Deep-rooted and completely erroneous preconceptions of our planet’s arid lands as sterile bit-players in the great game of the earth’s dynamic systems have long inhibited our scientific enthusiasm for, and understanding of, the desert. We are now beginning to catch up – take, for example, this recent headline from the American Geophysical Union:
The world's deserts may be storing some of the climate-changing carbon dioxide emitted by human activities, a new study suggests. Massive aquifers underneath deserts could hold more carbon than all the plants on land, according to the new research.
As described in a summary of this research on Science Daily:
Humans add carbon dioxide to the atmosphere through fossil fuel combustion and deforestation. About 40 percent of this carbon stays in the atmosphere and roughly 30 percent enters the ocean, according to the University Corporation for Atmospheric Research. Scientists thought the remaining carbon was taken up by plants on land, but measurements show plants don't absorb all of the leftover carbon. Scientists have been searching for a place on land where the additional carbon is being stored--the so-called "missing carbon sink."
The lead author of the report in the AGU publication, Geophysical Research Letters, is Yan Li, a desert biogeochemist with the Chinese Academy of Sciences in Urumqi, Xinjiang; he and his team examined the character of groundwaters in the gigantic closed system of the arid Tarim Basin, and came up with some fascinating – and provocative – results. Runoff waters from the surrounding mountains pick up, as a normal part of the carbon cycle, some CO2 dissolved from the rocks and soils through which the rivers flow. However, by the time that water ends up in aquifers, the underground reservoirs beneath the desert, it contains substantial amounts of DIC, dissolved inorganic carbon.
Being able to date the carbon, Li and his colleagues could distinguish between old carbon originating from the rivers and very young carbon added to the water as it seeped through the soils of the irrigated oases along the desert margins. These are poor soils, not in themselves sources of much CO2 - it originates from the respiration of the roots of crops and microbes in the soil. And because these crops are irrigated almost constantly, not only to keep them growing but to wash out the salts that, as in all desert agriculture, accumulate in the soil, most of the CO2 is transported downward into the groundwater moving out below the desert to be trapped in the deep aquifers. Importantly, because of the salts, these waters are saline and alkaline and the solubility of CO2 in saline/alkaline water is much higher than in pure or acidic water – the desert groundwater is a very significant CO2 sink.
Because of their ability to date the carbon dissolved in the waters, the researchers were able to establish that the levels jumped substantially in historical times as the development of the Silk Road enabled the beginnings of oasis agriculture. Man’s activities – irrigation and over-irrigation – have augmented the efficiency of this carbon sink by, it is estimated, a factor of twelve:
Based on the various rates that carbon entered the desert throughout history, the study's authors estimate 20 billion metric tons (22 billion U.S. tons) of carbon is stored underneath the Tarim Basin desert, dissolved in an aquifer that contains roughly 10 times the amount of water held in the North American Great Lakes.
The study's authors approximate the world's desert aquifers contain roughly 1 trillion metric tons (1 trillion U.S. tons) of carbon--about a quarter more than the amount stored in living plants on land.
And because this is a saline and alkaline aquifer, the water is completely unsuitable for agriculture – it will likely remain below the desert as essentially permanent carbon storage - undoubtedly not the only missing sink, but a hitherto unidentified one. As Li remarks: “The fact that such a huge carbon pool and active sink has been unstudied for so long may simply be because it is remote and hidden under deserts: out of sight, out of mind.”
[Image at the head of this post is of agriculture and dunes along the northern edge of the Tarim Basin, Google Earth; diagram of the process of carbon storage from Yan Li, Yu-Gang Wang, R. A. Houghton, Li-Song Tang. Hidden carbon sink beneath desert. Geophysical Research Letters, 2015; DOI:10.1002/2015GL064222]
Yet again, thank you NASA. As announced last month, their latest extraordinary earth-monitoring system is in orbit, commissioned and providing data: SMAP, the Soil Moisture Active Passive mission provides a high-resolution view of continuing changes in soil moisture across our world and allows understanding, analysis and planning in an unprecedented and fascinating way.
The map above is one of several already available:
These maps of global soil moisture were created using data from the radiometer instrument on NASA's Soil Moisture Active Passive (SMAP) observatory. Each image is a composite of three days of SMAP radiometer data, centered on April 15, 18 and 22, 2015. The images show the volumetric water content in the top 2 inches (5 centimeters) of soil. Wetter areas are blue and drier areas are yellow. White areas indicate snow, ice or frozen ground.
