Through The Sandglass

Tsunamis and earthquakes, wisdom ancient, modern, and dubious

 
"Always be prepared for unexpected tsunamis. Choose life over your possessions and valuables."

So reads an inscription on one of the ancient tsunami stones that dot the hillsides of Eastern Japan. Others bear messages such as “Beware the calamities of the great tsunamis,” and "If an earthquake comes, beware of tsunamis"; the two meter stone on which the latter wisdom was written was washed away by the tsunami of the 11th March. These stones can be hundreds of years old and can be found – worth taking note of this, you would think – high on hillsides and far inland from the coast.

It’s been a while since I fulminated about ignoring tsunami records, citing the historical and geological evidence that unambiguously showed that an event on the scale of the March catastrophe fell into what should be the “expected” category; more evidence has emerged in the meantime, more informed voices of reason have spoken out, but, alas, the idiocy continues. If we can’t take note of warnings from just a few generations ago, never mind science, what hope is there? Just what exactly is our attention span and how do we set our priorities? Well, the answer to the first question would seem to be about three generations. Andy Revkin, of the New York Times, recently wrote a couple of pieces that are well-worth a read and a follow-up of the links -  Limits to ‘Disaster Memory,’ Even Etched in Stone and ‘Disaster Memory’ and the Flooding of Fukushima. Fumihiko Imamura, a professor in disaster planning at Tohoku University in Sendai, a tsunami-hit city, observes that "It takes about three generations for people to forget. Those that experience the disaster themselves pass it to their children and their grandchildren, but then the memory fades." An obvious question, it would seem, at least to me, would be the extent to which dimensions of today’s society (politics, technology, the media, and greed spring to mind) perhaps accelerate the loss of “disaster memory.”

The rhetoric of the tsunami stones is straightforward, empirical, and pragmatic. But sadly these terms can hardly be applied to the rhetoric of today. In spite of the ancient advice to “always be prepared for unexpected tsunamis,” the nuclear authorities and operators in Japan continue to express their surprise. And, while I am an avid reader of the New Scientist, I know that I am not alone in feeling that, in recent years, its content has had to be increasingly scrutinised for statements that, shall we say, depart from scientific and objective rigour. Some of this originates from gently grinding editorial axes, some from quoting sources without checking the accuracy. For an unfortunate (and surprising) example of the latter, here’s a quote from a March article titled Seismic zones shake out nuclear problems, in which John Stevenson of the International Seismic Safety Centre of the International Atomic Energy Authority is quoted as describing how “nuclear energy authorities will want to re-examine the potential for tsunami risks to nuclear power plants,” and continues:

While power companies already estimate the risk from tsunamis based on the geological record, Stevenson admits that “there is always the chance that what has been experienced in the past will be exceeded.”

I simply don’t know where to start on this – the entire thing is erroneous and absurd from start to finish. Perhaps I should start by re-reading my earlier posts. But perhaps it’s also worth pointing out that Stevenson had clearly never heard of (or at least paid no attention to), Masanobu Shishikura. Dr. Shishikura, a geologist, works at the Active Fault and Earthquake Research Center in Tsukuba, a government-funded institute; a Wall Street Journal article earlier this month had the following:

The Man Who Predicted the Tsunami

After studying ancient rocks, a Japanese geologist warned that a disaster was imminent—to no avail

By PETER LANDERS

The giant tsunami that assaulted northern Japan's coast surprised just about everyone. But Masanobu Shishikura was expecting it. The thought that came to mind, he says, was "yappari," a Japanese word meaning roughly, "Sure enough, it happened."

Yes, he was one of the geologists analysing tsunami sands, in this case those from both historical events and those from thousands of years ago; I referred to some of this work in my first post on this subject. He and his colleagues had published their results and the work was brought to the attention of the authorities; Dr. Shishikura had an appointment on March 23 to explain his research to officials in Fukushima. But, a couple of weeks before, yappari. 

