Friday, April 26, 2013

Barchan dunes

Wind-blown sand in the desert can sometime build up into huge dunes. Quartz sand grains bump against each other and become rounded and well-sorted (typically medium-fine grained). The wind pushes them up the dune slope, a process called saltation, and when they reach the crest of the dune they fall down the steep front slope of the dune in the lee of the wind. This leads to a classic dune shape. In the presence of strong and persistent prevailing winds the dunes' shape reflects the wind direction. Individual dunes may develop "wings" pointing downwind. Looking like some alien space ship, they migrate across the desert on their way.

The ones shown here really are alien, they are isolated dunes seen on the surface of Mars, in the Noachis crater.

These crescent-shaped dunes are called barchans. Today they can also be seen on Earth, gliding across the arid wind-swept plains of desert environments.

Barchan dunes can be preserved in Earth's rock record too. Red sandstones formed in ancient Palaeozoic continental interiors often show barchan features. A fine example are the massive dune-bedded New Red Sandstones from the Permian rocks of Arran, North West Scotland. The lee slopes of the dunes are preserved as fine bedding planes, curving asymptotically to the palaeo-horizon. When further dunes pass over the top of them, they create huge cross beds. Measurements of the orientations of sets of these dune beds can give an indication of the palaeo-wind direction. In places the dunes show evidence of lightning strikes, with fossilised "fulgurites" … centimetre-scale hollow tubes where a lightning strike has earthed to ground in the desert, instantly melting the sand and then quenching to silica glass. Fulgurites stand proud on the outcrops of the Corrie shore in Arran, where they are more resistant to present day erosion on the shoreline. Elsewhere, around the Arran coast similar Permian dunes show evidence of burrowing residents, precursors of today's scorpions.

Image: NASA/JPL/University of Arizona

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