Through The Sandglass

Martian floods of tears - are the TSIs ESRs? Or is it just the total drag?

It’s always sobering to realise that we have a more detailed view of the surface of Mars than we do of the floors of our own oceans. And, while the evidence for water’s role in the history of Mars and its geomorphology (or, I guess, more correctly, martiomorphology) is more or less accepted, the interpretation of that planet’s surface features is almost exclusively pursued through the spectacles of terrestrial (on-land) terrestrial processes. We look to the processes of rivers and floods on our continents to explain the surface features of Mars. So there’s a certain satisfaction when geologists break out of this mindset and propose ocean floor analogues from Earth to explain the – now definitely subaerial – topography of Mars.

Whether they are proved to be right or not really doesn’t matter at this point: Lorena Moscardelli and Lesli Wood have made the vital contribution of opening up a debate, stepping out of the box, challenging conventional wisdom. Both from the Bureau of Economic Geology at the University of Texas in Austin, they have published a paper in the July issue of Geology titled “Deep-water erosional remnants in eastern offshore Trinidad as terrestrial analogs for teardrop-shaped islands on Mars: Implications for outflow channel formation.” The “teardrop-shaped islands” are the TSIs and the ESRs are Erosional Shadow Remnants – but so what, what is this all about apart from more acronyms?

Well, first of all, let’s review what we know and what we don’t know about water on Mars. What we know is obviously that liquid water plays no role in today’s processes – there is evidence for water locked up in Martian minerals and the permanent polar ice caps contain frozen water (unlike the seasonal ones that form exclusively from dry ice, frozen carbon dioxide) – but a flash-flood is not one of the hazards that a Mars Rover faces. Then there is evidence of ancient shorelines – almost certainly lakes, possibly oceans. But when I say “ancient,” of course I mean serious deep-time – the idea of a warm and wet period on Mars refers to a time around three billion years ago. And finally we come to martiomorphology – there are endless features of the Martian landscape, imaged in superb detail, that look exactly like geomorphological hallmarks of rivers, floodplains, water-scoured gullies, deltas, alluvial fans and so on. There are always alternative explanations for individual features, but the collective evidence suggests flowing surface water.

But. detailed though our images are, and even though the Rovers Opportunity and Spirit (now, sadly, deceased) have, with unexpected grit and fortitude, explored and sampled a few square kilometres, a geologist or geomorphologist has yet to stride the field on Mars, and definitive interpretation eludes us. Which is where the fun lies.

One of the most closely scrutinised areas of Mars with respect to water-sculpted landforms is the gigantic valley of Ares Vallis – see, for example, this European Space Agency site (where the image at the head of this post came from). Like so many landforms on Mars, this is a seriously gigantic feature – a couple of thousand kilometres long, with valley walls a couple of thousand meters high. Its floor and the many tributary canyons provide enough for researchers to chew on for decades.

And that’s exactly what Moscardelli and Wood have been doing. Like their colleagues in the Martian scrutinising business, they must always bear in mind what they know of their own planet, but they have expanded this approach to considering the floor of our own oceans.

The topography of Ares Vallis and its tributary canyons bears many of the hallmarks of catastrophic flooding – canyon headwalls that seem to have been the site of massive cataracts that constantly eroded the cliffs backwards up the canyon and downstream from the cataracts, scoured floodplains. These landscapes are reminiscent of those of the so-called channelled scablands of Eastern Washington State, bizarre geomorphology carved by a series of gigantic floods released from glacial lakes as the climate warmed. Where, on Mars, all this water came from, is another matter, the subject of debate. The September 2010 issue of Geology contained a well-documented paper on the “Retreat of a giant cataract in a long-lived (3.7-2.6 Ga) martian outflow channel.” The research of a team from University and Imperial Colleges, London, argues that periodic groundwater release caused a long period of catastrophic flooding and erosion of the Ares Vallis region. And, when they say “long-lived,” they mean it – Ga is a billion years, so this period of landscape development lasted 1,100,000,000 years.

BUT. Among the distinctive and enticing landforms are TSIs - “teardrop-shaped islands,” clearly visible in the image below, taken from the paper by Moscardelli and Wood.

What could these forms be telling us? The teardrop – streamlined - shape is common in nature, and shows up in a variety of contexts on our own planet, not to mention in the aerodynamic profile of an airplane’s wing. Now, wonderfully, given that much of this work is available only to subscribers (of which I am one), Geology saw fit also to publish in the public domain, open-access, a short review of these landforms and their possible meaning on the floodplains of Mars. The illustration below is taken from that review (written by Devon Burr at the University of Tennessee), and it really is fascinating. For a start, it shows yardangs, on which Evelyn and I recently (and totally coincidentally) wrote a joint post.  
Teardrop-shaped, streamlined, landforms are common terrestrially and extra-terrestrially, and can be shaped by water or wind – they seem to be one result of fluid flow, regardless of the fluid. And, as Burr explains, this is for good reason – the physics of drag:

Streamlined forms are shaped during flow as a tendency to minimize total drag. Total drag is the sum of form (pressure) drag and skin (friction) drag. Form drag around a blunt body arises largely from flow separation in the lee of an obstacle, so that elongation of the form through in-filling of the leeward separation zone reduces the form drag. Conversely, skin drag acting tangentially to the obstacle surface is minimized by reducing the surface area by making the feature geometrically more compact. Consequently, minimization of total drag is accomplished through a combination of elongation and compaction. The result of these countervailing tendencies is the streamlined form…. Both erosional and depositional streamlined forms are observed in terrestrial floodscapes.