The failure of “disaster memory” and the consequences of ignoring the science apply to the earthquake itself, as well as the tsunami. In Nature, Robert J. Geller of the Department of Earth and Planetary Science, the Graduate School of Science at the University of Tokyo, wrote a fascinating piece a couple of weeks ago titled Shake-up time for Japanese seismology; here’s the introduction:

Robert J. Geller calls on Japan to stop using flawed methods for long-term forecasts and to scrap its system for trying to predict the 'Tokai earthquake'.

Globally, in the past 100 years, there have been five subduction-zone earthquakes of magnitude 9 or greater (Kamchatka 1952, Chile 1960, Alaska 1964, Sumatra 2004, Tohoku 2011), which suggests that the upper limit on the possible size of a subduction-zone earthquake may not much depend on the details of the subduction modality. Large tsunamis have frequently struck the Pacific coast of the Tohoku district. The well-documented 1896 Sanriku tsunami had a maximum height of 38 metres and caused more than 22,000 deaths. The 869 Jogan tsunami is documented to have had a height roughly comparable to, or perhaps slightly less than, that of the 11 March tsunami.

If global seismicity and the historical record in Tohoku had been used as the basis for estimating seismic hazards, the 11 March Tohoku earthquake could easily have been 'foreseen' in a general way, although not of course its particular time, epicentre or magnitude. Countermeasures for dealing with such events could and should have been incorporated in the initial design of the Fukushima nuclear power plants.

Geller describes the bizarre and idiosyncratic way in which a generic but named earthquake (“Tokai”) was essentially the basis for all disaster planning and legislation, effectively dislocating the process from reality and the scientific evidence.

This is rational and evidence-based – rather like the declarations of the tsunami stones. But when it comes to describing the seismic event, we seem to have abandoned all temperance. Which brings me again, unfortunately, to the New Scientist. How do you like this for a headline in a serious scientific publication?

“Starburst megaquake: Japan quake overturns geology.”

The article begins as follows:

THE Tohoku earthquake that rocked Japan last month has sent geologists reeling. As the first analyses of what may well become the best studied earthquake in history start to filter through, there is already talk of rewriting the rule book on how "megathrust" quakes happen.

Yes, the event was, literally, extraordinary (but not unprecedented) – in magnitude, in the nature and complexity of the rupture, in the frequency and location of aftershocks; sure, the sheer volume of data collected in association with the earthquake and its aftershocks is unprecedented, and major revisions to our understanding of such events will inevitably result, along with the abandonment of some cherished hypotheses and models.  But “ sent geologists reeling,” floundering about in their overturned science by a “starburst megaquake”? Please, give us all a break.

And then there is the excitement – and newsworthiness in the “be afraid, be very afraid” category – of the idea that large magnitude earthquakes are connected, that the Japanese event was an aftershock of Sumatra, and so on and so on. Contributing to the smoke and dust around this idea was Simon Winchester, whose cataclysmic declarations have been thoroughly and thoughtfully dismantled by (amongst others) Chris Rowan in “How to (and how not to) talk about earthquake hazards in the media.” Our friends at the New Scientist declared that “discussions over the clustering of megaquakes will rumble on” after describing how workers at the USGS and the University of Texas had reviewed a 30-year catalogue of events and found “no significant evidence  that large quakes regularly trigger tectonic activity 1000 kilometres or more away.” And, as Ross Stein of the USGS was quoted as saying (in the same article), “You will get a lot of different answers from different people, but inevitably the ability of any one of those to convince everyone else that they’re right is going to depend on the statistics of very small numbers, and we’re never going to get anywhere.”

I was struck by many of the things Stein has to say, but I like this in particular (for obvious reasons), further discussing the idea of triggered events:

If you have a quake of, let’s say 6.2 or larger, every sand grain on the planet is moving to the music of that event 

I think that I’ll end this rant with that poetic side of science.

