“It would take more Planck lengths to span a grain of sand than it would take grains of sand to span the observable universe.”
Geologists tend to pride themselves on their grasp of scale, and, of course, I enjoy the endless roles of sand grains as measures of scale large and small, but I have to admit that I have great difficulty getting my head around this. It appeared in an article by Seth Kadish on Wired, an article I found fascinating but challenging. The Planck length is “believed by physicists to be the shortest possible length in the universe. Beyond this point, they say, the very notion of distance becomes meaningless” – it is approximately 10-35 meters. The article, titled “We all might be living in an infinite hologram,” is intriguing, not least for the revelation of the existence of a device called the Holometer – something I might request for Christmas.
Here’s the piece, with thanks to Wired and Seth:
Quarks and leptons, the building blocks of matter, are staggeringly small. Even the largest quarks are only about an attometer (a billionth of a billionth of a meter) in diameter. But zoom in closer—a billion times more—past zeptometers and yoctometers, to where the units run out of names. Then keep going, a hundred million times smaller still, and you finally hit bottom: This is the Planck length, approximately 1.6 x 10-35 meters, believed by physicists to be the shortest possible length in the universe. Beyond this point, they say, the very notion of distance becomes meaningless.
How small are we talking? It would take more Planck lengths to span a grain of sand than it would take grains of sand to span the observable universe.
Still, the idea of a finite size limit may seem bizarre. After all, if you can define a distance, you can just cut that distance in half—ad infinitum, right? Not necessarily. One of the great discoveries of the 20th century was that at small scales, many physical properties, like angular momentum and energy, can take only certain discrete values, or “quanta.” That principle—supported by decades of experiments—is the foundation of quantum mechanics.
Which leads to a rather weighty question: If the properties of matter can be quantized, what about the fabric of spacetime itself? Is the universe a smooth continuum, as described by Einstein’s theory of relativity? Or if we looked really close, would it all dissolve into a mosaic of shimmering pixels like a computer screen? Is the reality we observe just a hologram made up of the tiniest of dots?
Probing down to the Planck scale with a particle accelerator would take an instrument the size of our galaxy. But scientists at Fermilab, near Chicago, have a surprisingly modest new device called the Holometer that just might yield some clues. Using a pair of solid state lasers and some precisely polished mirrors, they hope to pick up the telltale jitter of those hypothetical pixels—what’s called “holographic noise,” after the fuzziness of holograms. If they find it? Welcome to the Matrix.
[The title of this post is from Virgil's The Aeneid, the image, Shadow of the Dark Rift, courtesy of NASA]
Between sleeping and awakening. We tend to think of the world’s dune fields as fearsomely awake, threatening and encroaching, creating vast sand and dust storms – and this is indeed the case for great stretches of our planet’s sand seas, the ergs. But far larger dune terrains are asleep, stable and vegetated. Or they are at least dozing, waiting to be re-activated, and understanding exactly what are the circumstances under which previously dormant dunes can spring back to life is critical to the livelihoods of the people who live in arid lands. Furthermore, the implications for atmospheric dust load and climate change are profound.
The image above is from the Great Sandy desert of Western Australia, where single linear dunes stretch for tens of kilometres. Yes, as the outback winds blow, the sand moves, as we can see from the ripples on the dune, but the scrawny (though remarkably diverse) vegetation is sufficient to stabilise the dunes themselves. The extraordinary morphology of these dunes is shown in this Google Earth image, the dune in the photograph above being the central one in the satellite view.
But it is only a snapshot and things can change remarkably quickly. As climate changes, what are the mechanisms by which slumbering dunes can be stirred into wakefulness? It’s clearly more complex than simply changing winds and precipitation, but how can we answer this question and make any predictions about the future? I discussed this intriguing topic a little in The Desert book (whose publication is now, by the way, delayed until February – not through any fault of mine), so here is a taster from Chapter 6, Ancient and Modern, Boom and Bust:
The deserts of today contain long histories of aridity (probably more than 20 million years in the Atacama, more than 7 million years in the Sahara). Of particular interest is that those histories are not ones of sustained levels of aridity, but rather a history of fluctuation, of ebb and flow between semi- and hyper-arid as the climate changed. The movement of active dunes was prevented as vegetation grew, the winds died down or precipitation increased, even slightly. When the climate dried out, the dunes resumed their march and these episodes of activity and stabilization are recorded in the sands. These kinds of changes can even be seen in historical times: the largest dune field in the western hemisphere can be found in the prairies of Nebraska, appropriately, part of the ‘Great American Desert’. These dunes are vegetated and immobile today, but a thousand years ago they were on the move and they were periodically resuscitated by the droughts before and during the pursuit of Manifest Destiny, causing problems for the settlers’ wagon trains. Navajo oral histories provide first-hand evidence of changing vegetation and fluctuations in dune activity. Dunes cover around five per cent of the global land surface today, but most are stable. [The map above] shows the current distribution of active and ‘relict’, immobile, ergs, a snapshot in time. The map would have looked very different a few thousand years ago and will change significantly in the future.
