A most unusual Sunday Sand

A pop quiz for Sunday – what mineral are these sand grains made of? A hint: check out some of the shapes and think of your kitchen.

They are salt crystals, halite, sodium chloride, NaCl, the stuff in the salt-shaker, tiny cubes. Normally, we think of sand as durable, the grains made of tough old minerals or long-lasting shell fragments; but then we recall that the definition of sand is purely one of size, and, while they are pushing the upper limit of the definition, these salt grains are sand. And they come from a beach – but one of the most bizarre beaches in one of the strangest places on the planet.

The East African rift valley marks one of the great rents in the earth’s crust, and where it runs into the Red Sea and the Gulf of Aden is one of the most tectonically active and bleak places around: the Afar Triangle, the Danakil depression. This area is a triple junction, a location where the edges of three plates meet; except that “meet” hardly describes it – they’re actually getting away from each other as fast as they can, the crust spreading and pulling apart at a rapid rate. Hence the volcanoes, the earthquakes, and the subsidence that are the hallmarks of the dramatic landscapes of this region. 

Close to the coast of Djibouti, in the midst of this tectonic mayhem, lies Lake Assal, and it’s from there that these grains come.

 
The lake is in the centre of this NASA image, surrounded by a swirling landscape of lava flows, and separated from the bay of Ghoubet Kharab and the Gulf of Aden by narrow barrier.

The starkly beautiful landscape is scored by northwest-southeast-trending cliffs and topographic ridges – the dramatic surface evidence of the active faults that are tearing the crust apart. Lake Assal is located in the centre of this segment of the rift; the barrier to the southeast is largely formed by the volcano of Ardoukoba which last erupted in 1978, filling the landscape with lava flows. This eruption was accompanied by earthquakes that marked the instantaneous widening of the rift by 1.8 meters. In this French geological map, the main rift faults are shown by the yellow lines, and the different ages of lava flows shown in different colours, becoming, not surprisingly, younger towards the centre, reflecting the growth of the rift.    As long as Ardoukoba continues to maintain this barrier, Lake Assal will continue to exist, but, once breached, the waters of the Gulf of Aden will flood in – for Lake Assal lies 155 m (509 ft) below sea level. The shoreline is the lowest point in Africa and the third lowest on the planet.

Temperatures routinely exceed 50 degrees centigrade in the summer and, apart from the occasional flash flood, there is no fresh water. The lake is fed by hot springs and seawater flowing through the fissures of the rift, and its waters are incredibly saline – indeed, these waters are the saltiest on earth outside Antarctica, reaching a concentration of 35% or more (the waters of the Dead Sea are a mere 33.7%; some hypersaline lakes in the dry valleys of Antarctica hold the world record). So it’s no wonder that the shores of Lake Assal are covered in salt and that salt crystals form the sands of its beaches. It has long been a location of the salt trade, the old camel caravans now being replaced by commercial-scale industrial operations.

These salt grains may be at the upper end of the size scale for sand, but they also come as supergrains, granules that can be found for sale from various “new age” outlets, billed as “African Pearls”; the purchasers are encouraged to dissolve them in their bath water, but quite what the difference is between doing so and simply raiding the resources of their kitchen is not explained.

[Lake Assal grains and granules kindly provided by Marco Bonifazi. East African Rift image from http://acces.inrp.fr/eduterre-usages/ressources_gge/afar/afar.htm. Satellite image from Wikimedia Commons, in the public domain as a screenshot from NASA’s globe software World Wind using a public domain layer, such as Blue Marble, MODIS, Landsat, SRTM, USGS or GLOBE. Lake Assal photos from Wikimedia Commons, released into the public domain by its author, I, Fishercd. A good summary of the geology of the region, but in French, can be found at http://www.jpb-imagine.com/djibgeol/asal/somasal.html; a description of the commercial salt industry, together with a geological description, can be found here, and is the source of the geological map above.]

Comments

[suvrat]

coincidentally, i saw a Discovery channel program on the Great Rift yesterday. It had a segment on the old caravan salt trade around Lake Assal.. what a dramatic landscape!

[Richard Bready]

"Tectonic Mayhem" would be a good name for a really loud band.

[Sandglass]

I like the idea, but the band would have to have real and diverse musical talent....

Any nominations for the "Tectonic Mayhem" symphony? Mahler? Perhaps we need to think Shostakovich?

Hendrix?

I feel a blog carnival coming on....

[Richard Bready]

Perhaps Magnus Lindberg? Not just for the cover image on http://www.amazon.com/gp/product/B0013AUW2M/ref=dm_sp_alb though that's apt. If you have a UK IP address, you can hear on Spotify that "Cantigas" has passages aiming for the Richter scale. And "Arena," also forceful, is appropriately named, with cover art resembling either lightning or magma: http://www.amazon.com/Lindberg-M-Corrente-Finnish-Symphony/dp/B002K0WEWU/ref=sr_shvl_album_1?ie=UTF8&qid=1302228804&sr=301-1

[Sandglass]

Excellent nomination. I, of course, was particularly impressed by the sample from "Arena" available on Amazon.com.

Out of curiosity, I put in "tectonic" as a search term for music on Amazon, and came up with a 2005 album titled "Tectonics" by a French one-man Industrial Doom band (a new term to me)named P.H.O.B.O.S. A free sample of, for example, "Engilfed in Subduction" can be heard at http://www.amazon.com/P.H.O.B.O.S./e/B001LHSDW8/ref=ntt_mus_dp_pel - BUT TURN DOWN THE VOLUME FIRST.... This is a really loud one-man band!

[Richard Bready]

MERCY that's loud! And it does sound like mayhem. Specialist reviewers agree: http://www.metal-archives.com/review.php?id=71435
It's far more seismic than "Tectonics" on http://www.amazon.com/Images-Earth-History-planet-through/dp/B00000G51B/ref=ntt_mus_ep_dpi_1 -- although that's an interesting concept.
Not in the same division, but an amusing cover: http://www.amazon.com/Sidewalk-Tectonics/dp/B004830ERE/ref=sr_1_1?ie=UTF8&qid=1302403239&sr=1-1-catcorr
Music does seem especially responsive to geological/palaeontological imagery. I have just discovered Birdsongs of the Mesozoic, described as "classical-punk-jazz-car-wreck music."