Sunday sand: Easter ooids

The Greek word for egg is òoion. Hence something that is egg-shaped – such as an egg, or an egg-shaped sand grain – can be described as an ooid; pronounce it as oh-oid. All (except one) of the ooids above are sand grains from the Bahamas where sweeping and undulating tapestries of sand banks are composed of such grains:

 
Being somewhat lazy (and slightly busy), I’ll simply quote the good description of ooids from Wikipedia:

Ooids are small (< 2 mm in diameter), spheroidal, "coated" (layered) sedimentary grains, usually composed of calcium carbonate, but sometimes made up of iron- or phosphate-based minerals. Ooids usually form on the sea floor, most commonly in shallow tropical seas (around the Bahamas, for example, or in the Persian Gulf). After being buried under additional sediment, these ooid grains can be cemented together to form a sedimentary rock called an oolite. Oolites usually consist of calcium carbonate meaning they belong to the limestone rock family. Pisoids are similar to ooids, but are larger than 2 mm in diameter, often considerably larger, as with the pisoids in the hot springs at Carlsbad (Karlovy Vary) in the Czech Republic.

An ooid forms as a series of concentric layers around a nucleus. The layers contain crystals arranged radially, tangentially or randomly. The nucleus can be a shell fragment, quartz grain or any other small fragment. Most modern ooids are aragonite (a polymorph of calcium carbonate); some are composed of high-magnesium calcite, and some are bimineralic (layers of calcite and aragonite). Ancient ooids can be calcitic, either originally precipitated as calcite (as in calcite seas), or formed by alteration (neomorphic replacement) of aragonitic ooids (or the aragonite layers in originally bimineralic ooids). Moldic ooids (or molds later filled in by calcite cement) occur in both young and ancient rocks, indicating the removal of a soluble polymorph (usually aragonite).

The article includes some fine images that include ancient examples of ooids in an oolite from the balmy tropical and carbonate-rich seas that covered Utah around 170 million years ago. A close examination of the surface of the rock reveals that it is made up almost entirely of little sand-sized balls, and make a slice of this rock thin enough for light to pass through and scrutinise it down a microscope and the exquisite structure of these grains is revealed:

The Wikipedia article goes on to describe what we know of how these structures grow:

Ooids with radial crystals (such as the aragonitic ooids in the Great Salt Lake, Utah, USA) grow by ions extending the lattices of the radial crystals. The mode of growth of ooids with tangential (usually minute needle-like) crystals is less clear. They may be accumulated in a "snowball" fashion from tiny crystals in the sediment or water, or they may crystallize in place on the ooid surface. A hypothesis of growth by accretion (like a snowball) from the polymineralic sediment of fine aragonite, high-magnesium calcite (HMC) and low-magnesium calcite (LMC), must explain how only aragonite needles are added to the ooid cortex. Both in tangential and in radial ooids, the cortex is composed of many very fine increments of growth. Some modern (and ancient) ooids partially or totally lack clear layering and have a micritic (very fine grained) texture. Examination of such micritic ooids by scanning electron microscopy often shows evidence of microbial borings later filled by fine cement.

But enough of aragonitic ooids and micritic textures. There’s a bit of romance in the word, a rolling-off-the-tongue satisfaction to it. Mark Twain certainly encountered and enjoyed the word, and wrote, in Life on the Mississippi:

In the space of one hundred and seventy-six years the Lower Mississippi has shortened itself two hundred and forty-two miles. That is an average of a trifle over one mile and a third per year. Therefore, any calm person, who is not blind or idiotic, can see that in the Old Oolitic Silurian Period, just a million years ago next November, the Lower Mississippi River was upwards of one million three hundred thousand miles long, and stuck out over the Gulf of Mexico like a fishing-rod. And by the same token any person can see that seven hundred and forty-two years from now the Lower Mississippi will be only a mile and three-quarters long, and Cairo and New Orleans will have joined their streets together, and be plodding comfortably along under a single mayor and a mutual board of aldermen. There is something fascinating about science. One gets such wholesale returns of conjecture out of such a trifling investment of fact.

Now, as was often the case with his geological facts, Twain got things slightly wrong (the age of the “Old Oolitic Silurian Period” is around 400 million years) but no matter – ooids have taken us from the Bahamas to the nature of science via Utah – happy Easter!

