Sand

• 1. Introduction

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  Sand is running out. Fifty billion tons of sand and gravel are dug up every year, according to a 2022 report from the United Nations Environment Programme. That makes sand, a material so ordinary it slips through your fingers, a non-renewable resource over the span of a human life. The grains you stand on at a beach took thousands of years to form. The grains in a desert, by contrast, are useless for the thing the world wants sand for most. So what exactly is sand, and why can one kind build a city while another kind cannot hold concrete together? The answers reach from coral reefs to quartz crystals, from an ancient Greek thought experiment to artificial islands in the Persian Gulf.

• 2. What Counts As A Grain

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  The diameter of a grain is what separates sand from everything else. The Unified Soil Classification System, used in engineering and geology, defines sand as particles between 0.074 and 4.75 millimeters across. Geologists draw the line differently, placing sand between 0.0625 millimeters and 2 millimeters. Below that range lies silt, with particles as small as 0.004 millimeters. Above it lies gravel.

  Archimedes counted grains in The Sand Reckoner, written around 240 BCE, and those grains measured 0.02 millimeters in diameter. That same 0.02-millimeter figure was treated as sand under the Albert Atterberg standard used in the early 20th century. A 1938 specification from the United States Department of Agriculture set the cutoff at 0.05 millimeters. A 1953 engineering standard from the American Association of State Highway and Transportation Officials raised the minimum to 0.074 millimeters.

  ISO 14688 grades sand as fine, medium, and coarse, running from 0.063 millimeters up through 2.0 millimeters. In the United States, five sub-categories run from very fine to very coarse. These divisions follow the Krumbein phi scale, where size is calculated as the negative base-2 logarithm of the diameter. On that scale, sand spans phi values from minus one to plus four. The simplest test needs no instrument at all. Sand feels gritty when rubbed between the fingers, while silt feels like flour.

• 3. The Many Colors Of Sand

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  Silica, in the form of quartz, is the most common constituent of sand in inland and non-tropical coastal settings. Quartz survives because it resists weathering, owing to its chemical inertness and its hardness. The second most common type is calcium carbonate. One form of it, aragonite, has been built over the past 500 million years by living things such as coral and shellfish, and it dominates places like the Caribbean where reefs ruled the ecosystem.

  Gypsum produces the bright white dunes of White Sands National Park in New Mexico, where the sand is a form of calcium sulfate. Selenite is another calcium sulfate variant, found at sites such as Salt Plains National Wildlife Refuge in the United States. The bright white sands of tropical coasts are eroded limestone, sometimes carrying coral and shell fragments, a sign that some sand depends on living organisms.

  Mineral content paints the rest of the spectrum. Sands rich in magnetite run dark to black, as do those from volcanic basalts and obsidian. Chlorite and glauconite turn sand green, as does basaltic lava high in olivine. Across Southern Europe, iron impurities locked inside quartz crystals give sand a deep yellow. Arkose is sand or sandstone heavy with feldspar, weathered from a nearby granite outcrop. Some deposits even hold garnets and small gemstones.

• 4. From Rock To Beach

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  Granite is the most common rock to form sand. Within it, feldspar minerals dissolve faster than quartz, which breaks the rock into small pieces. In high-energy environments, rock falls apart much faster than in calm ones, and the rushing pace leaves more feldspar behind because it has less time to dissolve. Geologists call sand made this way epiclastic.

  Rivers carry most of the sand used in construction, gathered from the riverbed or its flood plain. The demand has emptied many small rivers, harming the environment and the land beside them, because sand is removed far faster than it can return. The Sahara Desert tells a different story. Lying near the equator and starved of water and vegetation, it lets the wind strip away clay and dead organic matter, leaving sand and larger rocks. Yet only 15 percent of the Sahara is dunes, while 70 percent is bare rock.

  Beach sand forms where waves grind rock near the shoreline over thousands of years, helped by river deposition and by marine animals eating algae off the stones. Once enough builds up, the beach becomes a barrier against further erosion. Marine sand comes from sediment washed into the ocean and from eroded ocean rock, and it tends to gather closer to land. Europe is the main miner of marine sand, a practice that damages ecosystems and local fisheries.