And there’s a key point: such forms can result from erosion (as with yardangs), or deposition, in the downstream lee of an obstacle.

Cut to the deep ocean waters off the Orinoco delta. Using clever seafloor imaging, Moscardelli and Wood describe the features shown below (taken from their paper): 
The dark circular features are mud volcanoes (liquefaction phenomena common in loose sediments in a tectonically active area) that seem to form the “anchor” for the TSIs – which are interpreted as ESRs; the mud volcanoes form the obstacle to massive flows down the sloping sea floor, leaving the streamlined form downstream from them protected from the erosional power of the flow and shaped by the physics of drag. Now, compare these with an example from Ares Vallis, where the TSIs are “anchored by impact craters:

Interesting, eh? As the authors conclude (after a detailed analysis documented in the paper) that “These observations suggest that the teardrop-shaped islands [on Mars] might have been formed as a result of catastrophic submarine mass movements similar to those documented within continental margins on Earth.”

There is, of course, one key difference between Earth and Mars that drives the drag equations: gravity. The gravity on Mars is significantly lower than that on Earth, and so such streamlined forms may result from different conditions of fluid flow. Given the fact that streamlined forms also occur on Mars in regions where an ocean is unlikely ever to have existed, and yet that water flow can be invoked as a mechanism for their formation, Burr concludes, “If the physical sedimentology in outflow channels on Mars is indeed similar to that in submarine environments on Earth (Komar, 1979), then the submarine analogy revived by Moscardelli and Wood might extend beyond the limited number of circum-Chryse TSIs. In other words, this analogy may argue for the effect of lower gravity producing submarine-style processes in Martian outflow channels generally, regardless of any hypothesized submarine context.”

So, who knows really what’s going on? The answer is, of course, nobody, but Moscardelli and Wood have opened up the debate, pointing the way to a broader understanding of sedimentary landforms in extra-terrestrial – and terrestrial – environments. As Burr sums it up:

Testing between these two interpretations—that certain streamlined forms on Mars formed in a submarine environment, or that all streamlined forms on Mars formed in an outflow environment in which particle physics mimics that in submarine environments on Earth—will require morphometric data from ESRs for comparison to Martian streamlined forms, as well as examination of the geologic context for the Martian examples. The question extends beyond Mars to other worlds. Titan has even lower gravity than Mars, a 10-times thicker atmosphere than Earth, and also shows streamlined forms (Fig. 1E). Could subaerial processes on Titan mimic outflow processes on Mars and submarine processes on Earth? Pinning down the true cause for the similar appearance between terrestrial ESRs and Martian TSIs would contribute to understanding the effect of reduced effective gravity on sedimentary landforms.

I’ll leave (assuming that anyone has made it this far) with a favourite image that raises a favourite question: what planet are you from?

 

[The paper by Moscardelli and Wood was picked up by Wired Science; the full reference is: Lorena Moscardelli and Lesli Wood, Deep-water erosional remnants in eastern offshore Trinidad as terrestrial analogs for teardrop-shaped islands on Mars: Implications for outflow channel formation, Geology 2011;39;699-702; for the cataracts analysis, Nicholas H. Warner, Sanjeev Gupta, Jung-Rack Kim, Shih-Yuan Lin and Jan-Peter Muller, Retreat of a giant cataract in a long-lived (3.7-2.6 Ga) martian outflow channel, Geology 2010;38;791-794. Burr’s open-access summary is here. For the origin of my final image, have a browse around the Mars HiRise image collection. The image of Ares Vallis at the head of this post is from the European Space Agency Mars Express site.]

Penguin geophagy

I have to confess that, with the single exception of when my daughter was one in a pantomime many years ago, I have never understood the attraction of penguins. Having spent time in the Arctic, I have long had a yearning to see the other end of the planet, but this has remained – and probably will continue to be – unfulfilled. For two reasons: primarily of course, the enormous cost, but secondly because the only opportunities for visiting the Antarctic seem to have, as their primary focus, endless encounters with endless colonies of penguins. It’s not that I don’t recognise the validity of a penguin’s existence, it’s simply that their sheer numbers and prolific excretory habits bestow on their environment an unbearable stench, and that they don’t strike me as being creatures with a compelling intelligence that invites lengthy observation. Essentially, seen one penguin, seen ‘em all. And I do realise that this is not a popular perspective – penguins have, after all, become film stars and loveable icons of their pristine (if smelly) environment. And yes, I am aware that these ungainly birds display exquisite grace underwater, and yes, I have read of the clever communal wave that that they perform to stay warm, but even so….

It was therefore with some amusement, given my views on their ranking on the IQ scale of life on earth, that I read of the emperor penguin that turned left and ended up on a beach in New Zealand – a profound navigational error that, once made, seems to have been pursued without further reflection. Now, don’t get me wrong, I certainly felt sympathy for it – even felt sorry for it – but my first reaction was mirth and a sense that my view of the position of the penguin on the tree of life’s skills was confirmed. It is surely a good thing for penguins that polar bears were destined to roam the other one – survival of the fittest would inevitably have otherwise prevailed, and penguins would have been but a fleeting aberration of evolution (unless, of course, the stench is a clever defensive mechanism that even bears could not overcome). 