The soil moisture scale is in cm3/cm3 and demonstrates dramatically the often-forgotten fact that drylands, home to a third of our planet’s population, comprise over 40% of the land area.
As usual, the technology is extraordinary:
For an introductory description of how the radar and radiometer instruments conspire, see http://smap.jpl.nasa.gov/observatory/overview/. The NASA press release summarises the mission and the ways in which the data will be used:
Launched Jan. 31 on a minimum three-year mission, SMAP will help scientists understand links among Earth's water, energy and carbon cycles; reduce uncertainties in predicting climate; and enhance our ability to monitor and predict natural hazards like floods and droughts. SMAP data have additional practical applications, including improved weather forecasting and crop yield predictions.
A first global view of SMAP's flagship product, a combined active-passive soil moisture map with a spatial resolution of 5.6 miles (9 kilometers), is available at:
During SMAP's first three months in orbit, referred to as SMAP's "commissioning" phase, the observatory was first exposed to the space environment, its solar array and reflector boom assembly containing SMAP's 20-foot (6-meter) reflector antenna were deployed, and the antenna and instruments were spun up to their full speed, enabling global measurements every two to three days.
The commissioning phase also was used to ensure that SMAP science data reliably flow from its instruments to science data processing facilities at NASA's Jet Propulsion Laboratory in Pasadena, California, and the agency's Goddard Space Flight Center in Greenbelt, Maryland.
"Fourteen years after the concept for a NASA mission to map global soil moisture was first proposed, SMAP now has formally transitioned to routine science operations," said Kent Kellogg, SMAP project manager at JPL. "SMAP's science team can now begin the important task of calibrating the observatory's science data products to ensure SMAP is meeting its requirements for measurement accuracy."
Together, SMAP's two instruments, which share a common antenna, produce the highest-resolution, most accurate soil moisture maps ever obtained from space. The spacecraft's radar transmits microwave pulses to the ground and measures the strength of the signals that bounce back from Earth, whereas its radiometer measures microwaves that are naturally emitted from Earth's surface.
"SMAP data will eventually reveal how soil moisture conditions are changing over time in response to climate and how this impacts regional water availability," said Dara Entekhabi, SMAP science team leader at the Massachusetts Institute of Technology in Cambridge. "SMAP data will be combined with data from other missions like NASA's Global Precipitation Measurement, Aquarius and Gravity Recovery and Climate Experiment to reveal deeper insights into how the water cycle is evolving at global and regional scales."
It is, to me, wondrous that even a simple examination of the three successive images from which the one at the head of this post was taken reveals clear changes over the course of a few days – I can only begin to imagine what analysis of the real data will reveal.
The UN’s map and classification of global drylands drives home their importance to the way our planet works. The new SMAP data bring this to life in a dramatic way.
[SMAP image credit: NASA/JPL-Caltech/GSFC]
The possibilities surrounding the re-start of the Large Hadron Collider are endless, but what, of course, intrigued me in a recent piece in the New Scientist was the imagery of the desert. One of the aspirations for my book was to explore the many roles of the words “the desert” in our imaginations, and here they are as we continue our journey to unravel the secrets of life, the universe and all that.
The article begins:
THEY call it "the desert" – a vast, empty landscape separating us from a promised land that shimmers like a mirage on the horizon. A land full of answers, where we finally achieve a complete understanding of material reality.
Stop dreaming: we can't get to this nirvana. The way across the desert is too long and hot, and we have no vehicle to take us there. But if physicists' hopes are realised, a machine just waking from a two-year slumber could bring us a decisive step closer – and might even reveal answers closer to home…
Since February 2013, the LHC has been undergoing a comprehensive overhaul. Now it is gearing up again, more powerful than ever before, for a journey towards the desert – and the complete unknown. The excitement is palpable. "We are living in a once-in-a-lifetime experience, opening the curtains on a totally new energy scale," says Jim Olsen of the LHC's CMS experiment.
For most physicists, the conclusion is that the standard model is part of a bigger theory – one that brings us closer to unifying all forces and understanding matter at all energy scales. The problem is, although precise predictions vary, our best guess is that further bouts of force unification only lie at scales of trillions of TeV and above, that were last attained in the first trice of the universe – within 10-36 seconds of the big bang.