• 5. Reading A Single Grain

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  A single grain holds its own history. Quartz freshly weathered from granite or gneiss stays angular, and builders call it sharp sand and prize it for concrete. In geology the same material is grus, and gardeners spread it to loosen heavy clay soils. Grains carried long distances by water or wind come back rounded, marked with telltale abrasion patterns. Desert sand is typically rounded for exactly this reason, smoothed by the wind that shaped it.

  The people who study sand for pleasure have their own name. Collectors who gather sand as a hobby are called arenophiles. The organisms that thrive in sandy ground are called psammophiles.

• 6. A Thousand Uses

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  Glass begins as sand rich in silica, the principal component of common glasses. Quartz sand is also the raw material for producing silicon. Concrete leans on sand as a principal component, and bricks are made by adding sand to clay before firing. Mortar mixes sand with masonry or Portland cement and lime. Cob, an old building material of water, straw, lime, and subsoil, can be as much as 75 percent coarse sand.

  Sand abrades, filters, and protects. Before sandpaper, wet sand was an abrasive between rotating surfaces, used to make stone vases or strip old stain from copper pots. Graded sand still works as a sandblasting abrasive, and media filters and rapid sand filters clean water with it. Sandbags shield against floods and gunfire, cheap to ship empty and quick for untrained volunteers to fill. In hydraulic fracturing, rounded silica sand acts as a proppant, holding open the cracks the process creates.

  The gentler uses run just as wide. Sandy soils drain well and suit watermelons, peaches, peanuts, and intensive dairy farming. Aquariums use sand as a low-cost base, and saltwater reef tanks need aragonite sand to mimic the wild. Engine drivers and transit operators scatter sand to grip the rails, and road crews spread it for traction on ice and snow. Children build sand castles, proverbially impermanent, in the sandboxes of public playgrounds. In classical and medieval warfare, sand was once heated and poured down on invading troops.

• 7. The Cost Of Digging

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  Sand suitable for concrete is in high demand, yet desert sand cannot do the job. Grains rounded by the wind do not lock together to form solid concrete, while rough sea sand does. As populations and cities grow, the demand for the right kind of sand climbs while natural sources run low. In 2012 the French director Denis Delestrac made a documentary called Sand Wars about this shortage, tracing both the legal and the illegal trade.

  Hydraulic dredging pumps the top few meters of sand from the water into a boat for processing on land. Every marine creature pulled up with the sand is killed, and the damaged ecosystem can suffer for years. The losses ripple into fishing industries and the communities at the water's edge. Removing sand also raises the risk of landslides, which can swallow farmland and damage homes.

  The scale is enormous. The global demand for sand in 2017 reached 9.55 billion tons, part of an industry worth 99.5 billion dollars. China, Indonesia, Malaysia, and Cambodia have banned sand exports over these concerns. The 2022 UNEP report offered ten recommendations, among them a ban on beach extraction and a move toward a circular economy for sand and gravel. Manufactured sand, known as M sand, offers one alternative, made from crushed rock and more angular than river sand.

• 8. Quicksand And Imported Shores

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  Quicksand forms where pore water pressure runs high and sand meets salt water, creating a colloid hydrogel that behaves like a liquid. Creatures trapped in it often die from exposure rather than from sinking. Beachgoers face a quieter danger too. A hole dug too deep in the sand can collapse and cause serious injury or death.

  Sand also carries a workplace hazard. Bags of silica sand for sandblasting now warn users to wear respiratory protection, because safety data sheets state that excessive inhalation of crystalline silica is a serious health concern.

  When a place runs short, it imports the shore. In Dubai, the sand needed for infrastructure and for the Dubai Islands outstripped local supply, so sand was brought in from Australia. The artificial islands swallowed more than 835 million tonnes of sand, at a cost above 26 billion dollars, a reminder that the cheapest material on the beach can become one of the most expensive things a nation buys.