Geoscience Reference
In-Depth Information
8
Dune Fields, Sand Seas and Transport
Pathways
One of the most striking things about dunes is their
tendency to congregate in awe-inspiring expanses, sand
seas. These are reviewed in a paper (Wilson 1973) with the
deliciously laconic title 'ergs', invoking the Arabic word
often used to refer to a sand sea. Why should sand do this?
From the process perspective, just as for the formation of an
individual dune, the accumulation just means that sand has
entered a region at a higher rate than it has left.
For an active system (rather than a fossil one for which
see Sect. 8.2 ), this can be due to several factors (a good
discussion of the formation of sand seas can be found in Pye
and Tsoar (1990). First, topographic barriers can cause sand
to accumulate because the sand cannot move as easily
uphill; most Mars crater dune fields, and the Great Sand
Dunes in Colorado (Fig. 8.1 ) are of this type. Second, a
convergent wind regime can lead to reduced net sand
motion. Sand is brought into a region where it moves as far
forward as back each year, and thus accumulates: this is
generally the process by which linear and star dunes—the
largest types—form. The major sand seas are in this cate-
gory, although also smaller dunefields such as the Stovepipe
Dunes in Death Valley. Finally, winds passing through a
topographic constriction may fall in speed as they expand
away from it, causing deposition (the Coral Pink Sand
Dunes in southern Utah are one example): a similar effect
enhances deposition of sand in craters.
The geomorphology of dunes in the absence of topo-
graphic barriers may attest to the age of the deposit. Linear
and star dunes are forms that accumulate more-or-less
statically and thus can be very old, whether they are active
at the present day or not. On the other hand, transverse—
and especially barchan dunes—indicate a dominant trans-
port direction, and thus if the dunes are not fossilized, they
must be in motion (see Chap. 8 ) . If they are in motion, then
there must be a sand supply and active transport. Barchans
are often found in corridors, defining the transport pathway
from their source: an isolated patch of barchans implies that
the sand supply or the conditions to move it have been short
in duration.
8.1
Sand Sources and Sinks
The source of sands on Earth is usually river action, and
many sand seas have a direct association with river sources;
for example, the Namib sands are derived from rivers
draining into the Atlantic. On geologically short timescales,
sand may accumulate in some regions like depressions, or
may swirl around in closed paths, notably in the Sahara and
Australian deserts. In the long-term at the planetary scale,
one might expect most sand on Earth to eventually blow into
the sea. This might be thought of as a one-way trip, but over
geological time, tectonic movements and other changes
bring that sediment back into play either as re-exposed sands
and evaporitic deposits, as sandstone or limestone which can
weather out, or (via subduction) as volcanic rocks.
This rock cycle does not apply (as far as we know) to the
other planets. Mars may have had seas in the past, but at
present the sand is free to swirl around forever. In fact, most
dunefields appear to be in local depressions (notably impact
craters—e.g.,
Fig. 8.2 ;
see
also
Fig. 18.16 )
with
the
exception of the north polar erg.
Titan was expected to have seas, which would act as
sand sinks and led to some pessimistic early expectations
about dunes on that world (Lorenz et al. 1995). However, at
the present epoch at least, the seas cover only a few percent
of the polar regions, and the low latitudes are dry and
substantially covered in dunes.
A variety of forensic approaches can be used on terrestrial
dunes to ascertain the provenance of the sand. In addition to
remote sensing or mapping (from which the pathways can be
obvious—see Figs. 8.3 and 8.4 , and 12.6 ), the composition of
 
 
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