Today we have a special post here at
NOVA Geoblog. Author
Michael Welland joins us to answer a bevy of questions about the topic of his expertise: sand. Michael's book
Sand: The Never-Ending Story is now out in paperback, and this post is one stop on his "virtual book tour" through the geoblogosphere. I really enjoyed reading
Sand, and I reviewed it last year in
EARTH magazine (hardcopy only, I'm afraid: no link possible). I was tickled to see that
a quote was mined from my
EARTH review for the back cover of the paperback edition of the book, a first for me.
If you have a sand question to ask, leave it in the comments, and Michael can respond there, too.
This post also serves to kick off "Book Month" here at
NOVA Geoblog. All this month, I'll be blogging about the books that I have read recently.
All questions in this post come from my spring 2010 Physical Geology students. Enjoy! -
CBWhat are people who collect sand as a hobby called?"Odd!" ...They are also often called arenophiles, "sand lovers" but strictly, that's a mixture of Latin and Greek - "arena" is the Latin for sand. To be consistently Greek, they should be "psammophiles" but then that term tends to be used by biologists and botanists to describe critters and plants that live in the sand.
How come sand gets everywhere when you go to the beach? And by everywhere, I mean everywhere...sealed Ziploc bags, inside cell phones, places you don't want it...everywhere. Or, on a more scientific sounding note-beaches that are eroding. Is it due to sand being swept out by the ocean waves faster than it can be replaced? (How is sand formed?) If that's the case-why do some beaches erode faster than other? Is it because of the width between the dunes(?) and the ocean?Boy, do I know about sand getting everywhere! After my Sahara travels, my backpack pockets still contain sand, my camera zoom makes uncomfortable grating noises, and I had a hell of a time explaining to my cellphone company why there were sand grains under the keys (I'd stupidly used its calculator for map scale conversions when we were trying to figure out where we were). I guess this problem is just another of the strange behaviors of granular materials in that size range.
Now, as for beaches, they are just about the most dynamic environments on earth, changing every day, with the seasons, with every storm, and with changing sea levels. Check out the story of the wholesale move of the Cape Hatteras Lighthouse. Every beach has a sediment budget - incomings and outgoings that constantly change. Sand is added to the beach (but also carried away) by longshore drift and by the action of every wave. A storm will erode unimaginable amounts of sand - which is then deposited elsewhere. Sand blows off the beach and onto the dunes, or off the dunes and onto the beach - an incredibly complex system. Typically, at certain points along a coast sand will be swept into the head of a submarine canyon and flushed out into the deep sea - essentially never to return (check out, for example, the Monterey Submarine Canyon).
Here are a couple of interesting photos of where sand ended up after Hurricane Katrina:
And here are three photos of Dauphin Island, Alabama, before Hurricane Ivan, after it, and after Hurricane Katrina - spot the differences!
Over the course of several thousand years, can a black sandy beach turn into a white sandy beach? I have heard about green sand. Is green sand existent and where can you find it? What elements is sand made out of ? ...And why does its color change in other places? In talking about the definition of sand in terms of size rather than composition, and the different types of sand, I've begun to answer these questions. Just as the cuisine of a local restaurant is dominated by local ingredients, so is the composition of a sand in any one place. You wouldn't expect to find beaches of coral fragments in Greenland, or sand grains of old metamorphic rocks in Hawaii. The sands of a particular beach may have been carried a long distance by rivers and coastal currents, but they originate from the same system, a system that is stable over long periods of time. The river and beach sands of the east coast of the US tell the story of the erosion of the Appalachians and the effects of the Ice Ages. So no, a black sandy beach will not turn into a white sandy beach over the course of a few thousand years - but if, over the course of a longer period of time, different source rocks are exposed in the areas where the sand grains originate, or if there is a major reorganization of sand-transporting currents, then the beach composition will change to reflect that. We see this all the time in the geological record - changes in sand composition that tell us about changes in provenance, tectonic activity and so on.
