Did you know...

…that Namibia got dwarf dunes?

These are hardly known and were only explained more thoroughly in April 2018. In the journal Nature Physics my Israeli colleague, Dr. Hezi Yizhaq, and German scientists explained for the first time how these mini dunes form.

Anyone who thinks of Namibia will probably immediately see the mighty sand dunes of Sossusvlei (see round trip, day 5). These are up to more than 300 m high. On the surface they are mostly decorated with so-called ripples, hence small sand waves that one also knows from the shore area at the sea. But there is also an intermediate size – the so-called megaripples. In contrast to the only 1-2 cm high and 10-20 cm wide ripples, the megaripples are clearly higher and one meter wide. Therefore they were thought to be huge ripples. However, the first explanation of this phenomenon shows now that megaripples are more related to the dunes. So they are mini dunes or dwarf dunes.

Such dwarf dunes are quite rare because, unlike normal dunes, they do not consist predominantly of a uniform sand grain size, but of small and large sand grains. Whereas permanent wind over centuries pushes the sand grains of the same size from usual dunes to gigantic sand mountains like in the Sossusvlei, megaripples are formed by wind and the forward bouncing of the grains on the ground. The light small grains of sand fly further through the wind than the heavy and coarser grains that remain behind. Wind, erosion and the slow progress of the heavy grains of sand then form a kind of dwarf dune where the ripple spacing is defined by “hop length”. Megaripples have a strong vertical sand grading, with coarse grains forming a protective top layer that prevents the transport of the finer material underneath.

In contrast to large dunes, the dwarf dunes are very unstable because strong winds can deform the megaripples again. Megaripples occur, for example, near Torra Bay along the Namib Skeleton Coast, but also in Israel’s Negev Desert, in Jordan or in California. The next time you are on your way through Namibia’s dune landscape, just keep an eye out to see if you might find such dwarf dunes somewhere else.