In order to decipher the history of phases of erg activity, a means of determining the age of a particular layer of sand is required. For a long time this was simply not possible: the desert is far from an ideal environment in which to preserve the mineral and fossil materials that provide the geological clocks, and, even should carbonaceous remains be available, carbon dating only works for ages younger than 60,000 years. However, over the last couple of decades two remarkable clocks have been developed that have revolutionized our ability to decipher the stories of the deserts and transformed our understanding of how they work. Once again, isotopes are the key. Once a sand grain, typically quartz, is buried, it is bombarded from its surroundings by natural radiation from isotopes of potassium, thorium and uranium. The radiation strips away electrons from the mineral atoms but those electrons remain trapped in the crystal structure of the sand grain, a store of energy that can be released simply by shining light on to the grain. In doing so, the energy of the trapped electrons is released as light and the grain glows, it luminesces. The longer the grain has remained buried, away from sunlight, the more electrons are trapped and the more energy will be released. Since isotopes are predictable, we know the rate at which electrons are produced and so we have a clock. Optically stimulated luminescence dating (OSL) gives us a measure of how long our sand grain has been in the dark, for when it re-emerges into the sunlight the clock is re-set.
However, once the grain is back at the surface, it is bombarded by extraterrestrial cosmic rays which themselves re-organize some atoms into new isotopes, and the longer the grain remains at the surface, the more isotopes accumulate: now we have a clock that measures how long a grain has been exposed on the surface. As with all such measurements, data from one sand grain is far from sufficient, and huge numbers of analyses are required to come to a statistically acceptable conclusion. And nature creates all kinds of complications in the story of an individual sand grain that can cause ambiguities and complications in interpreting the data, but these methods have given us a means of quantifying episodes of desert activity and have provided extraordinary insights into the dynamics of arid lands.
For example, detailed studies of dunes from the Kalahari and the Namib Deserts have produced chronologies of periods of movement and stability over the last 80,000 years or so. There are some significant correlations of events between different types of dunes from widely different locations, in general correlating with the global history of pulses of glaciation. However, the data from different areas can show significant conflicts, and correlation of the timings of dune movements with proxies for temperature, wind and oceanic upwelling from the continuous archives of neighbouring ocean sediments raises some interesting and challenging questions. It seems that different parts of the same sand sea can be active at different times, and that dune movement is more a measure of changing wind strength than a reliable indicator of aridity: ironically, the same burial ages coming from large numbers of sand grains must reflect accumulation and therefore a reduction in the strength of the winds.
It’s a complex environment, the desert, but while we are only beginning to understand it, other forms of life have known it well for close to 500 million years…
The processes whereby dune fields can become hypnopompic (or, indeed, hypnagogic) remain elusive, but a great deal of work has been done in the sands of the Kalahari. The crucial role of vegetation – and the influence of the activities of homo sapiens – has recently been reported by a group of researchers from the universities of Virginia, California, and New Mexico: Sleeping sands of the Kalahari awaken after more than 10,000 years. Most of the Kalahari dunes had been stabilised by vegetation that survived in balance with the methods of the resident pastoralists. Until water wells were drilled. The dramatically increased water supply resulted in a complete change in the agricultural intensity, a growth of livestock numbers far in excess of the carrying capacity of the land, and, consequently, a severe loss of vegetation, crucially the stabilising grasses. The dunes began to stir.
It's unclear, say [the paper’s authors], whether the Kalahari's dunes hang on the edge of a tipping point between their current state--"vegetated fixed linear dunes"--or have moved to what researchers call a degraded state, "barren and active dunes."