But then, as I was writing this, I suddenly remembered an example of a black beach turning into a normal beach - overnight! If you go to any beach and look at the ripples in the sand, there will generally be smears of dark-colored grains emphasizing the forms of the ripples. These are grains of heavier minerals, often iron oxides, which are winnowed by the action of waves because of their weight. If this winnowing, by waves, currents, or rivers, takes place over a long period of time, then considerable concentrations of heavy minerals can result; these deposits, called placers, can be commercially important and are the sources of diamonds, gold and many other important mineral commodities. But on a beach, just as easily as such a deposit can form, so can it be removed by a storm; I described an example of this in the book:
...such an occurrence was the cause of one of Thomas Edison's many business failures. On a fishing trip with friends off the coast of Long Island, Edison put into shore for lunch and found the beach covered with a layer of black sand. He took some home and discovered that the black grains were a magnetic iron oxide mineral - magnetite - which stuck to a magnet while the common sand grains fell off. Edison's enthusiasm ran, as it often did, ahead of his business sense, and he immediately arranged for the purchase of the beach and the manufacture of separating machinery. Unfortunately, by the time he and his colleagues returned to Long Island, a winter storm had reworked the beach and completely removed the black sand.
So there's a case of a black sand beach disappearing!
Green sand - yes, it exists, most famously on Hawaii. Local ingredients again, the volcanic rocks contain crystals of the apple-green mineral olivine, and this can become concentrated on some beaches to become the main constituent of the sand - placers again. Here's an example from the Big Island:
Sand comes from sediments that are carried down from rivers that come from mountains. So is sand a mixture of minerals and dirt? ...Or just a lot of different broken down minerals? I just watched something about China's issue with being continuously battered by massive sandstorms. What I want to know is: Where is the sand coming from? Why is it hitting China, and what are the health risks/other consequences of these sandstorms? The breakdown of rocks by chemical and physical weathering and the transport of the debris by rivers is the most common origin of sand - but it's not the only one. Beaches in the tropics are made of sand that is biological in origin - shell fragments, bits of coral, and the shells of minute organisms. Which brings us to a key point: Sand is defined purely by size.
This is for a very good reason - granular materials that fall in this size range behave very differently from things smaller and things bigger. And those behaviours are often bizarre. It really doesn't matter what the sand is made of, its composition. And note that, reflecting the fact that nature works in multiples, each category is twice the size range of the next smaller one. So, it doesn't matter if the material is made up of 1 mm quartz grains, shells, diamonds - or sugar - technically it's coarse sand. It can be made up purely of quartz or purely of foraminifera shells, or a mixture of minerals and rock fragments, or a mixture of coral and shell fragments - it's all sand. Some beaches in the tropics are made up almost entirely of sand-sized pellets of dried fish shit.
Dirt is the non-technical (and mostly American) term for soil, and soil is the in situ material that results from the conspiracy of chemical and physical weathering and organic material and activity. Once it's eroded and transported by wind or water, it isn't dirt any more. So, technically, the dirt in much of Nebraska is sand (the Sand Hills support only very poor vegetation); once the dunes become active again (perhaps as a result of changing climate) that sand will be on the move once more.
And that's a big part of what's happening in China. More than 2.5 million square kilometres (a million square miles - more than four times the area of Texas) of the country is desert, and so sand and dust storms have always been a problem. But poor land management on a massive scale - removal of forests, over-grazing, and soil-depleting agriculture - has made the problem worse. What had been stable soil, dirt, is now exposed to the winds and on the move - very much like the dustbowl conditions of the American Midwest in the 1930s. The total area of China's deserts is growing at around 200 square kilometers (80 sq mi) every month, and every year tens of thousands of tons of sand and dust are blown into Beijing. China's capital has always suffered from dust storms, helped again by the ice age, when grinding glaciers wore rocks down to flour, technically known as loess, which, once airborne, blankets huge areas for long periods of time. But Beijing's dust storms are turning into sandstorms. It's not necessary to travel to the Gobi Desert to find encroaching sand; it's a mere hour's drive out of Beijing. The Great Wall, built to defend against invaders from the west, is proving no match for the onslaught of sand: whole sections are being destroyed by the storms.
So the sand is coming from the interior deserts, but more and more from degraded landscapes that are newly becoming desert. The problems for homes and infrastructure are enormous, but so are the health risks in populated areas from particulates - hence the dramatic measures taken at the time of the Beijing Olympics.
