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Beach Sand: What It Is, Where It Comes From and How It Gets Here by Dennis Adams Information Services Coordinator |
The sandy
beach at Folly Field on Hilton Head Island.
Photograph by Dennis Adams (August 8, 2002)
Though sand accounts for two percent of our planet’s crust, the Encyclopedia of Earth and Physical Sciences (Marshall Cavendish, 1998) reported that sandy beaches like ours on Hilton Head and Hunting Islands are "relatively rare," "considering how much coastline there is on Earth." Beaches are normally rocky (or, at best, gravelly) strips of land. Most of the world’s sand lies in rivers, desert dunes and on the floors of oceans.
| Sand is a mixture of fragments worn down from rocks and shells (throughout tropical Oceania, island sand comes from coral and other sea shells). Although the purest sands consist of quartz or coral material alone, most of the sand between your toes is a blend of about two-thirds quartz and other material (which can be up to 15% feldspar, 15% percent rock fragments, 5% clay minerals and smaller proportions of calcium carbonate, organic materials and other minerals). |
 Photograph by Dennis Adams (August 8, 2002) |
When a grain of sand appears rough-edged under a microscope, not much time has passed since it has been weathered from bigger rocks and shells. The grains become rounder as physical abrasion and chemical weathering take their toll. Desert sand has the roundest grains of all, because the winds keep the particles shifting and hitting against one another constantly. On the dunes of the Sahara, for example, the wind drives not merely ripples, but vast waves of sand.
Sand grains are smaller than gravel but bigger than particles of silt (should you care to measure the difference, sand grains range in size from 0.0025 inches to 0.08 inches in diameter – or "a pinpoint to pencil lead", by naturalist Peter Meyer’s rule of thumb). Fine-grained sand forms a firm beach surface, which can support the weight of an automobile. But beaches made of coarse sand require grittier vehicles like tractors and jeeps of those who dare to drive over them.
The surf hits
Folly Field Beach, Hilton Head Island
Photograph by Dennis Adams (August 8, 2002)
Our beaches owe their sandiness to the ocean currents. "As waves approach the shallow seafloor near the coast, they stir up sand and other bottom sediment," explained The New Book of Popular Science (Grolier, 1996). "When the waves are small, only the uprush is powerful enough to move sand – which then becomes part of the beach. Moderate-sized waves carry sand both to and from the beach, but generally deposit more than they can sweep away. It is only when waves become large, as during a storm, that they tend to erode more sand than they deliver."
Our barrier islands are really enormous offshore deposits of sand (like the smaller sandbars). Erosion occurs when the waves take back what they have given: the sand returns to the ocean, or ends up on another island. Because the process never stops, beach renourishment projects are only temporary "fixes". Fripp Island is not the Rock of Gibraltar.
Anyone who has dug into the beach has seen alternating layers of darker and lighter-colored sand. The blacker minerals are denser materials that the larger (and stronger) waves bring to the beach. Smaller waves carry less-heavy, light-colored sand in from the ocean. As strong currents follow weaker flows, they build up this "layer-cake" effect. Wherever wave action is most severe, lighter sand gets washed away, leaving a dark beach surface.
Outgoing waves leave low-lying ripple marks on fine-sand beaches like ours (according to The New Book of Popular Science , these ripples are "almost always 18 inches from crest to crest" and "are not found in coarse-sand beaches because the grains fall back into place after the water has retreated"). Ripple troughs hold pools of seawater, where mica left behind by the waves sparkles in the sunlight.
| Peter Meyer, in his Nature Guide to the Carolina Coast (Avian-Cetacean Press, 1991), wrote that sand dunes "are formed through the interaction of three key elements: sand, wind and plants. … Wind at the shore is nearly constant, whipping the sand about. Plants at the dune stop the sand as it is blown; the sand accumulates at the base of the plants, building the dunes." Beach Grass and Sea Oats thrive on the primary dunes (the first row directly facing the ocean). Other halophytic plants, adapted to a high-salt environment, can survive on the secondary dunes behind the primary row ("but not many", added Meyer). Sand dunes shelter the marshes and maritime forests at their rear when wind, water and storms rush in from the ocean. They are natural sea walls, but will shift and erode with the same natural forces that move the beaches. |
 Sea Oats on a dune behind Folly Field Beach, Hilton Head Island Photography by Dennis Adams (August 8, 2002) |
But to return to our basic question, where do our own, quartz-based sand grains come from in the first place? Meyer says that "very likely, they come from rocks and mountains on land as they erode due to wind and rain." Particles of quartz and feldspar blow or wash into rivers: the feldspar deposits in estuaries and is eventually carried offshore, hundreds of miles away from its diverse points of origin. The worn-down quartz piles up at the mouths of the rivers that carry it far downstate to the sea. Ocean currents then work to deposit all the minerals and shell fragments along the coastline.
Quartz, the source-rock, is an igneous mineral, formed when molten material has cooled and turned solid. It stands second only to feldspar as the most abundant mineral in the Earth’s crust (feldspar, another igneous rock, and amounts to 50% by volume, while quartz amounts to 12%).
Sand has some valuable commercial uses, too. The South Carolina Statistical Abstract lists all counties except McCormick, Edgefield, Barnwell, Allendale, and Hampton as places where sand is mined as a commercial mineral commodity in this state. Sand mixed with gravel and cement makes concrete.
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The purest
form of quartz-based sand (silica sand) becomes glass, abrasives,
furnace linings, and molds for scientific applications. Sandblasting
(with steam or compressed air) not only cleans large building surfaces,
but cuts metal stencils, carves designs into rock, and even etches
delicate glasswork. Sandstone – grains of sand bound together
by pressure or other minerals – includes the brownstone used
to build houses (some famous medieval cathedrals are also sandstone
structures).
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