Geology Reference
In-Depth Information
as it moves across a surface or from the accumulation of
debris derived from valley walls. Surface material often
coalesces into various moraines, which are linear depos-
its. Some material carried by the ice eventually reaches
the front of the glacier, where it can be deposited in situ
or carried away by melt water. The
finer material is often
transported by the strong winds which are generated
along ice margins. Coarser glacial deposits, termed
drift, may assume a variety of geometries, which pro-
vide clues as to the form and position of glaciers after the
ice mass has retreated. On Earth, glaciers have effected
extensive changes in the landscape. U-shaped valleys
(
Fig. 3.41
), grooves, and striations parallel to the
ow
of ice, and amphitheater-shaped erosional cirques in the
headward parts of valleys are indicative of glacial
erosion.
In periglacial zones (cold regions), soli
uction pro-
cesses arising from the slow downslope
flow of water-
saturated, unconsolidated materials can occur. Periglacial
processes involve the erosion of rock or soil by snow and
ice, frost action, and chemical weathering, leading to such
features as permanently frozen ground (permafrost), pat-
terned ground (
Fig. 3.42
), pingos (dome-shaped ice-
cored mounds that can be 70m high and 600 m across
on Earth), and thermokarst (melt-eroded) topography.
The occurrence of a periglacial region is not genetically
related to the proximity of glaciers or continental ice
sheets, contrary to what is implied by its etymology.
However, the presence of water is essential for most
periglacial processes to occur. This broader de
nition is
useful in that it allows us to consider the possible oper-
ation of periglacial-type processes on the surfaces of other
objects in the Solar System.
3.6 Summary
Planetary surfaces are shaped and modi
ed by four prin-
cipal processes: tectonism, volcanic activity, impacts, and
gradation. Each of these processes produces distinctive
landforms on Earth, where most of these processes have
been studied in detail. One of the goals of planetary
geology is to determine how these landforms might be
different in extraterrestrial environments.
Views of the Earth obtained from orbit show that, while
some processes can be identi
ed by remote sensing,
others cannot, thus introducing uncertainties in the inter-
pretations of images from other planets. Furthermore,
with increased knowledge of the outer planet satellites,
planetary geologists must assess the validity of applying
Earth analogs to bodies composed mostly of ice and hav-
ing markedly different environments.
Assignments
1.
Scan news media sources for the most recent accounts
of active natural disasters and document one example
for each of the four principal geologic processes that
shape planetary surfaces. Write a few sentences about
the event, identify the process involved, and provide
the news media source.
identify the planet and the process, and give a short
description of the feature.
4.
Both impacts and volcanic eruptions can lead to the
formation of craters. Discuss the fundamental differ-
ences in how these two processes differ in the forma-
tion of their respective craters.
2.
Explain why aeolian (wind) processes require stron-
ger winds to move sand grains on Mars than on
Venus.
5.
Planetary variables in
uence the morphology of vol-
canoes; identify one such variable and explain how a
volcano on the Moon might be different from a vol-
cano on Venus, all other factors (such as magma
composition and rate of eruption) being equal.
3.
Go to NASA websites that contain planetary images
and
find examples of landforms beyond Earth for the
four geologic processes that shape planetary surfaces.
Print at least one image for each process (a total of four
images); label the mission that obtained the image,
6.
Explain why so few impact craters are seen on Earth in
comparison with the Moon.