How come there is so much sand on the shores and coastlines of the earth, and absolutely none as you move toward the middle of a country or continent?Don't tell the residents of Nebraska that - a quarter of their state is covered in sand dunes! The Sand Hills are the largest area of dunes in the western hemisphere, covering 60,000 square kilometres and were active and mobile between 1000 and 1200 CE. They were formed originally from the debris of the glacial erosion of the Rocky Mountains. The hills were stabilized eight hundred years ago but have had episodes of reincarnation since: a long drought toward the end of the eighteenth century resuscitated dunes on the Great Plains, whose activity caused problems for the westbound wagon trains decades later.
You can find more imagery and information here. And think about the great active deserts of the world where huge amounts of sand are to be found - the Sahara, the Arabian Peninsula, essentially the entire interior of Australia, a quarter of the land area of China. But it's not just the deserts - every river bank and lake shore has sand. About the only places on the earth's surface where sand is rare are the very deep ocean floors - and it's rare but not absent. Sand can be dropped from melting icebergs and flushed out into the deep oceans by the tremendous energy of turbidity currents, slurries of water and sediment hurtling down the continental slopes and spreading out across the deep ocean floors; any of the great deep ocean currents can move sand around - and they do. And there are countless sand grains in your back yard, mixed up in the soil.
Here's a Google Earth image of Nebraska:
Hoping this question related enough to sand, because I'm still under the impression that all/most glass is made of sand. I've heard that in stained glass windows inside ancient churches, there is a "bulge" at the bottom of the glass, and that because of its disorderly atomic structure, gravity can "pull" it down a bit after hundreds of years. I've also heard that this is totally wrong, and that the bulge is just an effect of the way they made stained glass at the time. I'd love it if your friend could shed some light on the issue.
It's an old and common "urban myth." Although glass does flow, the timescale over which it happens is far too long for even the oldest windows to show any effect. Old methods of making glass did not create perfect sheets and, logically, if a piece is thicker at one end than the other, you would install the thick end at the base of the window. There's a good article about the myth on Corning's website.
What makes quicksand so powerful that it can drag a human down?
Another "urban myth!" There's a great episode of Mythbusters that examines "killer quicksand" and has them bobbing around in a giant tub of quicksand, trying to be sucked in. They bust the myth: "Quicksand is denser than water; the greater the density, the greater the buoyancy of objects within. Any victims found in quicksand likely died for some other reason (i.e. exposure to the elements)." You can watch the episode here.
Quicksand is an example of one of the strange behaviours of sand-sized granular materials - dilatancy. Here's a bit about this from the book:
Quicksand forms when there is sufficient water in between the grains to separate them - to push them apart through dilatancy - but the water is prevented from draining; the sand is in suspension. This can happen when an incoming tide scours large holes in the sand that are rapidly filled by the outgoing tide, trapping water and air in the sand. Or a subsurface spring or other source of water percolates upward through a body of sand, dilating it. The result is a slurry, delicately balanced between solid and liquid, switching instantly but briefly between the two states with the slightest disturbance. But being a mix of water and sand, quicksand is more dense than water, and the human body floats well in it. The problem arises when a person floating in quicksand tries to move too quickly; the movement destroys the dilatancy of the slurry and the grains reconvene and jam back into a solid, effectively cementing the unfortunate person in place. It has been estimated that the force needed to pull your foot out of jammed quicksand is about that needed to lift a medium-sized car. The key is to wiggle, allowing water to fill the space created around you, and then swim, very slowly. Quicksand is lethal because lone individuals die of exposure, starve, or drown when the tide comes in, not because they are sucked under.
What is the calculated estimate of the amount of sand on Earth?
Essentially impossible to calculate - particularly if you include all the sand grains in ancient sandstones. But that hasn't stopped people having a stab at it. I wrote a bit about this in the book:
In 1980, Carl Sagan, the enthusiastic popularizer of all things astronomical, kicked off one of the most enduring, entertaining, but quantitatively pointless debates about large numbers. He declared, in his television series Cosmos, that "the total number of stars in the universe is larger than all the grains of sand on all the beaches of the planet Earth." The calculations are ongoing and the debate rumbles on, particularly in the ethereal realms of the internet, and there are, predictably, two schools of thought. While estimates are always increasing, the number of stars is the easier number to calculate: anywhere between 1020 and 1022. As for the grains of sand - well, it depends. What are the assumptions in terms of grain size and, indeed, what counts as a beach? Only the areas of sand above high tide, or areas underwater as well? Depending on how you choose to do the calculation, you can derive a number that is larger or smaller than 1022. And if all the sand grains of the Earth are included, not just those on beaches, then it's again a different matter.
