Originally published in Seven Mile Times' Spring 2015 Issue, April 1, 2015
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In every outthrust headland, in every cu r ving beach, ...
THERE’S HISTORY IN EVERY GRAIN By Dr. Lenore Tedesco of The Wetlands Institute
Sweeping beach view – Stone Harbor Point
and evokes wonderful memories. Burying your toes in the sand, sandy shoes on the back porch, building sand castles, or the glory of waves lapping up on the sand, all have special meaning. The mere mention of the word “sand” can bring a smile to one’s face, a relaxing calm, and a longing for the shore. Hidden on every sandy shore is a mystery waiting to be solved. Each shoefull of sand holds many secrets. The story of sand is remarkable. How far did it travel and through what lands did it pass? Did it come from a blazing volcano, a wave-battered coral reef, or an ancient mountain? Sands betray their complex history to a skilled sleuth. A careful observer can read this history and tell the source of the sand, the climate at its source, the distance it traveled, and the character of the place where it was found. A close look at sands from around the world reveals their basic origins and how they vary with differences in climate and energy. There are four common sources for sand: weathering of continental granitic rocks; weathering of oceanic volcanic rocks; skeletal remains of marine life; and chemical grains precipitated from water. Each source supplies different basic building blocks that will be changed by their journey to the sea. Continental granitic rocks are rich in pink and white blocky feldspars, platy and shiny flakes of mica, and glassy spheres of quartz. The first step in granite weathering produces a blocky particle called grus, where each grus particle is composed of several crystals. Grus quickly breaks down to individual crystals as it starts its journey. Along the way, some minerals break down to become soil, while others become physically abraded, smoothed and polished. Physical abrasion gradually causes grains to smooth and become spherical. They may become polished or matted. April - May 2015
Blackened shells mixed (old and buried for a long time) with new shells on Seven Mile Beach.
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Photographs of sands slightly magnified but to same scale. 1) Angular quartz (clear), pink feldspar and polished shells of a young sand in Mount Desert Island, Maine. 2) Fine sand dominated by clear quartz, black shell bits and heavy minerals at Stone Harbor Point. 3) Round quartz (clear) and polished shell weathered by long transport in Venice Beach, Fla. Shark teeth added by weathering of ancient phosphate sands (black spheres). 4) Fresh volcanic (basalt) glass from local source in Punalui, Hawaii.
Quartz is the most stable mineral, so sands derived from weathering of granites gradually become enriched in quartz as the other grains are dissolved and broken down. This chemical purification or increase in maturity may require thousands of miles of travel and may take thousands of years! Some
sands have their journey interrupted by millennia when they are deposited in an environment. Pure quartz sand beaches belie their history – they are made up of the very old and wise travelers. Usually, quartz sand beaches also include other minor mineral types. In granites, the minor minerals are visually darker and
usually denser. The so-called heavy minerals are different for the different source rocks and can be used to trace sands back to their origins. On many beaches, heavy minerals are selectively concentrated and give rise to the beautiful streaking patterns found in the intertidal zones. On the East Coast of the United States, beach sands come from weathering of the various old mountains that extend in broken chains from Maine through the Appalachians. Grains are transported from these source areas by rivers until they reach the coast. Sands newly arrived on the shore include a mixture of minerals, and the grains are angular and unweathered. The journey then continues southward along the beach for thousands of miles. A grain of sand on Seven Mile Beach has its origins somewhere to the north along the coast. It could have come down the Hudson River or it could have come from as far away as Maine. These well-traveled sands will be finer, rounded and dominated by quartz. Many sand grains on Seven Mile Beach are in the middle of their journey – merely bouncing on by – headed further south. The sand shoals of Hereford Inlet are an intermediate stop-off point for much of the sand on Seven Mile Beach. There, it gets buried and shells of marine organisms get added to the mineral sands. Buried skeletons turn black as organic material embedded in the shells becomes chemically altered in an environment without oxygen. The blackened shells we find on our beaches are shells that have been returned to the beach from these older buried sands. Storms can erode these sands and shells and bring them back on shore, but more likely it is the work of the dredges that is periodically mining these sand shoals and adding a short excursion back north continued on page 38