Yes, it’s complex, and the work examines recovery of grasses after grazing has been halted. However, exactly what stimulates an “irreversible shift to a stable degraded state” – permanently awake and active dunes – has yet to be understood. The rate at which change from stable to active can take place is clearly revealed by spending a little time on Google Earth and its wondrous historical imagery tool. Here are three images from one the sites in Botswana that the project describes – incredible, visible fluctuations over a period of less than ten years.
The full report can be read at the Ecological Society of America’s Ecosphere journal website.
And I feel that I should warn readers that “hypnopompic dunes” is not standard terminology, but rather a flight of my sometimes-overactive imagination. But then again, it does have a certain ring to it…
[Map of active and relict ergs from: Peter G. Fookes and E. Mark Lee, ‘Desert environments: landscapes and stratigraphy’, Geology Today, 25/5 (2009).]
Via Geoff Manaugh at BLDBLOG and the US Military, a spectacular digital/analog sandbox!
Yes, it's developed for war games and military planning and operations, but wow, does it look like fun. The digital dimensions are powered by Microsoft's X-box Kinect sensor, it can be networked, and any kind of digital landscape imagery can be projected onto the sand. As Manaugh comments, it "would seem to have some pretty awesome uses in an architecture or landscape design studio" and just think of the applications (and the fun) in education at all levels. It's interesting that comments on the BLDBLOG post point out that the sandbox and the associated technology was originally developed by Oliver Kreylos at UC Davis. Herewith, a photo from the website of the Augmented Reality Sandbox at the W.M. Keck Center for Active Visualization in the Earth Sciences:
There's an inspiring video of Oliver Kreylos explaining and demonstrating the sandbox on YouTube.
Now, go to Military.com for another compelling (but inevitably warfare-focussed) video of this thing in action. It's top of my list for Santa...
"Since October 1998, the American Geosciences Institute has organized this national and international event to help the public gain a better understanding and appreciation for the Earth Sciences and to encourage stewardship of the Earth. This year's Earth Science Week will be held from October 12-18 and will celebrate the theme Earth's Connected Systems."
I have celebrated this annual event in previous posts (although I seem to have overlooked last year) with a specific focus on the admirable Earth Science Literacy Initiative and their "Big Ideas" summary. This captures the fundamentals of our understanding of how our planet works and our relationship with it - the basics of our engagement with geology.
In addition to what we know, what keeps science alive and fascinating are all the things that we don't. This year, for readers who have not come across them already, I would like to draw attention to the superb series of recent posts on GeoLog, the official blog of the European Earth Sciences Union. Titled "The known unknowns - the outstanding 49 questions in Earth sciences", these summarise the basic questions that continue to vex our profession and stir controversy and debate, together with valuable links to appropriate resources. The link is to the third in the series, and I have so far counted 25 questions, so there is clearly more intrigue to come. As the introduction to the series states:
Science is about asking questions, as much as it is about finding answers. Most of the time spent by scientists doing research is used to constrain and clarify what exactly is unknown – what does not yet form part of the consensus among the scientific community. Researchers all over the globe are working tirelessly to answer the unresolved questions about the inner workings of our planet, but inevitably new answers only lead to new questions. What are the main questions that will keep Earth scientists busy for many years to come?
What I would like to think is that these kinds of initiatives will provide accessible and compelling materials that will stimulate young folk to become geologists and not-so-young folk to enquire further.
Meanwhile, a note about my absence for the last few weeks - I have been traveling. The image at the head of this post might provide a clue as to where, as might the photo below. More will, inevitably, be revealed in the near future.
We are all familiar with the endless (and expensive) foolishness of ‘beach nourishment’, of the idiocy of Dubai’s artificial sand islands and the general ravages of ‘sand wars’ around the world, but now it seems that sand-shifting is taking on an ominous geopolitical role.
“There was this huge Chinese ship sucking sand and rocks from one end of the ocean and blasting it to the other side using a tube.” This is the description by Pasi Abdulpata, a fisherman from the Philippines, of what he saw happening in the South China Sea towards the end of last year. He was quoted in an article in Bloombergearlier this year (which I missed), and the issue has now been picked up by the BBC. Abdulpata was fishing around the Spratly Islands, a sprawling archipelago of only-just-islands and only-just-submerged reefs.