Is sand from different places unique enough for someone to determine where it came from?
Absolutely - to the extent that sand stuck under a vehicle or in the sole of a shoe can and is used in criminal forensics. Simplest if I quote the section on this from the book:
With the sophisticated microscopic diagnostics now possible, the character of soil and sand as evidence in a wide variety of criminal cases has taken on increasing significance. There are crimes that rarely make the headlines, such as cactus smuggling, that can be routinely solved by being able to point to the origin of sand clinging to the roots of the contraband. Investment scams where evidence for a new gold prospect is "salted" with grains of gold from elsewhere can be uncovered by a microscopic look at those grains.
A significant amount of the world's gold supplies comes from the sands of ancient and modern rivers. In 1997 a shipment of these grains of gold worth $3 million was made from mines in the interior of Ghana to the coast and then on to London for processing. After a dispute over the arrangements and cost, the shipment was moved on to Canada via Amsterdam. Canada was the first place where the crates were tagged and given new seals. When they were eventually opened, they contained ordinary sand and iron bars. Where on the shipment's circuitous route had the substitution taken place? The sand was examined by Richard Munroe, a Canadian forensic geologist and policeman. If the substitution had been made in London or Amsterdam, the sand would likely bear the imprint of its northern European origins - particularly the action of ice from the glaciers that had so recently sculpted the continent. But none of those signs were there. Instead, the grains bore the distinctive features of being subjected to a tropical climate, and their composition was typical of the geology of the interior of Ghana. While local security difficulties prohibited making an exact match of the sand grains, any Canadian involvement was ruled out and the insurance claim filed by the mining company was dropped. Sand is a popular material in crimes of "substitution"; in the lively commerce between North and South America, sand has been substituted for, among other goods, cigarettes going south and perfume going north. The genetic fingerprint of the sand involved has pinpointed the location of the crime and helped prove innocence and guilt.
Sand and soil found in the soles of shoes, on clothing, or on tires can place people or vehicles in a particular place - however much they may deny it. Geology has become a standard tool in the kit of government forensic laboratories the world over, but it has been around for some time. The fictional Sherlock Holmes claimed to be able to describe an itinerary from mud splashes on trousers. In real life, evidence from sand has been used for over a hundred years. In 1908, in Bavaria, a poacher was suspected of murdering a young woman. His wife had cleaned his shoes the day before the murder, but they now had three layers of sand and soil stuck on their soles. As part of the investigation, one Georg Popp, a local chemist, applied his geological expertise to these layers. He reasoned that the layer next to the sole of the shoe was the oldest; it was made of the same materials as those outside the suspect's house. The second layer contained red sand and other materials identical to those from where the body had been found. The last and most recent layer contained brick fragments, cement, and coal dust that matched samples from where the suspect's gun had been found. What this layer did not match was the soil from the fields where the suspect claimed to have been walking at the time of the murder. The prosecution case was complete.
On a dark, rainy night in September 2002, a black truck parked beside the Shenandoah River in Virginia. Another truck pulled up, and the window rolled down to reveal the barrel of a shotgun. The driver of the first truck was killed at point-blank range. The murderer left in a hurry, the wheels of his truck spinning in the sand and gravel. After a preliminary investigation, the police had a suspect but insufficient evidence to prove guilt. When the suspect was seen starting to wash his red pickup truck, the police swooped. The truck was spattered with fresh mud: time to bring in the forensic geologists. The mud contained some very distinctive sand grains, a variety of minerals that could only have come from a local quarry. While the quarry was not where the murder had taken place, water washed debris from the quarry into the river, which carried it downstream, mixing and diluting it with the other sand and mud in the river. At low water levels, these were dumped in sandbanks along the river's edge. Geological sleuthing demonstrated that each successive sandbar downstream from the quarry contained less and less quarry debris, and the only one that precisely matched the material from the suspect's pickup was the scene of the murder. The suspect pled guilty in the face of this incontrovertible evidence.
