Sand - Dune Worlds: How Windblown Sand Shapes Planetary Landscapes - page 16

Fig. 2.1 We deliberately
challenge the reader's
expectations by starting the
illustrations in this sand section
with a dune made from material
that we do not typically call sand,
but that meets our criterion of
'dune-forming'. Here, the lee of a
stack of supply cases on a
meteorite-hunting expedition in
Antarctica has allowed blowing
snow to form a somewhat linear
dome dune overnight. A slight
halo of saltating snow can be
seen above the dune against the
tent background. Photo Jani
Radebaugh

Dunes can form on Earth in volcanic ash, a couple of
celebrated examples being a dark ash barchan that has been
marching away from the Ol Doinyo Lengai volcano in
Tanzania at about 10 m per year, and the colorful painted
dunes in Lassen Volcanic Field. Indeed, cross-bedding
textures can often be seen in ash deposits; some may be
caused not by conventional winds but by the strong out-
flowing surge when an ash column collapses. Bedforms are
not limited to forming from sand-sized particles: pumice
gravel in the Puna of Argentina forms the largest wind-
ripples known on Earth. These hard-to-move lumps define
the other end of the dynamical spectrum of particulates—
stuff that needs the strongest winds to be launched into the
air, and is only modestly affected by the airflow once that
happens. This is true, generally, for sands on Mars, many of
which have a volcanic (basaltic) composition.
Some sands are quite literally grown. Shell fragments of
mollusks are a common component of many beach sands, and
limestones in particular (but carbonate rocks on Earth in
general—e.g. Brooke 2001) may have a substantial amount
of tiny shells, which if eroded out and transported by wind can
be referred to as an eolianite. When these rocks break down,
these tiny fossil shells are the natural result; oolitic limestone
is defined by spheroids ('ooids', from the Greek word for
'egg') between 0.25 and 1.25 mm. The tiny but resistant
silica corpses of smaller living things yet, hard-shelled algae
known as diatoms, form sediments (diatomite, or diatoma-
ceous earth) which can break back down into hollow particles

typically 10-200 l across. Their small size and very low
density makes them easy to transport by wind, and makes
them responsible for the dustiest places on Earth, like the
Bodélé Depression in Chad (which also has some of the
fastest-moving dunes made of these little shells, see Chap. 8 ) .
At the densest end of the spectrum, sorting by fluvial or
aeolian processes can concentrate minerals. Often, bands of
dark magnetite can be found on sand dunes, and Gay (1999)
describes a small dune in Peru where aeolian sorting led to
it having a concentration of some 46 % magnetite. The
density of magnetite is some 4900 kg/m 3 , around double
that of quartz. Of course, human activities are better yet at
sorting: there are doubtless piles of metal ores at mines and
foundries worldwide that beg for aeolian experimentation.
And in a cruel imprint of human history on geology, the
sands of certain beaches in Normandy have a high fraction
of steel particles.
With such a wide range of 'sand' and formation processes
on Earth, one might expect that an even wider range needs to
be considered for other worlds. Yet in fact, by and large what
we know of Mars at least suggests the processes that make
sand, and the resultant compositions, are the same as, but just
a subset of, what happens on Earth. Titan, at least, likely has
sands of an exotic composition (probably containing such
organic chemical compounds as phenanthrene, coronene and
other exotica), although they may be processed into saltating
sand in much the same way as evaporitic sands on Earth.
However they are made, it is important to adopt a wide