The bird (an adolescent, of course), now popularly christened “Happy Feet” (oh Michael, bite thy tongue…) and of so far unknown gender (how difficult can penguin-sexing be?) is, as I write, recovering after an endoscopy. It required this procedure because it had taken to eating sand.

Now many birds need to intake grit or sand to assist with the grinding process in their gizzards – and penguins are reported to pursue this activity. When humans do the same, it’s referred to as geophagy. “Bizarre stories abound of such diets. An eighty-year-old woman in India is said to eat a kilogram of sand before breakfast. Japanese inhabitants of coral reef islands are said to have eliminated the need for doctors through eating the calcium-rich sand. All this is arguably patent nonsense, but pica, from the Latin for “magpie,” is the medical term for an appetite for nonnutritional substances; it is a medical disorder.” (Welland, 2009).

But within Happy Feet was found enough sand to fill its stomach and esophagus – it had clearly gone to excess in terms of gizzard assistance, and it is suggested that, since in the Antarctic penguins eat snow and ice, perhaps the hapless creature mistook sand for snow. I rest my case.

At the time of writing, Happy Feet was reported to be recovering – and I do hope that this continues to be the case. But if, in the interim, a tragic reversal of he/she/its condition has occurred, I respectfully ask that my readers do not resign in protest.

Sunday Sands - Guest Post

Pete Newman is a geologist and an old friend and colleague; as an arenophile, he was not only an early and enthusiastic supporter of my idea for the book, but also provided me with some treasures from his granular collection, some of which featured as the first colour plate in the book, others of which have been the heroes of various blog posts (see below). I have been badgering him for a while for a guest post, and I’m delighted to publish it here – along with original artwork.

The Granule

Hi Mike—

Congratulations on your new job and on getting back to Jakarta!  Based on my own experiences there, I feel like you’re a lucky man.  Not only in general terms, but for an Oilman and Sandman like yourself, the grease and the granules must beckon strongly.  When I heard that you’d be posted there, I dug out my Indonesian sand samples to see which would be different enough to interest you.  You’re already aware of the unique foraminiferal beach at Sanur and the beautiful glassy quartz grains of the Toba beaches---those will be hard to top---but there is just so much variety in that country that you can’t have seen it all.  Let’s see if any of the following three granule types will expand your sand catalog.

       1) I believe that I’ve previously mentioned the cute little “hopper” quartz crystal grains that I found in sand bars along the Bikis River in Kalimantan Timur.  Now I’ve just found some nice hand-drawn illustrations of these in my files, originals that were done by an Indonesian illustrator, and I’m attaching one here as illustration #1.  (You’d be amazed how little I paid for this work, way back in those immediate post-Sukarno days).  This is one of those cases where a picture is worth a thousand words, but I never got to do a geological report where I could justify using the drawings.  But just imagine a sandstone where every quartz grain was a hopper crystal, how much that would enhance the porosity beyond the theoretical maximum!  Yah, dream on. 

       2) Illustration #2 is of a sand-sized grain of uncertain identity, one from a Kaltim beach.  The characteristic crystal shape and high density (sp gr >3.3) mean that this mineral shouldn’t be that hard to identify, but I was never able to nail it down (and didn’t have too many extra-company resources to consult in Djakarta).  The drawing was done by me.

       3) Now we come to my absolute favorite sand grain, out of what must be the thousands I’ve examined.  Illustration #3 is a sand-sized glass ball, one that came from Samudra Beach in South Java.  This specimen turned up in a heavy mineral study I did to determine the source area of the Cibulakan sandstones---and thus their source direction---to learn where the thicker and coarser portions of these sands might lie.  (I was trying to figure out how to do acreage turnbacks of our undrilled areas.  And just for your geo-interest, the answer was a mixed [north and south] source: our reservoirs have a hypersthene/glassy quartz minor [southern] component plus an abundant zircon/tourmaline/anhedral quartz component that indicates the majority of the grains came from the Sunda Shield.)

        When I began sampling the south Java beaches, I found that the sea had done my heavy mineral separation for me: the swash of the waves had separated the quartz grains from the heavy components (which were overwhelmingly magnetite, occurring in beach sand layers that were pure and as much as 15” thick).  But how, you may wonder, did that help me find this grain?  Well, this grain and many other glass grains were actually of a denser-than-normal glass (> 3.0 specific gravity) which meant that they were mixed in with the magnetite.  Using a magnet, it was easy to separate out the non-magnetic “source fingerprint” minerals---no expensive heavy liquids required!  And in this residue---a tiny fraction of the total amount of beach sand---there were a number of grains of colorless to greenish glass, often in spherical form.  I separated the spherical ones from everything else by rolling them downhill---just tilted our dining room table, sprinkled the grains, caught the round ones in a sheet at the bottom.

       One (and only one that I ever saw) of those grains had a “tail” that was apparently drawn out when the glass was soft---this one is my fave and the one that’s shown in the attached sketch.  I’d like to say that I know the origin of this li’l beauty but all I can do is suggest three possible ways it could have formed:

   1) An eruption from one of Java’s volcanoes?  If found on the “Big Island” of Hawaii, this little ball would probably surprise no one as some very fine glass, including spheres, is erupted there.  But are those glass balls as dense as this one?