Forensic geology has played a part in a wide range of criminal cases worldwide, but perhaps the most high-profile, yet disappointing, example was the murder of the Italian prime minister Aldo Moro. In May 1978, the body of the kidnapped prime minister was found in a car in Rome. Sand from his clothes and shoes, and from the car, was shown to have come from a particular stretch of beach near the city, yet searches of the area provided no evidence. Other forensic work confirmed the association with this beach, yet the connection with the suspects could not be proved. Years later, the kidnappers declared that they had planted the beach sand as a decoy - whether this is true or not remains unclear.
The world's first database of sand grains has been assembled from soils in the United Kingdom, specifically for police forensics. This database contributed key evidence for one of the country's particularly appalling recent criminal cases, the murder of two young Cambridgeshire schoolgirls in 2002. Once again, distinctive soil under the murderer's car tied him to the location where the victims had been
buried.
I know we use sand for glass (and in turn all products that use glass), however, are there any odd or interesting uses for sand that people don't usually know about? Are there any surprising or "out there" uses for sand?
Well, how about computer chips? And all the important minerals (as well as gold, diamonds, sapphires, rubies and garnets) that are found as placer deposits? These include titanium, tungsten, tin, platinum, and niobium. Sand is used as a filter, as a casting method in foundries, in different specialist sports surfaces. Silica and silicon products are used in pharmaceuticals, cosmetics, paper, and paint. Oh, and don't forget concrete.
Is sand a good medium for fossilization to occur in, and if so what signatures would a fossil show in relation to being formed in sandy soil?
Sand is not a great "fossilizer" compared to mud, for example, simply because sand is deposited in very dynamic and high energy environments - beaches, rivers and so on - and is constantly being re-transported by currents and moved on. Organic remains are easily damaged and broken up. That said, a lot of fossils are found in sandstones and trace fossils, footprints, tracks, trails and burrows can be quite common.
Because of the large size of rocks, it can be easy to use distinguishable features to date and classify them by. Can sand grains be dated in the same fashion as a large slab of granite, or do they need a more precise way to measure how old they are?
Dating a rock like granite involves finding the age of cooling and crystallisation of individual minerals through the radioactive isotopes or fission tracks they contain. Some minerals are better for this than others - quartz doesn't contain much in the way of radioactive components, so isn't much help for this. But some minerals that end up as sand grains are - zircon grains are the classics. They are tough as old boots and preserve a very good record of their cooling history. The oldest earthly possessions that we have are zircon sand grains from sandstones in Australia - they're up to 4.2 billion years old. But that's the age of the mineral grains, not the sandstone that now contains them.
One thing we can do with quartz grains is measure how long they have been exposed to cosmic radiation - e.g., how long a sand grain has sat on the surface of a place like the Atacama Desert. There are various different methods all of which are referred to as luminescence dating. It takes an awful lot of work but the results can be amazing - the Atacama has been a desert for far longer than we thought, for example. The USGS has a very good section on the different methods. It's a powerful method for unravelling the history of a landscape and for archaeological research. One example that I particularly like relates to prehistoric rock paintings in Australia. There are no materials that allow dating of the paintings, so their age is often a mystery. However, there are places where now-vanished wasps built their nests over part of a painting, using sand grains in the process. Those sand grains can be dated using luminescence methods and thus give a minimum age for the rock art. I came across exactly the same thing in the Sahara:
I remember sculptures of sand being struck by lightning can occur on beaches at times. My question focuses on how lightning makes sand form into random shapes of "glass" like structures? And when lightning does strike sand, does it have to be a certain type of sand?
It really is glass - the instantaneous and intense heat from the lightning fuses the sand grains into a solid structure, often a tube, and the result is called a fulgurite (from the Latin for lightning). This can happen in essentially any kind of sand and they can be really big - the record is one 17 feet long. They can also be found in ancient sandstones - lightning strikes from a couple of hundred million years ago. Silica glass was also formed by the energy of the first nuclear test explosion at White Sands in 1945 - the sandy soil was fused into glass by the heat. Fulgurites come in all shapes and sizes - the one here is fairly typical:
CB again: Have any additional sand questions for Michael? Leave them in the comments! If you found this blog post interesting, I recommend Michael's book. I have a copy I can loan to NOVA students, or better yet, you can buy your own!
Labels: books, nova, sediment