 

[Bahamas image thanks to NASA; images of the Carmel oolite and Bahamas ooids from Mark A. Wilson, College of Wooster, via Wikimedia Commons]

A sand sculpture memorial

Created at Cape Canaveral a few days ago by Florida artist Todd Brittingham, this sand sculpture celebrates Earth Day, but also marks the end of the space shuttle program through the fourteen stars, each dedicated to one of the astronauts who lost their lives on the Challenger and Columbia.

The volunteers who help create the sculpture have arranged themselves as more stars around the work.

 

[Photo by Craig Rubadoux, FLORIDA TODAY]

Sunday sand: the green, green grains of Wissant

 
The mineral glauconite, so named from the Greek glaucos, meaning, oddly enough, blue-green. Glauconite is an iron-potassium member of the phyllosilicate group of minerals, the micas, whose crystalline form appears as thin sheets  - if you insist, glauconite’s empirical formula is K_0.6Na_0.05Fe³⁺_1.3Mg_0.4Fe²⁺_0.2Al_0.3Si_3.8O₁₀(OH)₂

In a previous Sunday Sand post, I described the green grains of Groningen, and the mineral responsible for the colour – and here it is again, even greener, even bluer, this time from the coast of France. Glauconite takes on a variety of mineralogical guises, but these are typical sand-sized pellets or rounded aggregates, rolled around on the floor of an ancient tropical sea. It’s glauconite that is the pigment for many of the green sands around the world (and for some of the hues of Russian icons). Around Northern Europe, beneath the great chalk cliffs of Dover, Calais and Denmark, lies a pile of sand and clay layers, many of which are glauconite green and that tell stories of the seas along whose shores the dinosaurs frolicked.

It’s from one of these clay layers that today’s grains have come, weathered out along the coast of the Pas de Calais region, not far from the town of Wissant:

 
This clay, well-known in the UK as The Gault, formed in moderately deep seas around 100 million years ago; it’s a “stiff clay” that has often been used for brick-making; however, its major role has been in revealing the geological story of that era, for it contains a treasure trove of fossils – everything from minute foraminifera to ammonites, crustaceans, reptile bones, and sharks’ teeth. Here’s a spectacular Gault ammonite from the Fossils of the Gault Clay site, a comprehensive review of these spectacular collections:

 

The importance of the geology along the stretch of coast around Wissant was first recognised in the first half of the 19th century by the great and gloriously named French geologist,  Alcide Charles Victor Marie Dessalines d'Orbigny. D’Orbigny was a gentleman naturalist, typical of those times, who made significant contributions not only to geology and palaeontology, but also to archaeology and zoology. It was d’Orbigny who first recognised and named the wonderfully diverse, microscopic critters, the foraminifera, who have featured in a number of Sunday Sand posts. And it was at the Pas de Calais coast that d’Orbigny was the first to decipher the clays and green sands and see that they represented a key paragraph in the chapter of the earth’s history that is the Cretaceous Period. The name “Cretaceous” derives from the Latin word for “chalky” – remember the white cliffs of Dover or the vineyards of the hills around Austin, Texas or Champagne. The Cretaceous period began around 145 million years ago, and lasted 80 million years, culminating with the climax of the extinction of all those frolicking dinosaurs. The glauconitic sands and clays of Wissant mark a key stage within the Cretaceous, a time named by d’Orbigny in 1842 as the Albian, after Alba, the River Aube in France where further key exposures of these rocks occur. The Albian has, so to speak, withstood the test of time, and is today a paragraph of our planet’s life recognised internationally as marking the end of the Early or Lower Cretaceous:

Time would move on from the Albian, the seas would become shallower, and the great thicknesses of chalk would be deposited over so much of the earth during the Cenomanian. It was the green sands and the glauconitic clays that underpinned this monumental pile – and also underpinned the Channel Tunnel, as its engineers will testify.

 

[Photograph of the Wissant coast from http://www.geog.sussex.ac.uk/BAR/images/Pictures-France%202003/Blanc_Nez/Page.html. I'm grateful, as always, to Carla Lagendijk for providing the sand grains.]