   2) A microtektite?  In size and shape, it bears similarities to those, and the Southeast Asia location is right.

   3) A man-made product, from construction of the Samudra Beach hotel  (which sits right behind this beach)?

And though I’ve recently read about glass micro-balls being created by D-Day explosions on Omaha Beach (EARTH magazine, June), I know of no record of ordnance exploding on the south coast of Java.

       So, not knowing anything real about these little balls, I certainly couldn’t say that they helped me in my work.  But as you know more than most, geological fun shows up in many unexpected places.  Studying geology in school, how could I have guessed that it would lead me to rolling sand grains down a table while our Indonesian cook rolled her eyes thinking “What now?”

Your sandy-haired friend, 

Pete

  
[The “hopper” quartz grains from Kalimantan were featured in a post from a couple of years ago, the Lake Toba ones as “a few of my favourite grains,” and Pete’s Bali forams in another Sunday Sand post. Plate 1 from the book - Pete’s samples in a daily pill container – is shown above together with the hopper quartz grains. Thanks, Pete, for all your help and support – and for this post!

Oh, and the sand from the D-Day beaches that Pete refers to – see also my post and article link from the anniversary.]

Nourish or retreat?

 
25 million people – one out of every eleven inhabitants of the United States - live within 50 miles of the 127 miles of the open ocean coast of New Jersey. Coastal tourism contributes billions of dollars to the state every year. Hundreds of millions of dollars have been spent on moving millions and millions of cubic yards of sand around to “restore” the coast, and the state’s fund for continuing to do so stands at $25 million a year.

Does this make sense? I have ranted a little in previous posts about “beach nourishment,” demographics, and the rights of folk to live in some of the most unstable environments on the planet while others (and the environment) pay dearly for their privilege. Being somewhat pressed this week, I will simply refer you to an excellent piece from the  Earth Institute at Columbia University that addresses the issues around nourishment versus “managed retreat” in dealing with the insistence of shorelines – specifically those of New Jersey - to change. See also the report of a Beach Erosion Workshop from the USACE Philadelphia District Coastal Planning group, “Beach Nourishment: The New Jersey Experience” (which includes an entertaining section titled “Unexpected consequences”), a summary of New Jersey’s shore protection program, and extensive articles on beach nourishment from Coastal Care.

I would also recommend whiling away an enjoyable hour or two examining the offerings of Google Earth’s historical imagery along new Jersey’s coast – for example, Hereford Inlet, just up the coast from Cape May and a playground of shifting sand, in 1995, 2006, and 2010:

 
And I’ll leave you with a nice photo of beach nourishment (courtesy of Coastal Care).

 

Sunday sand: a clever little piece of kit

This week has taken on a theme of microscopy, so today I will continue with it and answer a couple of readers’ questions about what I use for my – admittedly amateur – sand images. Back in London, I have a binocular microscope and an attachment that allows me to screw on my old digital camera (not a full-blown SLR, but a fixed lens with an excellent telephoto and threaded for filters and microscope adapters). However, I didn’t want to haul all this out to Indonesia. I am currently awaiting the arrival of a small shipment - no, it’s not coming by sailing barge, but it could not even leave England until I had work and residence permits, a somewhat prolonged bureaucratic process….. In that shipment is a full-sized Celestron digital microscope that I much look forward to playing around with, but to keep me going, and to fill in at the lower magnification range, I decided to bring with me a small hand-held digital microscope.

I had first seen one of these devices used on a television science lecture, and was impressed. I now have one and remain impressed. I don’t do commercials on this blog, but it’s obviously necessary to give some details: it’s a Celestron 44302 microscope, incredibly available for less than $65 in the U.S., and, unusually, for a fair exchange rate equivalent price in the UK. It’s small and light, and comes with a simple bit of software so that after installation it’s just USB plug-and-play. And play with this thing you really can. It has a built in LED light and the magnification of 10-40x and a higher value around 150x is controlled by a large ribbed knob. You can take snapshots and video using the built-in 1.3mp camera, and the software includes some kind of image organisation capability. In reality, I prefer to simply use “alt-print screen” and transfer the image directly into Photoshop where I can do what I want with it.

 

The stand that it comes with is extremely useful, certainly at low magnifications; it takes quite a bit of playing around and fiddling with the focus (plus taking lots of images and deleting many – the wonders of digital photography) to obtain satisfactory results. One problem is that there is a finite time delay between adjusting the focus and the result showing up on the computer screen, plus moving the focus dial easily nudges the microscope and you lose where you were. In some situations, particularly at higher magnifications, I find that I have to hold the microscope rather than using the stand and have a willing assistant press “alt-Prt Sc” on my command. And the colour and contrast treatment is not great in the digital imaging system – hence the value of Photoshop. For 3D objects, such as a great variety of sand grains of different sizes nestling together, depth of field is always going to be a problem – it was for my set-up back in the UK and it certainly is with this. When I am reunited with my full-size microscope, I will look into software that I believe exists that takes a series of images taken in sequential focus on a 3D object and combines them into a single, completely in-focus, image. But take a look, for example, at Gary Greenberg’s microphotography of sand grains for what can be achieved with highly sophisticated and specialised technology.

So, this little device certainly doesn’t fall into the category of highly sophisticated technology, but it sure is clever, it serves my purpose – and it’s fun. It would be a wonderful addition to any school classroom and any home with a kid with even a hint of an interest in science – for the price, I think it’s amazing.  