The Spratly Islands would not figure on the global geopolitical agenda if it were not for the fact that they occupy a significant area of the South China Sea, all of which is claimed by China, and some of which may harbour reserves of oil and gas. There are, however, a number of countries – five to be exact – who have different ideas about who owns what and where the international boundaries should lie. This map, from another Bloomberg article, summarizes the ‘anxious archipelago’:
The sand-sucking was going on at the Johnson South Reef, occupied by China but claimed by Vietnam, and the site of the 1988 ‘Johnson South Reef skirmish’ in which more than 70 Vietnamese died, two Vietnamese boats were sunk and the Chinese took over. They built some kind of structure on the reef, purportedly for erosion protection, and it has been the strategic destination of fishing and patrol vessels ever since. But the sand-sucking dredging and pumping activities are something new, and have been dramatically documented by satellite imagery and the Philippines Government, as described in Jane’s earlier this year:
China is attempting to bolster its presence in the South China Sea by creating an artificial island on a reef in the disputed Spratly Islands.
Satellite imagery provided by Airbus Defence and Space corroborates images released by the Philippine Ministry of Foreign Affairs that shows major land reclamation on Johnson South Reef, which is claimed by Manila as Mabini Reef, as Chigua Reef by China and Gac Ma by Vietnam.
Johnson South Reef was at the centre of a 1998 confrontation between China and Vietnam that left more than 70 Vietnamese personnel dead. After taking control of the reef China built a concrete platform and installed radio and communications equipment.
The images released by the Philippine Ministry of Foreign Affairs show that since February 2013 there has been extensive dredging of the atoll to create an islet around the platform. Other concrete structures have also been constructed.
The ministry said the construction appeared to be designed to support an airstrip and said it was "destabilising and in violation of the Declaration on the Conduct of Parties in the South China Sea (DoC) and international law. Mabini Reef is part of the Kalayaan Island Group (KIG) which is part of Philippine territory".
The article contains this series of images which show the large-scale ‘nourishment’ of the islands:
As The New York Times has reported, in May US Defense Secretary Chuck Hagel scolded China for “land reclamation activities at multiple locations” in the South China Sea. Is this the beginning of a further – and dangerous – episode of ‘sand wars’?
[Image at the head of this post: Philippine Department of Foreign Affairs, via Associated Press, as included in The New York Times article. The BBC also has an 'immersive story' which is well-worth a look.]
These images are from the November 1968 edition of the now sadly defunct Desert Magazine, a special on Death Valley. The “Riddle of the Racetrack” refers to the enduring mystery of the “sailing stones” of Racetrack Playa, the remote dry lake in the northern part of the valley, and begins:
OFF the beaten path in the northwest corner of Death Valley National Monument lies a hidden valley—and a mystery. The valley contains a dry lake approximately one-and-a-quarter miles wide and three miles long. The Racetrack Playa at first glance appears like any other of hundreds of such dry lakes in the southwest.
It has one different and mystifying feature; rocks and other objects on its surface have been known to shift, move and skate about! No one has actually seen any of these objects move but the tracks left from such movement are obvious.
There are many theories explaining the phenomena. Some say it has to do with the earth's magnetism, while others claim it is related to the sunspots. Still others suspect the gravitational pull of the moon producing an effect similar to the ocean's tides. Under scientific examination, however, most of these theories can be dismissed.
Sunspots and magnetism are only a couple of the conjectures for the motivation of the sailing stones - the supernatural, aliens playing chequers (sorry, ‘alien tractor beams’) and teenage pranksters have also featured. But, while the debate has continued ever since the mystery was first observed a century ago, the preferred explanations are entirely natural. However, that debate is now – largely – settled, thanks to the work of a team led by Scripps Institution of Oceanography/UC San Diego palaeobiologist Richard Norris in collaboration with Ralph Lorenz, a planetary scientist at the Johns Hopkins University Applied Physics Laboratory.
In a video accompanying the description of the work, Norris relates how they conducted what one of his colleagues described as “probably the most boring experiment ever.” Now, this is surely open to discussion. Experiments set up in the 1930s and 40s to record the fall of a drop of pitch (average wait eight years, and winner of the Ig Nobel Prize for physics in 2005) must vie for the honour. But it’s certainly true that the Racetrack Playa project required patience and modern technology. The Desert Magazine article commented that
It appears that the mystery will remain unsolved until some hardy soul camps at the playa's edge all winter, and waits with a movie camera for the action to begin. Any volunteers?
The Scripps team had the advantage of sophisticated weather stations, telemetry, time-lapse cameras, and GPS, so they could get things set up and leave, returning periodically to see what might – or might not – have happened. It took six years.
