomorphic
targets. Tripod-mounted laser scanners are
also gaining increasing popularity. Based on
the same physical principles, these instruments
permit topographic reconstructions with milli-
meter- to centimeter-scale uncertainties to be cre-
ated of individual hillslopes, fault scarps, or river
beds; and, with a series of topographic scenes
over time, the details of landscape changes can
be recorded and quantified. The vastly improved
accuracy of these new topographic views, rang-
ing from global topography to individual
hillslopes, has underpinned a new view of the
Earth and the processes that mold its surface.
The new world
Over the past two decades, technological
advances have dramatically improved our view
of the Earth. As the diversity and availability of
digital topography has expanded, our ability to
visualize the shape of the Earth's surface has
become much easier and more accurate. For
11 days in February 2000, NASA's Space Shuttle
used an active radar system to map the
topography of the world between 60
°
N and
°
60
S (Farr
et al.
, 2007). The digital elevation
model (DEM) derived from this mission now
provides almost complete elevation coverage
with a spatial resolution of 90 m. Still more
recently, a higher-resolution, 30-m DEM that
covers the world between 83
°
°
S has
been developed from satellite stereoimages
and is freely available (http://www.gdem.aster.
ersdac.or.jp/). The success of Google Earth in
merging such topographic data with remote
sensing images has provided tectonic geo-
morphologists with an unprecedented opp-
ortunity to explore the Earth's surface. As
high-resolution (
N and 83
Some modern controversies
≤
1 m) imagery is increasingly
incorporated into Google Earth, individual fault
scarps, uplifted marine terraces, and channels on
actively growing folds in previously inaccessible
areas can now be visually explored from a
computer almost anywhere in the world!
In the temperate mid-latitudes, however, where
forests blanket much of the landscape, even
high-resolution imagery does not commonly per-
mit a clear view of the actual land surface. But,
this restriction is also changing. A new technology,
lidar (light detection and ranging), which is also
At present, many lively controversies animate
studies in tectonic geomorphology. Some of
these provide an interesting backdrop for
reading subsequent chapters of this topic. For
example, the history of Cenozoic cooling of the
rocks within many mountain ranges has been
interpreted to suggest that the ranges experi-
enced accelerated rates of uplift during late
Cenozoic times. With increased recognition that
this apparent increase was approximately coeval
with the onset of the “Ice Ages,” it was commonly