Oh yes – this sand. It’s from Bradenton Beach (thanks, Lori!), a barrier island on the Gulf Coast of Florida. It’s dominated by tiny, clean, quartz grains and large shell fragments – plus some little black grains; I’m not really sure what the latter are – my guess is that they are of organic origin. The sweeping expanse of Bradenton beach is shown below; it’s an image taken from a real-estate site, and close examination shows why – housing and development crowds the shores yet again on one of the most unstable and shape-shifting, storm-prone environments on the planet. I have asked why this makes any sense in earlier posts, so I won’t bother repeating the question today. 

Guest post: A report from the microscopic front line

As always, a great pleasure for me from this blog is the people who just get in touch out of the blue. Karen Locke first posted a comment on my Sunday Sand post on Bali earlier this year. I was intrigued and followed up with her, discovering to my delight that she is a graduate student scrutinising sand grains in detail – and in earnest. I am always interested in hearing from people whose relationship with sand is professional, hence the “front line” reference in the title; here are folk who are convincing sand grains to tell their stories and for whom the translation of those stories is important. I asked Karen if she would be willing to contribute a guest post, and now here it is – I hope that my readers enjoy this as much as I did, and thanks Karen!

Sand in Thin Sections

Thanks to Michael for allowing me to guest-post. I suppose I should start with a little bit about myself: I’m an older MS grad student at San José State University in San José, California, USA. I’m just finishing up a thesis on the composition and provenance of sands extracted from wells drilled in the Santa Clara Valley, where San José is located.

The Santa Clara Valley has had a violent history. It’s surrounded by mountains that were raised (and are still being raised) by thrust and strike-slip faults. The mountains are made of a fascinating array of different types of rocks. The valley itself is filled with sediment from the mountains, and is subsiding under its own weight, as valleys do. By studying sand from various depths in the wells, I get a picture of how the sediment changed over time, and I’m finding clues to the paleoevolution of this valley over the last 800 thousand years or so. I’m asking questions like, did the streams always flow the same way? What was the composition of the rocks that ran down the middle of the valley but have eroded away, and does that tell me about how they might have moved in the past? And the big question, which I can’t answer but can provide a few more clues about is: how did the faults around the valley move to create the changes I see in the sands, and what does that imply about what they might do in the future?

But that’s a thesis; what I really want to talk about today is how I studied the sand. By studying the sand, my goal was to identify the composition of the sand grains. I wanted to know what kind of rock each one came from. Now, sand grains are tiny things. Even under a microscope, it can be deucedly hard to identify them. So I chose to make thin sections, and study the sand under a petrographic microscope.

Thin sections are created by gluing a 0.03 mm slice of rock (or in this case, a slice of sand set in epoxy) to a glass slide. In such a thin slice, most minerals are translucent or transparent. Yes, you can see through solid rock if it’s thin enough. Thin sections are made to be viewed with a petrographic (polarizing) microscope, because lots of minerals have interesting properties when viewed with polarized light – properties that can help identify them. Rocks are made of minerals, and the first step in identifying a rock is identifying its minerals.

The petrographic microscope has a polarized light source that shines through from beneath the slide, rather than illuminating it from above. Why polarized light? Many visual properties of minerals change as you rotate them over polarized light, and it’s the change with rotation that makes them significant. Color is one interesting property; many minerals rotate through two or three colors, while other minerals are steadfastly one color or colorless throughout the rotation. Another interesting property is relief (how tall a mineral crystal appears relative to the ones around it). Yes, the minerals can appear to be different heights, even though the slice is uniformly thick, and the apparent relief can change as you rotate the slide.

Playing with a petrographic microscope messes with your sense of reality. J

Other very important diagnostic properties of minerals in thin section are cleavage and growth habit. Cleavage is the way a crystal breaks internally; some minerals break at very specific angles, while others do not. Habit is how a single mineral tends to grow, or how multiple crystals of a mineral aggregate together. Long and skinny or squared off, aggregating in sheaves or long strings or just growing along side one another – there are lots of possibilities, and often they’re very diagnostic.

Then there’s another critical, and very cool, tool on a petrographic scope. Between the slide and the eyepiece, these scopes have a second, or upper, polarizer that polarizes perpendicular to the one on the light source. This polarizer slides in and out. Now, polarizers cause light waves to move in a plane. So with the upper polarizer out, you’re looking at your slide illuminated by plane light. Putting the upper polarizer in, or crossing the polars, means you’re now looking at the light that came through your slide in one plane, and attempting to look at it in a perpendicular plane. But that generally doesn’t work. Light in one plane can’t get through a polarizer perpendicular to it. Try taking two pairs of polarizing sunglasses and holding the lenses perpendicular to one another; the result will just be black. So looking at your thin section with crossed polars, what you should see is black nothing – unless the minerals themselves perturb the initial polarization. Which of course is what some of them do.

Minerals are crystalline in structure. Some have very regular, simple structures; others have elaborate arrangements. When light goes through a mineral, it can’t simply go through the atoms in the crystal, it has to run down channels between the rows and columns, if you will. Now, when the crystal structure of a mineral is simple and rectangular, it takes the light the same amount of time to run down a row as up a column, and the light comes out the other side of the crystal together. This is called an isotropic mineral, and it really does turn completely black when the polars are crossed. But when light hits a more elaborately-organized crystal structure, it can actually take more time to move through the crystal in one direction than in another. When this light comes out of the crystal, the difference in the outgoing waves produces light that’s not in the original plane, and so it gets through the upper polarizer. This effect is called birefringence, and can be extremely diagnostic.