The winter season was clearly critical: decades of occasional observations recorded rocks being in different places from where they had been before the winter, all apparently having ploughed up the distinctive furrows in the mud of the lake bed, and some being truly huge.
The mysterious winter activity had long stimulated theories of the moving force being ice. This may be the same location of searing summer desert heat, but the playa lies at an elevation of over 1100 metres, and the winter nights are cold: any water that accumulates will freeze and, perhaps, the ice will shove the rocks around. The potential role of ice had been treated seriously over decades of research, including an important study published in the Bulletin of the Geological Society of America in 1955 (I have a PDF if any reader is interested). The author, George M. Stanley, meticulously measured the tracks of multiple stones (including an 11 kg example that had travelled 265 metres), together with the movement of ice. Although he did not directly witness any activity, his conclusions were as follows:
(1) Certain groups of stone trails on Racetrack Playa have identical signatures over areas nearly 500 feet across. They must have been formed by blown ice floes which held numerous small stones or scribers in generally fixed positions, and which rotated during travel.
(2) In several cases a single, moving ice floe released a small stone and picked it up again after a few feet of travel.
(3) Many tracks favor origin by ice floes blown across the playa by wind rather than by wind blowing the lone objects.
(4) Mathematical treatment applied to stone trails within a signature group demonstrated that the scribers moved in accordance with trigonometric relationships between separated loci in a rotating, planar body.
(5) There are ice ramparts and other signs of ice-floe action on Racetrack Playa, aside from the stone trails.
In other words, he saw ice as the motivating force, but most likely picking up the stones, blowing across the playa, and dumping them on melting There are tracks with no stone at the end, and this would seem a reasonable explanation, but Stanley noted that even lightweight objects such as the droppings of wild jackasses also created tracks and that such biodegradable materials might leave no trace. Stanley suggested that the high altitude of Racetrack Playa, and therefore the common occurrence of winter ice, explained why this phenomenon was so much more common there than on other, lower, desert playas. But he did note other, isolated examples, and a further fascinating piece of evidence in support of the power of ice on desert lakes:
On December 3, 1952, the central transcontinental telephone line east of Reno, Nevada, was suddenly disconnected for unknown causes… and a repair crew was sent out to find the difficulty. After following the line eastward from Reno the crew came to Toulon Lake, a part of Carson Sink covered with a year-round body of shallow water…
At the lake, partly ice-covered, the repair crew discovered that 3000 feet of line was missing; this included 20 poles loaded with 4 arms and 40 wires on 150-foot spans. Some poles had been set in caissons of corrugated metal 10 feet in diameter and 4 feet high, filled with boulders to protect the poles against waves on the lake.
Ice had formed for 3000 feet from the west shore and 2 or 3 miles along it, with open water to the east; the ice was 4 inches or more thick and floated on about 2 feet of water. Emergency repairs were made by dragging an insulated cable across the ice with a propeller-driven ice boat. The uprooted poles and remnants of wrecked caissons were found 300 feet south of the original position of the line. After two days the wind reversed direction, and the ice started to move north, tearing out the emergency repairs; it continued to move north under the influence of a 30-mile-an-hour wind a total distance of 700 feet and left the wrecked poles and caissons 400 feet north of the original line. Ice was pushed up several feet on the east shore.
This is really quite entertaining: another instance in which the severing of telephone cables revealed geological phenomena. It was the sequential cutting of the transatlantic submarine cables on the Grand Banks off Newfoundland in 1929 that led to our understanding of the processes and power of turbidity currents…
Investigations at Racetrack Playa continued, and, in the 1970s, Robert Sharp, then a geologist at the California Institute of Technology, and Dwight Carey, a geology student at the University of California, Los Angeles, labelled large numbers of rocks with, for some reason or another, women’s names and tracked their positions over the years. As the National Geographic reported:
Hortense (R) moved 820 feet (250 meters) in one winter. Karen (J), a 700-pound (320 kilograms) rock at the end of a 570-foot-long (174 meters) track, didn't move at all during their seven-year study and disappeared years later. Karen showed up again in 1996, when Paula Messina, a geologist at San José State University who had been mapping the paths of all the sliders on the Racetrack, found her far north of where Sharp had last seen her. "When I told him I had positively identified several of his original rocks, his reaction was a little like one would expect from a man who was just told I found his children."
