Geology Reference
In-Depth Information
precipitation patterns. The most recently devel-
oped of these is WRF, the Community Weather
Research and Forecasting (WRF) model. WRF
models formally ingest NCEP's (National Centers
for Environmental Prediction) reanalyzed
weather data, and are capable of downscaling
these results to 1-km resolution. Given that 1 km
is indeed the scale of much valley and ridge
topography, we now have a tool to model the
precipitation patterns on digital landscapes at
the proper spatial and temporal resolution to
resolve weather events.
Clever application of these weather models
will be needed when coupling them to land-
scape evolution models operating on long
time scales. The spatial scales now match quite
well for many applications (data, for example,
are at sub-hillslope scales for mountain
ranges), but the time scales are seriously mis-
matched. The WRF models, for example,
require model updates at time scales of
seconds in order to honor the atmospheric
physics. Landscapes evolve on time scales of
thousands to millions of years. Development
of robust methodologies to bridge these time
scales represents a significant challenge to the
tectonic-geomorphic community in the coming
years.
evolving subglacial hydrologic system, models
of glaciers have parameterized sliding in
several ways. The problem is analogous to the
climate versus weather problem that we
discussed above in that the processes that
appear to govern sliding operate on short
(sub-seasonal) time scales, whereas we wish
to address problems of landscape change that
occur on time scales of tens to hundreds of
thousands of years.
One-dimensional models of long-valley profile
evolution were initiated by Oerlemans (1984)
and were recast by MacGregor
et al
. (2000). The
more recent models (MacGregor
et al
., 2009)
have incorporated at least a crude hydrologic
sub-model that allows the water system to
evolve seasonally. These models demonstrate
the essence of glacial erosion: glaciated valleys
flatten down-valley of the long-term equilibrium-
line altitude (ELA) and steepen in their head-
walls relative to their fluvial initial profiles. This
profile change simply reflects the pattern of ice
discharge in glacial systems (Anderson
et al.
,
2006): it increases from zero at the headwall to
a maximum at the ELA and then declines to zero
at the terminus. Note that this discharge pattern
contrasts with fluvial systems in which the
typical water discharge increases monotonically
to the final sink of a lake or ocean. Water cannot
leave the river system except through ground-
water loss or evaporation (which are usually
minor), whereas ice can leave the glacial system
by melting.
Two-dimensional models of alpine glacial
systems are now becoming more common and
can be tested against moraine patterns that
record the occupation of the landscape by
ice at the Last Glacial Maximum (Kessler
et al.
, 2006). These two-dimensional models
are beginning to be used to address the role
of glaciers in modifying two-dimensional
landscapes (e.g., Tomkin, 2007). Kessler
et al.
(2008) demonstrate how the topographic steer-
ing of ice through gaps in rift-related escarp-
ments serves to erode these gaps, defeat
drainage divides, and promote the growth of
major fjord systems that subsequently deliver
much of the ice from continental-scale ice
sheets to the ocean.
Glacial models
In the first edition of this topic, we reported
the lack of attention to the role of glaciers in
landscape evolution models. Glaciers are
important agents of landscape modification in
high alpine settings: exactly the settings to
which the tectonic geomorphologist is drawn.
The situation has improved significantly in the
decade since that writing. In order to erode the
landscape, a glacier must slide against its bed
and, thereby, abrade or quarry the underlying
rock (Iverson, 2002; Hallet, 1979, 1996). Recent
attention to the sliding of glaciers has been
motivated both by its geomorphic import and
its role in delivering ice rapidly to the oceans at
the fringe of the Greenland Ice Sheet. Although
the sliding process is quite complicated, in that
it is intimately intertwined with the seasonally