Finally, birefringence gives me one last property I can use to identify minerals: extinction. As the slide is rotated, the birefringence effectively fades from bright to dark and then brightens again; when a birefringent mineral goes dark, it’s said to be extinct. Different minerals with similar birefringence go extinct at different angles to their growth habits, so angle of extinction is another identification tool.

So, now I have color, relief, cleavage, and habit in plane light, and birefringence and extinction in crossed polars. There are other mineral properties one can look at using the petrographic scope, but these are generally the ones I used to identify the minerals in the sand grains – at least, those mineral grains that were big enough to see. Some mineral grains were so tiny that even under a 40x power lens I couldn’t make them out. But that’s helpful in its own way, because identifying minerals is just one step on the road to my original goal: identifying rocks.

Rocks aren’t just random aggregates of minerals, they’ve got recognizable textures. It’s the combination of identifiable minerals and texture that identifies the rock. And now that I can talk about the rocks, I can show you some pictures. They’re all taken with a digital camera built into a petrographic scope. They’re all pictures of sand grains that are between 250 and 425 micrometers in size.

These are photos of a grain of argillite, a metamorphosed siltstone. The picture on the left is taken in plane light, and the picture on the right is taken with crossed polars. (The blue cast to the crossed polars picture is an artifact of the camera; under the microscope it’s much more grayscale.) Argillite consists of tiny grains of silt (mostly quartz or feldspar) in a brown mudstone matrix. The bright white crystal in the middle on the left is black on the right; in this case, that’s an example of extinction. Note that what seem to be whole crystals in the left picture don’t look whole on the right. That’s because in the process of metamorphosis, they’ve been recrystallized; one larger crystal has become many small ones, each with its own different angle of extinction. The next pair of pictures show that more clearly.

 
These photos are of two quartz sand grains. They’re not quite 100% quartz; the dark markings on them in the plane light photo (left) suggest they have some small amount of some opaque, probably metallic, mineral attached to them. Though it’s cracked, the upper grain is more-or-less a single crystal of quartz. You can tell that because the whole grain has the same birefringence in the crossed-polars photograph (right). The lower grain doesn’t look that much different from the upper grain in plane light, but crossing the polars reveals it is now composed of many smaller quartz crystals, each having its own angle of extinction. Again, this is recrystallization under metamorphosis. Under metamorphic pressure, the crystal, in many different areas at once, tends to reorganize into the tightest structure it can manage, creating many smaller crystals out of one larger one. Note that because quartz grains tend to be on the boxy side with irregular edges, extinction can’t be compared with growth habit to identify the mineral. I can’t show it in the picture, but quartz has a very unusual extinction habit that makes it trivially easy to identify: as the slide turns and the crystal goes dark, the color change proceeds in a wave across the crystal.

For something very different, here’s a pair of pictures of a metavolcanic sand grain:

A metavolcanic is a metamorphosed rock that was once lava. Lava is a very, very fine-grained rock, and even under the petrographic microscope it’s hard to make out the grains; it’s called a groundmass. But during metamorphosis, some of the tiny grains will organize into crystals. The long, skinny crystals in this grain are mostly feldspars; the stuff between them is still pretty much groundmass. Feldspars don’t necessarily grow long and thin, but they seem to do so during metamorphosis of volcanic rocks.

Finally, here are photos of my favorite rock, serpentinite. Alas, still photos can’t do it justice.

 
[For an early report on the project Karen's research has contributed to, see http://pubs.usgs.gov/sir/2004/5250/; for resources on microscopic petrography and the spectacular appearances of minerals in thin section, see http://serc.carleton.edu/NAGTWorkshops/mineralogy/optical_mineralogy_petrography.html and J.M. Derochette’s superbly illustrated site.]

Sunday sand: small details, large effects

As you may have guessed by now, I find sands and sandstones fascinating - fascinating and vital in so many different ways to our daily lives. But while, more often than not, they co-operate and fulfil the roles we need them too, sands can misbehave and frustrate our expectations and best-laid plans; today’s sand is one of those pesky ones, and a current source of some personal frustration.

All over the world, ancient sandstones deep below the earth’s surface act as reservoirs – vast storage containers for supplies of water, oil, and gas. In the case of water, such a reservoir is referred to as an aquifer, from the Greek for “water carrier.”As I wrote in Sand, in the infamous Chapter 9:

How many people in the world rely on water supplies flowing or pumped from underground? Controversial though the matter may be, how many of us rely on oil and gas flowing or pumped from underground? These critical resources do not occur in subsurface lakes, rivers, or caverns; they inhabit the microscopic holes in buried rock, very often the spaces between sand grains. Any rock that, like a sponge, is sufficiently porous to hold significant amounts of water or hydrocarbons is called a reservoir…. Ninety-five percent of America’s freshwater is underground, and a large proportion of it—30 percent of the water used for agriculture in the United States—is contained in one of the world’s great aquifers, the Ogallala. The Ogallala, the main part of the High Plains aquifer, is formed from essentially unlithified sands, silts, and gravels that were carried off the eroding Rocky Mountains during the last twenty million years. Ancient sand dunes form parts of the aquifer, and the Nebraska Sand Hills are built on the Ogallala sediments. The reservoir is huge: it underlies an area of about 450,000 square kilometers (174,000 sq mi), covering parts of Colorado, Kansas, Nebraska, New Mexico, Oklahoma, South Dakota, Texas, and Wyoming. It is close to the surface, easily accessible, and 300 meters (1,000 ft) thick in parts, with an average thickness of 60 meters (200 ft).