But still, while plenty of witnesses had reported the before and after positions of many of the stones, nobody had actually caught them in the act – until now. Early this year, the Norris team’s patience was finally rewarded. As Lorenz reports in a New Scientist article this week:
On a Sunday night in January, a park ranger forwarded an email reporting that a tourist had seen the rocks moving. I dropped everything and went to Death Valley. There I met the Norris team, who had also seen movement. We drove up to the playa, which was partly flooded and frozen over, to retrieve our instruments…
We were standing on the cliffs, watching the morning sun melt the thin floating ice sheet, when it happened. A gust of wind, no more than 4 metres per second or so, picked up. And then we heard the crack, and saw the ice sheet slowly glide, bulldozing some rocks along and leaving others.
The action – sedate and sporadic, lasting a total of around 18 seconds – was captured in a series of time-lapse images (blue arrows, stationary rocks for reference, red arrow, the moving rock; darker areas are ice):
The team had developed small GPS devices which they embedded in a selection of rocks at the beginning of the experiment, and so the long-term data could be collected – “The largest observed rock movement involved 60 rocks on December 20, 2013 and some instrumented rocks moved up to 224 m between December 2013 and January 2014 in multiple move events.”
But one of the remarkable things about all this is that the rocks are moved by thin ‘windowpane’ ice, unlike the thick stuff that caused the destructive events on Lake Toulon. Nevertheless, some rocks achieved velocities of ten centimetres per second and travelled 60 metres. The results have been compiled into this helpful graphic in the New Scientist article:
The work is reported in Plos One, is open-access and freely available online – it makes for a fascinating read. Herewith, a captivating image from that paper, together with the description:
View from the ‘source hill’ on the south shore of Racetrack Playa. View is looking north on December 20, 2013 at 3:15 pm. Steady, light wind, 4–5 m/s has blown water to the northeast exposing newly formed rock trails. Lower image shows overlay of lines to emphasize the congruent shape of adjacent rock trails as well as the proximity of rock trails to rocks that did not move. Image has not been enhanced.
[Thanks to Hans Begrich and Walter Vogelsberg for giving me the ‘heads-up’ on this work. It has been widely reported elsewhere (linked in the above), including on other geo-blogs, but I thought that a little of the historical background would be of interest. Wikipedia also has a good overview of the ‘sailing stones’ mystery, and, for fascinating videos etc. of this new project, the Scripps site is well-worth visiting. Image of boulder and track, James Gordon, Attribution-NonCommercial 2.0 Generic (CC BY-NC 2.0), https://www.flickr.com/photos/james_gordon_losangeles/8440177460/in/photostream/]
In his classic and wonderful book Desert Solitaire, the original eco-warrior, Edward Abbey, described the landscapes of Utah’s Arches National Monument:
… here all is exposed and naked, dominated by the monolithic formations of sandstone which stand above the ground and extend for miles, sometimes level, sometimes tilted or warped by pressures from below, carved by erosion and weathering into an intricate maze of glens, grottoes, fissures, passageways, and deep narrow canyons.
At first look it all seems like a geologic chaos, but there is a method at work here, method of a fanatic order and perseverance…
But exactly what the method actually was that sculpted the arches and other bizarre examples of natural land art has long been a topic of debate. Native Americans saw their origins in the work of the Great Sky Father, early settlers thought they were prehistoric native carvings, and conventional wisdom has ascribed these landforms in a general way to the erosional work of wind, water and salt. Fine, but why arches? Clearly, anisotropy within the body of the sandstone must be part of the equation, perhaps related to fractures and differential stress distribution; an interesting paper along these lines was published in the Geological Society of America Bulletin twenty years ago, and the pre-publication version is freely available online.
Just last month, a fascinating piece of work titled “Sandstone landforms shaped by negative feedback between stress and erosion” was published in Nature Geoscience and described clearly in an article from The Smithsonian. Jiri Bruthans and his colleagues in Prague use a series of physical experiments (see the fascinating video on the Smithsonian site), mathematical modelling and ‘ground-truthing’ to demonstrate that the gravitational load on a pile of sandstone can cause a differential stress distribution within the rock and the consequent ‘locking’ of sand particles along trajectories that, as a result, are stronger and more resistant to weathering and erosion. Furthermore, once the looser grains are removed the remaining locked volumes become even stronger, hence the feedback component. By introducing imperfections in the sandstone (small cuts, fractures and so on) they can reproduce an impressive variety of exotic landforms – including arches:
(illustration from the Nature Geoscience paper)
This is intriguing in itself, but what I couldn’t help wondering is that surely there must be a connection between this work and the strange world of granular physics. This is an inexhaustible topic that first fascinated me as I was researching the Sand book and has had me in its grip ever since. Explaining the behaviours of the ‘simple’ material, sand – dry, wet, damp – poses challenges to cutting-edge physics and engineering research worldwide. Among these strange behaviours is jamming, the sudden locking up of otherwise free-flowing grains. The (apparently) simplest example is sand, or any other granular material, flowing through a funnel: periodically, unpredictably, and frustratingly the grains will lock and the flow stops. A critical part of the design of a sandglass is the size and shape of the aperture in relation to the sizes and shapes of the sand grains – continuous flow without jamming has to be guaranteed. And then, among the myriad challenges presented by the requirements of handling industrial granular materials, is the flow of grain from a silo through a hopper – something that turns out not to be at all simple.
