The Life of Sand
Sand is the consummate shapeshifter, marking time as it sifts through an hourglass, forming ephemeral wave patterns on a beach, squeezing between one’s toes or even morphing into the concrete of our buildings. Yet few realize sand’s fascinating story. What is sand, and where does it form?
Boulders to dust
No omnipotent being gave tablets to humankind specifying how to group rocks by size, so geologists have filled the void and created categories for different “grain” sizes.1 Sand grains range from very fine (about as wide as a thick human hair) to very coarse sand (equal to the thickness of a US nickel).
Smaller still are silts and clays. The smaller grains can be roughly distinguished by how they feel when moist and rubbed between fingers – sand is gritty; silt is slippery; and clay is sticky and thick.
In the beginning
Sand begins as larger rocks or hard biologic material like shells or corals. However, the image of a large boulder tumbling down a mountainside until tiny grains remain is misleading – fragmentation by wind, water, freeze-thaw cycles, or other erosive actions are the dominant processes.
Sand’s birthplace is often far from where it is found. Rivers, streams, and wind transport grains eroded from distant mountains to the coasts or deserts.
Granite is one of the most common sources of sand, and its granular nature favors eroding directly into small pieces.
The many faces of sand
The most common sand is abiogenic – that is, of nonbiological origin, like granite – and primarily composed of white or translucent quartz grains. Quartz is a hard mineral and will outlast other softer materials, even if the parent rock contains other minerals. For example, granite contains quartz, feldspar, and mica. Young granitic sand will reflect granite’s mineral composition and include grains of the constituent minerals. Feldspar and mica are relatively soft, so these grains will disappear over time. Younger grains viewed with a hand lens tend to have sharper edges (compare recently broken glass to a piece of smooth, well-worn sea glass).
Other abiogenic sources form beaches of green olivine sand, black lava sand, and red garnet sand. Olivine sand is rare despite being a common mineral because it is comparatively unstable when exposed to the environment.
Biogenic sands – having living or once-living components – aren’t always obvious. Without a hand lens, the remains of the organisms that make up the sand may be too small to see. For example, coral fragments that pass through a parrotfish’s digestive tract or sea urchin spines on a Maine beach may go unnoticed by a casual observer.
These sands tend to be much softer and will breakdown much more rapidly than abiogenic sands. This is balanced by the relatively rapid formation of hard biological remnants like shells. The end result is that these soft sands don’t simply disappear – they’re replenished as quickly as they erode.
Up close and personal
Mineral sand from Maine that contains primarily white or translucent quartz grains. These grains are about 0.5 mm (0.02 inch) in diameter.
Quartz sand can have other colors when coated with a thin layer of minerals like the red hematite common in Utah’s red sandstone
Biogenic sand from Maine has animal remains – shells and green sea urchin spines – and quartz and pink feldspar grains. The green spine is 2-3 mm long.
In the Caribbean Sea, poop from parrotfish grazing on coral contributes to the formation of coral-sand beaches.
Dark, reddish-black magnetite sand from Grand Manan Island in Canada has a high iron oxide content. These grains are about 0.3 mm (0.01 inch) in diameter.
Olivine sand on Papakōlea (Green Sand Beach), Hawaii, comes from a nearby, eroding volcanic cone. The large gemlike grain is nearly 5mm in diameter. [Note that it is illegal to collect sand from Hawaii.]
Many sand grains have characteristics that a hand lens won’t reveal.
Magnetite is an iron ore that forms a black sand that is attracted to magnets. Some species of bacteria make tiny magnetite crystals that orient the bacteria like a compass needle in the earth’s magnetic field. Similar crystals have been proposed as the basis for the internal compasses that organisms like sea turtles and some birds use to navigate. The jury is still out on this idea.
One can use this characteristic to separate magnetite grains from other minerals in a sand sample.