 

Mining the sands of Anak Krakatau

“Anak” in Indonesian means child, and Anak Krakatau is the child of the old volcano that explosively self-destructed in 1883. The kid was born in 1927 and has indulged in bouts of adolescent rage ever since. When I was living in Indonesia many years ago, I had the opportunity to visit the island on two separate occasions, both of them quite exciting.

On one trip, tossed about in a leaking old tub by the swells of the Sunda Strait, and hanging on to my young daughter for dear life to stop her sliding overboard, we had the delightful experience of being “rained on” by sand. The wind had carried the cloud of drifting ash from the kid’s latest outburst to where it simply fell on us with a gentle pattering sound. I still have a small bottle of the product, and there it is in the image at the head of this post – newly born volcanic sand.

But this is not just any old sand – it’s rich in iron and titanium and therefore of some considerable interest to illegal sand miners – and yes, sand-smuggling is an occupation pursued the world over. During the last two years, the extraction of volcanic sand from the shores of Anak Krakatau has gathered pace. We’re not talking about a bunch of guys with shovels, but huge barges, dredges, and pumps, removing hundreds of thousands of tons of sand. For some time now, arguments over making this legal and issuing the required permits – for mining in a designated conservation area – have rumbled on in, I have to say, a very Indonesian way. In February, the Jakarta Post reported as follows:

Anak Krakatau’s natural exploitation opposed

Oyos Saroso H.N., The Jakarta Post

The South Lampung administration’s plan for sand mining around Mount Anak Krakatau has been met with opposition as green activists have said the activities threaten the environment.
Opponents of the plan also claim that the alleged mask of disaster mitigation is not justifiable.
Regent Rickyo Menoza was reported to have made the proposal for the permit with the Forestry Ministry, Lampung Natural Resources Conservation Body (BKSDA) and the Volcanic, Geological Disaster Mitigation Center.
Environmental watchdog Walhi feared the administration’s plan to dredge the sand would threaten the environment. The project, however, is expected to augment the administration’s income.
“The on site disaster mitigation efforts are only tricks to obtain permits that will allow entry to the Anak Krakatau area to dredge up the sand. On the other hand, the volcano and its surroundings are a designated conservation area that has to be protected,” Walhi Lampung chapter chairman Hendrawan said.
Sutono, a government official, denied the accusation.
“We are conducting these activities as part of a far-reaching program. We will build a drainage system to allow for lava flow so that the impact would not be severe in the case of an eruption,” Sutono said.
Hendrawan said he would file his complaint with police headquarters and the Corruption Eradication Commission (KPK) if the permit was issued and the sand mining operations were carried out.

Hmmm - “disaster mitigation efforts” – I’m not really convinced by that either. I’ll keep you posted.

My other visit to Anak Krakatau was one of those enormously enjoyable trips that could well have been my last. The volcano was in a particularly explosive mood – no lava, but ejection on a grand scale. If you look closely at the image above, you can see very large lumps flying through the air and cascading down the flanks of the volcano. Similarly in this photo below (taken, I hasten to add, with a good telephoto lens).

 
 
I recently discovered that, a while ago and completely forgotten, I had written up a brief account of this encounter with the bratty kid:

The critics liked it: a controlled performance, but one of passion and spontaneity. The performer was Anak Krakatau the small child of the monumental volcanic parent, Krakatau. We were correctly, west of Java, in a flotilla of three small Indonesian boats, out on a day trip to see the performance, swim, eat, drink a few beers and enjoy a rare opportunity for socialising between the Indonesians and expatriate Brits from the office.