Silos of grain occasionally, and without warning, explode, despite design specifications that theoretically far exceed the stresses of the contained material (a tragedy when, as has happened in Scotland, the grain should have been used in the production of single malt whisky). Research to explain such events has demonstrated that the distribution of stress within a pile of grains is far from uniform, that particularly high stress is carried along very specific trajectories – ‘force chains’ – within the pile and those forces can spontaneously structure themselves to act against the walls of the container: the silo collapses.
The ever-shifting and re-forming force chains that are typical of granular flow through a hopper have been cleverly modelled. The illustration at left is by Dennis C Rapaport of the Department of Physics at Bar-Ilan University in Tel Aviv and the animation can be seen on his website. And here from a YouTube video by George Lesica:
In both, the shape-shifting force chains can be clearly seen, and it is these that, if they happen to organise themselves properly, contribute to the material jamming. Junyao Tang and Bob Behringer of the physics department at Duke University have made a compelling video of this happening:
So, differential stresses, force chains and jamming in granular materials seem to me to be intimately related to the work published in Nature Geoscience, and the sculpting of bizarre landforms. For these phenomena are not restricted to flowing granular materials, but occur in sand piles – I find this illustration from the work of Radoslaw Michalowski at the University of Michigan intriguing:
While the term arching has been accepted in the geotechnical literature, the concept does not relate to a formation of a physical arch (as seen in karst formations), but rather a re-distribution of stress (or variation in the stress field), where stiffer components of the system attract more loads. The description is still elusive, and research toward prediction of arching is carried out. Arching in a model of a sand heap is illustrated in the figure, producing a stress ‘dip’ at the center of the base… While the appearance of the stress dip may be a curiosity problem, arching associated with it is a phenomenon of interest and importance in geotechnical engineering.
Ring any bells, this fanatic order?
[Image at the head of this post "Double arch" by Hustvedt - Own work. Licensed under Creative Commons Attribution-Share Alike 3.0 via Wikimedia Commons. For an excellent introductory video of some of the wonders and challenges of granular materials I suggest this from the National Science Foundation.]
One of the problems with writing a book that tries to describe the current state of any science is that, by the time the book is published, the current state will have moved on and there’s always something new and wonderful in that science. And so it is with The Desert.
As we struggle to come to grips with the startling roles of our planet’s arid lands in the way the earth system works – landscapes, surface processes, atmosphere, oceans, climate and ecosystems – the more we appreciate how much catching up we have to do. Our understanding of arid lands lags significantly behind that of their more accessible and pleasant temperate equivalents. The sheer scale of deserts and desert processes makes comprehensive data collection and integration a serious challenge – it’s not that long ago that Ralph Bagnold, ‘the man who figured out how deserts work’, had to give up on his pioneering analysis of wind-blown sand simply because the data on winds in the desert did not exist.
Dust is a great player in earth system games and the Sahara is the greatest source of natural dust. Yet the role of dust is only now being understood, and it is full of surprises, some good, some less so. This key topic is something that I try to reflect on in the final chapter of the book, looking at the challenges and the future of arid lands, but the science moves on. It has long been known that Saharan dust travels across the Atlantic and plays a role in the rainforests of the Amazon and the ecosystems of the tropics. I was intrigued to read that Saharan dust is responsible for soil development – and therefore agriculture - in the Caribbean region, and duly reported this startling fact. But now it seems very possible that African dust has been, and continues to be, responsible for the building of the Bahamas and biological activity in that otherwise nutrient-poor region.
