No idea why the question of sand suddenly came to mind while walking the beach on Thursday afternoon. My feet have passed over those same flats, runnels and dunes for close to 2,000 miles in the past twenty-two months and while admiring, complaining, examining close up and photographing the endless stretch of it all, yesterday was the first time it occurred to me to wonder how and why. After taking dozens of pictures and shaking pounds of the stuff out of shoes and clothes, it seemed strange that my thoughts never once turned to the science of something that appears in every window and could even be called a front yard.
The only explanation I had, and an anemic one at best, was to say that sand in its gazillions of grains is something produced by the erosion of rock. But is it the same everywhere, the sand of coastal beaches as well as that making up inland deserts and river beds? Characteristics are not difficult to list about the sand along Florida’s east coast and how it shifts according to season, but those particulars don’t get to the bottom of the question.
Sand accounts for two percent of the earth’s crust and considering how much coastline there is along the earth’s land formations, sandy beaches are relatively rare. More normal are the rocky beaches, or shingle beaches familiar to much of the English coast. Most of the world’s sand is found in rivers, desert dunes and on the ocean floor. Unsurprisingly, the formation of a sandy beach is the result of ocean currents. As waves approach the shallow seabed near the coast, they stir up sand and other sediment. When wave action is small, only the surge or uprush is powerful enough to carry sand far enough to become part of the beach. Moderate-sized waves carry sand both to and from the beach, but generally leave behind more than is carried back to the water. The trouble comes when waves are large and storm driven, for it is then that they erode more sand than they deliver. My home is on the seaward side of a barrier island that is basically an enormous offshore deposit of sand, much like the sandbars seen offshore and in rivers.
Sand dunes are formed through the interaction of sand, wind and plants. Wind along the coastline is nearly constant, forever whipping and swirling the sand about. The plants growing in a dune halt the sand as it blows, and it falls to accumulate at the base of the plants. Gradually this cycle builds dunes. Beach grass and sea oats thrive on the primary dunes, those facing the ocean. Other plants adapted to a high-salt environment thrive on the secondary dunes behind the primary row. Dunes are natural sea walls and shelter the land when wind, water and storms rush in from the ocean, but will shift and erode with the same natural forces that reshape the beaches.
Sand is a mixture of fragments worn down from rocks and shells. Tropical island sand in particular is the end product of eroded coral and sea shells. Although the purest sands consist of quartz or coral alone, most of the sand between your toes is a blend of about two-thirds quartz and other materials including feldspar, rock fragments, clay minerals, small amounts of calcium carbonate and organic trace minerals.
Near impossible to see with the naked eye, but under a microscope a rough-edged grain of sand is evidence that not much time has passed since it was weathered from larger rocks and shells. The grains become rounder as physical abrasion and chemical weathering continue. Desert sand has the roundest grains of all, because wind keeps the particles shifting and hitting against one another constantly. On the dunes of the Sahara wind drives not mere ripples, but vast waves of sand.
Many thousands of years ago much of the sand here in Florida had its origins in the quartz from inland rocks and mountains eroded by wind and rain. Imagining the process, particles of quartz and feldspar are blown or washed into rivers, the feldspar is deposited in estuaries and eventually carried offshore hundreds of miles away from points of origin. Worn, granulated quartz piles up at the mouths of rivers that carry it far downstate to the sea. Ocean currents then work to deposit all the minerals and shell fragments along the coastline.