Earth as a Planet: Surface and Interior 199
FIGURE 9 Desert drainage networks in Chad,
North Africa. Shown here are deeply incised
canyons on the southwest slope of the Tarso
Voon Volcano located in the west-central part of
the Tibesti Volcanic Range, in northern Chad
(20.5◦N latitude, 17◦W longitude, approximately
3400 feet above sea level). Characteristic
steep-walled theater-headed canyons form as
overlying relatively soft Tarso Voon ignimbrites
are stripped back over more resistant basement
schists, through the action of groundwater
seepage and surface runoff during infrequent
storms, or during previous eras of wetter climate.
Such differential erodability very likely also
played a part in the formation of ancient complex
ramified canyon networks on Mars of similar
scale and appearance, and may reflect the former
presence of more abundant supplies of
near-surface water during warmer periods on
Mars in its distant past. The ASTER image was
acquired on 12 January 2003. Spatial resolution
is 15m/pixel and the image as shown is a RGB
composite of three visible bands (1N, 2N,
3N–0.52 to 0.82μm). (Courtesy NASA/GSFC/
METI/ERSDAC/JAROS, and U.S./Japan
ASTER Science Team).landscape is not dominated by impact scars. Plate tectonic
processes are, in part, responsible; however, fluvial erosion
is probably the dominant factor for subaerial landscapes in
this regard. Also, without constant tectonic reinforcement,
rainfall would probably reduce a Himalayan-style, or Alpine
range to Appalachian-style mountains within 10 Myr or so.
On the Earth, when tectonic forces subside, constant fluvial
erosion wins out and hilly landscapes are flattened.
Other erosive processes, independently or in concert
with fluvial activity, also clearly play a role on the Earth,
including seepage-induced collapse (called “groundwater
sapping”) which can result in networks of steep-walled
gulleys and canyons. In addition, the chemical action of
groundwater can form landscapes of caves and sinkholes
in limestone areas (called “karsts”). Whereas groundwater
sapping and karst formation on the Earth may be relatively
less important than fluvial erosion, the opposite case may
be true for Mars. Another process regime that dominates
arid and polar deserts on the Earth, and apparently is highly
active, even today, on Mars, is that of wind-driven erosion
and transport of fine dust and sand (called “aeolian” the
Roman god of the winds). On the earth, aeolian processes
are dominant only in certain restricted areas, such as the
desert sand seas of Africa and Asia (Fig. 10a). On Mars,
however, fine dust and sand dune and drift morphologies
appear everywhere and can reveal important information
on current wind regimes and on the constitution of the fine
material based on observations and models of terrestrial
dune morphologies.
Another important terrestrial geomorphic process is
weathering—the breakdown of consolidated material into
constituent grains. Rock can be broken down in several
ways. Chemical weathering can occur when natural acids
act on carbonates in susceptible rocks, such as limestone
or sandstones, releasing the residual silicate grains. Me-
chanical weathering of rock can occur when the hydrostatic
pressures of ice in freeze-thaw cycles overcome rock brittle
strength thresholds at microscopic and macroscopic scales.
The formation of salt crystals also exerts mechanical energy
to break up rocks and can chemically weather rocks. Oxi-
dation of minerals, particularly iron-containing minerals, is
another form of chemical weathering. Biological weather-
ing occurs through chemical weathering caused by biogenic
acids, particularly in tropical areas. It can also occur me-
chanically, by bioturbation of soils and sediments, as well as
by the physical pressure of root and stem turgor in cracks
and fissures within solid rock. It is of significance that on the
Earth, all three major forms of weathering are enhanced or
enabled by the ubiquitous presence of water.
Perhaps some of the most dramatic forms of nonvolcanic
landscape alteration that we see on the Earth today fall into
the category that geomorphologists call mass wasting. Gen-
erally, the term mass wasting is applied to processes such as
landslides, creep, snow and debris avalanches, submarine