The Meiobenthos4 - tiny lifeforms in the sand
Beach sand provides habitat for many organisms – birds, sea stars, crabs. Sand also teems with less familiar, minuscule life – the meiobenthos. Communities of these tiny organisms live between grains of sand under the influnce of tiny water currents and movements in their environment. They are barely visible, roughly 0.045-1 mm in size, and include animals like the charismatic waterbears (tardigrades), very young clams and marine worms.
Meiofauna include representatives from nearly two thirds of all animal phyla and serve as critical links in food webs nestled between smaller organisms they consume like bacteria, and larger animals like crabs and small fish that prey upon them.
The mollusk in the photograph below is 0.5mm long and is the size of the nearby quartz grain. Protozoa also live in these spaces; many move so quickly they are often seen as a tiny blur.
Where is all the sand, and where does it go?
At least 10 million cubic miles of sand exist, or enough to cover all the land to a depth of about 280 meters ( 920 feet). That’s a lot of beaches. But beaches don’t contain most of the world’s sand. The bulk of oceanic sand accumulates in shallow water offshore. Finer grains like silts or clays collect in deeper waters – most of the ocean bottom is not sandy. The lion’s share of the earth’s sand is in deserts and underground deposits.
Sand doesn’t stand still. Giant dunes hundreds of feet high form and move, pushed by desert winds. Beaches move with ocean and river currents.5 Ultimately, sand may weather further and become smaller particles or even dust. Dust from the Sahara Desert can be seen in satellite photographs blowing across the Atlantic Ocean and has wide-ranging influences on global weather patterns.
Sand may solidify into sandstone and cycle back to sand – one can imagine a fortunate, reincarnated grain that is compressed into sandstone, uplifted into a mountain range, eroded back to a sand grain multiple times. The physicist James Trefil notes that a handful of sand may contain the first grain of sand ever produced on earth.6
If transported deep into the intense heat and pressure of the earth’s mantle, the sandstone may melt and transform into other types of rock, thereby entering a different part of the rock cycle.
With all this said, we may be running out of beach sand. Annually, we use about 50 billion tons of sand, mostly from oceans, rivers, and lakes,7 to make things like concrete in the building industry. In some countries, demand for sand has spawned black markets centered around illegal sand mining.
Sand versus stars
Stars and grains of sand epitomize vast numbers. Stargazing under a dark sky elicits the question, “How many stars are there?” A walk on a beach leads one to ask, “How many grains of sand are there?” And then, “Are there more grains of sand than stars?” The question may be as unresolvable as “How many angels can stand on the head of a pin?” Not surprisingly, many philosophers and scientists have asked the question and reached different conclusions.
Rachel Carson wrote, “In every curving beach, in every grain of sand, there is a story of the Earth.” So, the answer to the sand versus stars question is less important than the story itself. The next time you walk on a beach or hike in a desert, take a moment to look closely at the sand that is supporting you – there is a world in it.
1 “Grain” can refer to a virtually invisible clay particle a thousandth of a millimeter (~4 ten-thousandths of an inch) in size to a boulder greater than 256mm (~10 inches) in diameter.
2 Sandstone is a sedimentary rock formed from sand under pressure and heat.
3 Using a hand lens with polarized light is difficult. These images were taken using a polarizing microscope with the grain between two polarizers.
4 Pronounced “my-oh-benth-ose”
5 Despite our attempts to limit beach movement with jetties, seawalls, and other beach stabilization structures, beaches continue to change. Our actions can interfere with natural beach movements degrading local ecology and the beaches they were designed to protect. Some communities spend millions of dollars to dredge offshore sand for beach restoration.
6 Admittedly, the likelihood of this occurring is vanishingly small.
7 The vastly more plentiful desert sand is not used in concrete because the rounded grains from deserts don’t make concrete as strong as that made from the sharper and irregular watery grains.
A scientist at the Seashore – James Trefil
Special thanks to Zoe Weil’s editorial expertise