Geology Reference
In-Depth Information
temperatures below freezing. Water-ice exists on Mars, is
a major constituent of most of the outer planet satellites,
and is thought to be present in permanently shaded parts of
the Moon and Mercury.
Surface features on an icy or ice-rich body result largely
from processes of
flow and fracture. Although ice is often
modeled as a Newtonian viscous
fluid, experiments indi-
cate that some ice can be considered a pseudo-plastic
fluid.
that deforms by creep. In a Newtonian
fluid, the rate of
strain is linearly proportional to the applied stress, and the
viscosity is the ratio of the strain to the stress raised to
some power. Thus, as the stress level is increased, the
material deforms more and more rapidly. The result is
that ice appears to become less viscous at higher rates of
strain. However, under very rapid strain rates, such as
during an impact event, ice behaves more like brittle
elastic material than a
fluid.
On Earth, precipitation of snow in the area of accumu-
lation (the headward part of a glacier) forms a deposit that
is about 20% ice and 80% air. Melting and refreezing, plus
compaction, converts the snow to spherical ice particles
called
rn. As the
firn accumulates, further compaction
causes recrystallization to form the main ice mass, typi-
cally having less than 10% air. Glaciers are classi
ed as
valley glaciers or as piedmont glaciers (coalesced valley
glaciers at the base of a mountain range), or as ice sheets
(also called continental glaciers, or ice caps) if they are too
large to be contained by valleys. All glaciers move down-
slope or outward, leaving distinctive terrains. A
retreat-
ing
”
glacier simply means that melting and ablation of ice
exceed the rate of forward movement by the glacier.
Most glaciers also incorporate rocky materials within
the ice mass. This material can include dust and other
airborne particles and chunks of rock gouged by the ice
“
Figure 3.40. Yardangs are wind-sculpted hills that resembled
overturned boat hulls, re
ecting their streamlined shape,
exempli
ed by these features in Peru (US Geological Survey
photograph by J. F. McCauley).
Figure 3.42. These ice-wedge polygons on the Alaska sea coast near
Barrow are 7
15m across and form by periglacial processes working
on unconsolidated sediments (US Geological Survey photograph by
R. I. Lewellen).
-
Figure 3.41.
-shaped valleys, such as Deadman
Canyon shown here in Tulare County, California,
are typical of erosion by glaciers (US Geological
Survey photograph by F. E. Mathes).
“
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