The Ogallala contains a staggering volume of water – enough to fill Lake Huron – but it’s not inexhaustible, and the resource is being used up faster than it is being naturally replenished.

So our resources of underground water and hydrocarbons don’t reside in great caverns, pools or lakes, but rather, very commonly, in the minute spaces, or pores, between the grains of an old sandstone. How well it behaves as a useful reservoir depends on both the volume of those spaces between the grains – the porosity – and how easily the fluids can flow out of those spaces – the permeability. The trouble is that, once a sand is buried below the surface, subjected to pressures of the overlying sediments, heated up, and saturated with waters laden with various concoctions of dissolved minerals, all kinds of things can happen to clog up the porosity and bung up the permeability. Today’s sand is a lesson in exactly what can happen – as revealed by the stunning details of the technology of imaging by a scanning electron microscope (best to click on the image to enlarge).

The sandstone in question is around ten million years old, and was deposited as sand banks in the shallow seas of ancient Sumatra. Just as it does today, Sumatra then lay on a major plate boundary and was routinely shaken by earthquakes and blasted by volcanoes. A variety of rock and volcanic mineral fragments mixed with the quartz sands carried to the seas, and warm waters with sometimes exotic chemical recipes coursed through the sands as they were buried – hence my problem. Ever since the early part of the last century when the Dutch discovered oil in Sumatra, many of these old sandstones have proved to be effective reservoirs – but they can be fickle and unpredictable.

The image on the left at the head of this post shows the grains of one such sandstone – a very fine-grained one, each grain less than a tenth of a millimeter across (the scale shows 10 micrometers, a micrometer being one thousandth of a millimeter – a human hair is typically between 20 and 80 micrometers in thickness). The grains are mostly quartz, but they are far from being smooth, clean and sparkling; instead, they are coated in what looks like crushed cornflakes. Look at those coatings in more detail in the image on the right (ten times more detail), and they resolve into masses of individual flakes and fragments – mineral grains. Beautiful clusters of flat, flaky crystals of mainly clay minerals. The sand originally contained mud and clay, but chemical cooking once it was buried has changed much of that clay, experimented with the volcanic material, and grown new clay minerals – which have effectively clogged up the pores of this sandstone. All that remains of initial porosity between the grains is the occasional pore space labeled as “IP”; in a very few places, original minerals have been dissolved away to form new, secondary, pore spaces – “SP.” The quality of this sandstone as a reservoir has been severely – if not totally – degraded.

Clay minerals come in an astonishing variety of forms, but they are dominantly aluminium phyllosilicates – mixtures of Al and silica, together with a wide variety of other elements such as magnesium and iron, water molecules bound into their structure. The “phyllo” part of “phyllosilicate” comes from the original Greek for “leaf” (as in today’s “phyllo dough”) – these minerals form the thin, flat, leaf-like plates that are clearly visible in the right-hand image. They are very much like the platy mica minerals, and the spaces between the molecular sheets can absorb all kinds of other molecules. Here’s a typical formula for a clay mineral:

(K,H₃O)(Al,Mg,Fe)₂(Si,Al)₄O₁₀[(OH)₂,(H₂O)]

Some clays, such as kaolin, so important for ceramics and other uses, are relatively well-behaved, but others, such as the evil-sounding smectite, absorb water and expand, bunging up the porosity further; as far as reservoir quality is concerned, smectite is, indeed, evil.

So our sandstone contains all kinds of clays, including kaolin (K) and smectite, plus “Sid” – siderite, iron carbonate that completely changes the electrical conductivity of the rock; since one of the ways in which such a possible reservoir is evaluated is through measurement of its electrical conductivity (water being more conductive than oil or gas), Sid plays vicious games with such measurements and makes them almost impossible to interpret.

So, sand misbehaving: clogged-up porosity, plugged-up permeability, and screwed-up electrical properties. One of my colleagues has a three-year-old granddaughter who classifies her world into things that are “good” and those that are “not good.” This is definitely “not good."

Dammed if you do

 
Several posts here have been devoted to talking about dams, the complex ways in which they change the surface processes of our planet, human relationships with them, and the controversies surrounding their construction and operation. As we read of the approval to construct the world’s third-largest dam in Brazil, the profound problems now emerging with China’s Three Gorges dam, and transnational issues around a planned series of dams on the Mekong River, I have, thanks to Science Daily, just come across an extraordinary resource.  The Digital Water Atlas is one of those inspiring international collaborative projects, the results of which are available for all to access:

The Digital Water Atlas is an activity of the Global Water System Project (GWSP). The Atlas is implemented at the GWSP International Project Office (IPO) that is funded by the German Federal Ministry of Education and Research (BMBF) and the Ministry of Innovation, Science, Research and Technology of the State of North Rhine-Westphalia (NRW). The maps and datasets included in the Atlas are in-kind contributions from GWSP partners (see credits for more details). We would like to thank our funders and our partners, especially the Center for Development Research (ZEF ) at the University of Bonn as our host organisation, for their kind support.