No accelerator on Earth could conceivably achieve such energies. In this picture, what lies between us and the unattainable promised land is a desert devoid of interest. It makes the LHC's upgraded collision energy of 13 TeV seem a rather forlorn gesture.
Not so, says theorist Ben Allanach of the University of Cambridge. If the favourite candidate for a next-generation theory is right, the sliver of new territory we are about to enter could contain particles and phenomena that will take us a decisive step closer to an ultimate answer.
The theory in question is supersymmetry, or SUSY to its friends.
… gravity's true strength might be such that the promised land of unification lies at energies much closer to where we are now, perhaps even within the LHC's reach. Then the desert would be no desert, but full of a host of strange objects such as miniature black holes, which the LHC might be able to squeeze into existence by warping and pinching space-time in its collisions.
So the hope is that the territory about to be explored is a lush forest bristling with particles that give us clues to the nature of the desert – and beyond.
Over the life of this blog, I have written a number of times on the remarkable and innovative work carried out at Georgia Tech’s Crab Lab, investigations into how critters get around in loose sand and how this can be applied to robotics (see, for example, here, here, and here). Crab Lab is headed up by Dan Goldman, and I was recently delighted (and flattered) that he contacted me about the “Sand” book. I asked if he and his colleagues would be willing to contribute a guest post – and here it is. Courtesy of Henry Astley, a postdoc “who loves all things snakes” and collaborator Joe Mendelson: the state of the art on the physics of sidewinding.
In spite of their barren reputation, deserts around the world teem with life, including a wide array of animals from beetles to camels. While much has been written about their adaptations to deal with two of the most notable characteristics of deserts, extreme temperatures and scarce water, far less attention has been paid to their interactions with the other distinguishing characteristic of many deserts: sand.
The ability to move from place to place is crucial for animals to find mates and resources, regulate their body processes, avoid predators, and colonize new environments. But sand makes locomotion difficult, whether moving on it or through it. Particularly problematic is that sand will behave as a solid under certain loads, but will yield and flow like a fluid under others, and very small differences in foot placement and movement can be the difference between moving and becoming hopelessly stuck. Animals deal with this challenging substrate in a variety of ways, whether by anatomical changes or selecting the best movement patterns.
Perhaps the strangest movement pattern of desert animals is the famous “sidewinding” locomotion, seen in certain snakes of sandy deserts around the world. While prominent herpetologist Clifford H. Pope wrote in 1955 “A study of [sidewinding] is recommended to anyone who likes to be confused,”, the underlying motion is quite clear when examined in detail. The snake lifts its head and moves it forward, placing it on the ground, then repeats this motion in a propagating wave down the body – see Science paper supplementary movies.
This produces a characteristic trackway consisting of a series of parallel lines, with the imprint of each scale on the belly clearly visible, showing that the snake does not slip (which would smear the tracks and erase the fine scale imprints). Ultimately, this seemingly complex motion can be reduced to a pair of waves producing vertical and horizontal body undulation, +- 90 degrees out of phase, which propagate together down the body. This simple model may be a “neuromechanical template”, a simple model of a motion which captures all the essential features, potentially serving a simple “target” for the animals to control their locomotion (see Sidewinding with minimal slip: Snake and robot ascent of sandy slopes). This two-wave template of sidewinding also produces sidewinding locomotion when applied to a snake robot, allowing the robot to move on sand effectively.
Screen-grab from Science supplementary movies
This provides a great experimental tool, because the physics of sand have yet to be reduced to simple systems of equations (as has been the case in fluids for almost 200 years), making computer modeling of results difficult and time-consuming. However, the snake robot provides a “physical model” for movement in sand, allowing us to test hypothesized biological mechanisms. Further observations of biological snakes have revealed the modifications of the two-wave template responsible for effectively sidewinding up inclines and turning (see the recent PNAS paper), which in turn have further improved the effectiveness of the robot.
In spite of these insights, the serpents of the sand still hold many mysteries. Why do some snakes sidewind, while others don’t? Why can some species move effectively on sand, while others fail? How do sidewinders deal with obstacles? Can we reconstruct the evolution of this remarkable mode of locomotion from the tracks it has left, or has this history been lost in the sands of time?
[My sincere thanks to all at Crab Lab for their work and this post. Photographs by Henry Astley]
“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.
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/]