The Great Bahama Bank is great indeed – over a hundred thousand square kilometres of carbonate sediments that have accumulated over at least the last 100 million years to build up a thickness of several kilometres. And it’s essentially all mud. Not shell fragments, not coral debris, mud.
To say that this has long posed a conundrum for geologists is something of an understatement. Arguments and hypotheses have swirled around the academic world like the sediment and atmospheric patterns around the bank itself, but now a research team from the Universities of Miami and Amsterdam has proposed a radical mechanism for the build-up of the Great Bahama Bank: Saharan dust. From the University of Miami press release last week:
MIAMI – A new study suggests that Saharan dust played a major role in the formation of the Bahamas islands. Researchers from the University of Miami (UM) Rosenstiel School of Marine and Atmospheric Science showed that iron-rich Saharan dust provides the nutrients necessary for specialized bacteria to produce the island chain’s carbonate-based foundation.
UM Rosenstiel School Lewis G. Weeks Professor Peter Swart and colleagues analyzed the concentrations of two trace elements characteristic of atmospheric dust – iron and manganese – in 270 seafloor samples collected along the Great Bahama Bank over a three-year period. The team found that the highest concentrations of these trace elements occurred to the west of Andros Island, an area which has the largest concentration of whitings, white sediment-laden bodies of water produced by photosynthetic cyanobacteria.
“Cyanobacteria need 10 times more iron than other photosynthesizers because they fix atmospheric nitrogen,” said Swart, lead author of the study. “This process draws down the carbon dioxide and induces the precipitation of calcium carbonate, thus causing the whiting. The signature of atmospheric nitrogen, its isotopic ratio is left in the sediments.”
Swart’s team suggests that high concentrations of iron-rich dust blown across the Atlantic Ocean from the Sahara is responsible for the existence of the Great Bahama Bank, which has been built up over the last 100 million years from sedimentation of calcium carbonate. The dust particles blown into the Bahamas’ waters and directly onto the islands provide the nutrients necessary to fuel cyanobacteria blooms, which in turn, produce carbonate whitings in the surrounding waters.
Persistent winds across Africa’s 3.5-million square mile Sahara Desert lifts mineral-rich sand into the atmosphere where it travels the nearly 5,000-mile northwest journey towards the U.S. and Caribbean. The paper, titled “The fertilization of the Bahamas by Saharan dust: A trigger for carbonate precipitation?” was published in the early online edition of the journal Geology.
The conclusions of the paper are as follows (don’t, as I was initially, be confused by the term ‘whiting’ – it’s not the fish, but, as explained above, ‘white sediment-laden bodies of water produced by photosynthetic cyanobacteria’):
In this paper, we demonstrated a strong similarity between the distribution of whitings and the Fe in the sediments of the GBB. We believe that the Fe originates from dust deposited either directly or washed in from dust deposited on the adjacent Andros Island. The input of Fe helps induce the precipitation of CaCO3 through the photosynthetic activity of cyanobacteria. Cyanobacteria also fix N2, which is utilized by all the biological communities on the GBB and is evident in the d15N signature, which is close to zero over the entire platform. Such whitings might be responsible for helping to produce vast amounts of sediments, not only within recent times, but also during previous periods of geological history. Evidence of long-term dust deposition is present in other records and has been postulated to account for the accumulation of soils through the Caribbean and the southern United States (Muhs et al., 2007). Such production might be significantly increased during periods of high dust input and could account for variations in rates of accumulation and platform progradation. This model suggests a modification to the paradigm that proposes that high concentrations of nutrients are detrimental to the growth of carbonate platforms. Rather we propose that certain nutrients may promote platform growth, particularly ones dominated by nonskeletal carbonates and the formation of mud by whitings.
This is fascinating stuff and of potential significance way beyond the Bahamas and the present time – as the paper notes:
This phenomenon might be responsible for the formation of vast amounts of sediments in the oceans, not only within recent times, but throughout geological history, particularly in the early history of the Earth prior to the existence of calcium carbonate–secreting organisms.
This didn’t make it into the book, but then again, that’s what this blog is for.
[Thanks to Suvrat Kher at Rapid Uplift for bringing this to my attention. Images courtesy of the NASA Goddard Space Flight Center. The original paper: P. K. Swart, A. M. Oehlert, G. J. Mackenzie, G. P. Eberli, J. J. G. Reijmer. The fertilization of the Bahamas by Saharan dust: A trigger for carbonate precipitation? Geology, 2014; DOI: 10.1130/G35744.1]