Deposits of Sand: Ripples Versus Dunes - Dune Worlds: How Windblown Sand Shapes Planetary Landscapes - page 54

Fig. 5.6 Lobe-shaped avalanches on the slipface of a barchan dune in
the United Arab Emirates. The slip face is about 2 m high. The
alcoves, with vertical gouges, from which the avalanching sand started

are clearly visible—evidently some sorting results in the surface sand
having a slightly different color. Photo Jani Radebaugh

Individual avalanches typically occur over fairly narrow
spans of a dune, forming a narrow lobe (see Fig. 5.6 ).
Avalanching is sometimes accompanied by a hissing sound,
or sometimes much louder sounds and seismic vibrations (see
Chap. 10 : Booming dunes). Such avalanche lobes have been
observed on Mars (e.g., Horgan et al. 2012; see Fig. 5.7 ).

terrestrial dunes, where the iron mineral magnetite is both
appreciably denser than the quartz that makes up the bulk of
the dune, but is also much darker and so accumulations are
readily visible (Fig. 5.11 ). In the Namib sand sea, dark
patches seen on dunes are concentrations of not only
magnetite, but also ilmenite and garnet (e.g., Schneider
2008).

5.3 Grain Sorting

5.4 Dunes Versus Ripples

A final issue is that sand can often be a mixture of materials
or particle sizes. Since these are transported with different
effectiveness by the processes of saltation and reputation for
any given windspeed, they can be segregated, leading to
different concentrations of them on different parts of an
aeolian bedform. It is, for example, easy to observe that the
crests of dunes often have coarser sand. The sorting is most
evident on ripples, which often form (see Sect. 5.4 ) when
fine sediment can be armoured by coarser grains. The coarse
grains tend to accumulate on the upwind face, and espe-
cially the crest, of the ripples (e.g., Fig. 5.8 ). Exactly the
same phenomenon is observed on Mars (e.g., Figs. 5.9 and
5.10 )—see also Jerolmack et al. (2006).
The preferential accumulation is due to the reduced
mobility of the larger particles. Similar differential mobility
can result from grain size or the density of material from
which

What is the difference between a ripple and a dune? Typi-
cally, but not always, there is a scale difference—ripples are
small (e.g., Fig. 5.12 ). But the largest ripples on Earth can
be as large as small dunes, and the distinction may blur even
further on other worlds. Ripples do not have slip faces (and
although no bigger than terrestrial ripples, it was the pres-
ence of slip faces on dunes formed under Venus-like con-
ditions in a wind tunnel that led to the bedforms being
called 'microdunes'—see Chap 14 ). However, since many
dunes also lack slip faces, this is not a discriminator either.
The challenge here is that the different terms are really
genetic rather than purely morphologically descriptive.
Both features are the result of sand grains being transported
by the wind, but the processes involved in the formation of
these two features are quite different. To avoid falling into
the trap of misidentifying features on Mars without knowing the grain is made. This is often noticeable in