Just click the “credits” link to see how truly international this project is. Browse the selection of dozens of maps and you will be fascinated. The data includes not only the global data base of dams (the map shown at the head of this post and the technical documentation available here - click on the image for a higher resolution), but a remarkable range of maps and data that document water issues on a global scale:  “The purpose and intent of the ‘Digital Water Atlas’ is to describe the basic elements of the Global Water System, the interlinkages of the elements and changes in the state of the Global Water System by creating a consistent set of annotated maps. The project will especially promote the collection, analysis and consideration of social science data on the global basis.”

There is no need for me to say more, other than encouraging you to browse – oh, and, of course, showing this map: “Sediment Trapping by Large Dams.” The description is as follows:

A total of 236 regulated basins with 633 large reservoirs (> 0.5 km3 maximum storage capacity), which collectively represent about 70% of registered impoundment storage volume was used to estimate basinwide relative loss of suspended sediment destined for the world's oceans. For the purposes of display, the basins include both discharging and non-discharging portions of the land mass. A total of approximately 25-30% of pre-disturbance sediment flux is sequestered by modern impoundments, most built since 1950.

The residence time of water in large reservoirs and subsequent sediment trapping efficiencies is calculated as a measure of the impact of these man-made structures on the characteristics of river flow and sediment discharge to the ocean. Estimations of water removed from basins as diversions (i.e., interbasin transfers and consumptive use) also provide information on the impacts of diversions on river flow and sediment transport.

Note that this massive disruption to the global sediment budget reflects only suspended sediment – the bed load of rivers is challenging to measure (but also massive).

Dam builders and busters would do well to reflect on the data in the Digital Water Atlas. 

 

[Maps reproduced courtesy of  the "GWSP Digital Water Atlas (2008). Available online at http://atlas.gwsp.org."]

Sunday Sand: grains of tragic truth, June 6, 1944

In Sand, talking about the genetic history of families of grains, I wrote the following:

A traveler expects to sample a local cuisine that has its origins in local ingredients, and it’s no different with sand. Relaxing on a beach in North Carolina or the south coast of England, looking at the sand between your toes, you would hardly expect it to be made of bits and pieces thrown out of a volcano. Trekking along the coast of Greenland, you would be surprised if the sand were composed of the debris from a coral reef. What you do find are sand grains of predominantly local parentage. On the Normandy beaches where D-day landings took place, you will find sand-sized fragments of steel.

In 2009, for the first time, I visited the Normandy beaches – and the memorials and the cemeteries – and posted a brief report in September last year. I mentioned again the fragments of steel, shrapnel sand grains, but have never convincingly located any in the samples that I have looked through. Now the latest issue of Earth Magazine carries an article titled “D-Day's Legacy: Remnants of invasion linger in beach sand.” In it, two sedimentary petrologists, Earle F. McBride and M. Dane Picard, describe their visit to the D-Day beaches and the analysis of the sands from Omaha after their return to the laboratory – “Little did we know what we would find when we got home and started studying the sand.”

 A thin section of the sand revealed a large number of angular opaque grains that were magnetic. Shard-like, they were only slightly rounded. Some were well-laminated. These grains were also associated with small spherical beads of iron and glass. At first, we were uncertain of what we were looking at. However, in a few days, we concluded that the metal and glass particles were human-made — particles generated from the explosions of munitions during the Normandy landings. After further testing, we determined the sand does indeed contain 4 percent shrapnel and trace amounts of beads of metal and glass. Because waves and currents on any given day can selectively concentrate sand grains of a given specific gravity, we cannot be certain that our sample is representative of the entire beach — that shrapnel grains make up 4 percent of Omaha sand as a whole.

They go on to describe the likely origin of the glass grains as being from explosions in the sand – with the critical contribution of seawater salts as a flux that lowered the melting point of the quartz. Read the whole article, it’s well worth it.

So tomorrow, we remember – please – the events of that day in 1944. And perhaps should you visit those broad calm beaches as they appear now, think of that granular legacy, those witnesses beneath your feet. 

Strange and wonderful designs

 
Just what exactly is going on here? There is, as far as I know, no answer to that question – and that simply adds to the sense of wonder, of mystery. I first used this image in Sand, as an illustration of the intricate and unfathomable artistry of nature and sand; it came from Martyn Gorman who has the good fortune to live close to  a wild stretch of dunes on Scotland’s east coast. His photographs are spectacular, and showcased on his site, where he writes that:

I am fortunate to live within sight of the fabulous Sands of Forvie on the northeast coast of Aberdeenshire in Scotland; a complex of mobile sand dunes and beaches of great beauty and scientific interest. I never cease to be fascinated by the constantly changing forms and patterns to be seen in the sand and by the plants and animals that live there.

This particular photo was taken in 2006; recently he wrote to me with the words “They have returned!” and attached this image:

So this is clearly not a one-off phenomenon: something is going on, a conspiracy of sand and physics - and maybe a little chemistry - that inevitably results in these complex and compelling sculptures. But I can only repeat the question – what exactly is going on here? Isn’t it wonderful?

 

{Images copyright, and reproduced with thanks with the permission of, Martyn Gorman; make sure you visit his site.]