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Wave-particle duality

Geoblogger Brian Romans has declared it Dune Week (here's part of his tweet), so I thought I'd jump on the bandwagon with one of my favourite dynamic dune examples illustrating the manifold controls on dune shape. 

Barchan dunes and parabolic dunes both form where there is limited sand supply and unimodally-directed wind (that is, the wind always blows from the same direction). Barchans, like these in Qatar, migrate downwind as sand is blown around the tips of the crescent. Consequently, the slip face is concave.

Location: 24.98°N, 51.37°E

In contrast, parabolic dunes have a convex slip face. They form in vegetated areas: vegetation causes drag on the arms of the crescent, resulting in the elongated shape. These low-amplitude dunes in NE Brazil have left obvious trails.

Location: 3.41°S, 39.00°W

The eastern edge of White Sands dunefield in New Mexico shows an interesting transition from barchan to parabolic, as the marginal vegetation is encroached upon by these weird gypsum dunes. The mode transition runs more or less north–south. Can you tell which side is which? Which way does the wind blow?

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Herrmann and Duràn modelled this type of transition, among others, in a series of fascinating papers including this presentation and Durán et al  2007, Parabolic dunes in north-eastern Brazil, in arXiv Soft Condensed Matter. Their figures show how their numerical models represent nature quite well as barchans transition to parabolic dunes:

Geophysicists especially might note the wave-like nature of dunes, and indeed most other fluid-dynamical sedimentary structures. Like a surging crowd, granular particles exhibit collective behaviour that echos phase transitions of matter: solid, then fluid; particle, then wave. 

Other don't-miss blogs posts on dunes 

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Reader Comments (4)

Another wave-like phenomenon : flocks of geese. I admired a large flock over Stonehaven at the weekend, noting how they resembled my crosshole seismic gathers from the 1990s. P-wavefront, S-waves branching off, occasional reflections. But then they mess up the analogy by showing free will and the laggers play catch up or switch their connection to the leading group.
It's fun remembering we are Natural Scientists.

November 22, 2011 | Unregistered CommenterPeter Rowbotham

@Peter: Thanks for the comment. Nice analogy... I can picture it perfectly. For sure there's geophysics everywhere once you start looking. And once you start seeing geophysics in geese, you know you're well ahead of the game!

November 22, 2011 | Registered CommenterMatt Hall

Hello, liked me this article. It is wonderful and very educational. I have a question for several years: can a tsunami cause accumulations of sand with a shape resembling a dune? If this is possible, I'd like to know that type of way acquire, i.e. to which type of dune resemble...

Thanks a lot...

A very cordial greeting,

August 4, 2013 | Unregistered CommenterAriel

@Ariel: Apologies, I seem to have missed this comment over the summer. I don't know a lot about tsunami deposits, but I gather it's thought the obvious 'tongues' of sand along the SW coast of Australia were left by tsunamis. Hopefully that's enough to go on to start answering your question.

September 5, 2013 | Registered CommenterMatt Hall

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