Our pleasure craft hardly fell into the category of luxury yachts – indeed they were of quintessential local character, weather-beaten, patched, in need of a makeover, their mechanical systems hardly throbbing with vitality. But they seemed to work, their crews were cheerful, the sun was out, the sea was blue, and the volcano beckoned. Anak Krakatau has emerged from the sea in the midst of the wreckage of its parent’s explosive end, a substantial and very active volcanic island surrounded by a small jungle-covered archipelago which is all that remains after Krakatau finally overdid its incandescent rage. Entering the outer ring of islands we could see in detail what had been apparent for the entire trip from the mainland – Anak Krakatau was erupting regularly and spectacularly, but, by mainstream volcanic standards, modestly. Roughly every twenty minutes a great cloud of ash and small rocks welled into the sky from the crater, the rocks tumbling down the side in plumes of dust and ash, the ash cloud driven by the wind and raining volcanic dust in its path. We had had an excellent opportunity to build a simple and informal statistical database as we approached, timing the episodes, observing where the ash fell and comparing the size of each event. Rigorous analysis of these data supported what we had determined before our departure – the eruptions were small and regular and circumnavigating the island (at a circumspect distance) was a perfectly reasonable thing to do. So we did it.

Staying a couple of hundred metres out from the shore gave a spectacular view, along with the sound effects, of a couple of eruptions, and allowed collection of the fine volcanic grit as it drifted out of the sky onto the boats (and us). This was, you will understand, an intensely romantic experience for a geologist. Steam rose from the sea where newly-minted hot rocks continued to build the island. Unfortunately steam also rose from the engine of one of the boats. The other two gathered around and, while we watched the display, water was passed over to the invalid. It took about twenty minutes for the fever of inadequately-cooled internal combustion to be assuaged, and the sounds of coughing and decaying plumbing announced that we could get under way. It was approaching lunchtime and the decision was made to head off to anchor for sustenance and recreation next to one of the parental vestiges a kilometre or so away from where we had been stopped.

The sound of opening beer cans was overwhelmed by a particularly violent volcanic expostulation which accompanied, when we turned to look back, an equally, and unusually, violent eruption. We raised our beers to nature red in tooth and claw and were making the appropriate ooing and aarhing sounds when we began to realise just how big this one was. The cloud of georubbish was being hurled much higher than before, and the rocks were not landing harmlessly on the slopes of the volcano but were plummeting into the sea.

Including one, approximately the size of a bus, which created a huge plume of water as it landed at the same spot that all three boats had occupied twenty minutes before.

Recently, Anak Krakatau has grown into a more fluid phase of adolescence, erupting copious amounts of lava (photo Tom Pfeiffer, at Explore Volcanoes Now). I shall perhaps have to bring some prudence and discretion to planning my next visit.

Sunday sands - heading East

 
So, I’m off for another adventure. Wednesday, I head East for a sojourn in Jakarta that I fully intend will include forays to far-flung parts of the huge archipelago. The estimate by the government is that the nation of 240 million people consists of 17,508 islands of which perhaps half have been named. These islands are spread over an east-west distance of 5,400 kilometres – 3,400 miles – which, combined with the dramatic, dynamic, and diverse geology, produces a vast spectrum of landscapes – and sands. This spectrum is beautifully captured by the selection above, one of the images on Loes Modderman’s website.

Reefs, volcanoes, rainforests, and fascinating cultures – I shall report….

Sand and ice in Queens

 
Late last year, I reported on the strange interactions of sand and ice on the shores of Lake Superior and 500-million-year-old beaches. Recently, I heard from my friend, Larry Deemer, beach-side resident of New York and meticulous observer of his environment’s shifting moods and forms, describing phenomena that are reminiscent of these bizarre structures. I included some of Larry’s spectacular photographs of sand patterns early in the history of this blog, and his report of his winter beach ramblings was accompanied by further examples. As he wrote:

Last month I discovered an unusual sand formation after the snow and ice on the beach melted. What remained was a very thin crust of sand that was suspended over more hard pack sand underneath. The formations were large, almost as if they were advancing bulkheads, like a glacier.

It seems to me that these slabs of encrusted sand reflect freezing and winnowing processes similar to those occurring on the beaches of the Great Lakes – perhaps with the behaviour of salty water as an additional component in the equation. Larry also enclosed photos of similar “fragile, self-supported designs” but “on a smaller and flatter scale” from another morning following a freezing night:

 
Thanks, as always, Larry, for your intriguing reports and great photos.

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.]