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
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Photographs of sands slightly magnified but to same scale. 5) Weathered green olivine sands, basalt (black) plus white shell in South Point, Hawaii. 6) Worn white skeletal fragments with abundant forams (spiral discs) transported from offshore reefs in Big Pine, Florida Keys. 7) Colorful skeletal fragments with snails and sea urchin parts from nearby lagoon at Santa Cruz Island, Galapagos. 8) Very fine, fresh, polished skeletal material including pink skeletal sands in Great Guana Cay, Bahamas. 9) Spherical grains (ooids) precipitated from seawater at Joulters Cay, Bahamas. 10) Pebble beach with very round quartz grains (Cape May Diamonds) that come from eroding of 100,000-year-old dunes at Sunset Beach on the Delaware Bay. continued from page 36
for some of the sand. Unfortunately, the sand is headed back south again on its long journey. Some of the most beautiful and exotic beaches are composed of sands derived from volcanic rocks. The black basaltic rocks of volcanic eruptions are usually glassy because they cooled so quickly, robbing the crystals time they need to grow. Basalts may contain some larger crystals if they were formed deeper in magma chambers from which erupting lava was derived. These crystals are usually chemically unstable and dissolve away quickly. In dry climates, however, unstable grains such as green olivine may survive in preference to the glassy rocks in which they were contained. The blacksand beaches of Hawaii attest to the very young and close source of the rocks, while the green sands attest to very dry climates that preserve the unstable sands. Thus, two beaches in Hawaii have very different sands. Skeletal sands are derived from the shelly remains of organisms. Many invertebrate animals and some algae secrete calcium carbonate skeletons. These grains are usually born near where they are deposited so that their presence is a record of nearby environments. This is in contrast to sands weathered from granites and basalts. Who they are largely determines what type of grains they will add to the beach. Skeletons may be large and durable (coral frag-
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ments) or fine and fragile (sea urchins or sand dollars). Many break down quickly after death so that they contribute only shell fragments or they dissolve away completely. Calcium-carbonate grains are physically less durable than most land-derived grains and their abundance on many beaches testifies to the tremendous amount of skeletal material being produced nearby. Calcium carbonate is more stable in tropical settings, which is why tropical beaches are better known for their skeletal sands. The beautiful pink sand beaches of Bermuda and the Bahamas are due to the inclusion of pink shells of small animals called forams common on the reefs. Grains derived from chemical precipitation from water are a fourth type of material that can be found on beaches. In many saline inland lakes, water has evaporated enough for gypsum or even halite (salt) to precipitate. Normal ocean
water only contains a concentration of salts sufficient to precipitate calcium carbonate. Warming ocean water on shallow tropical banks enhances this process. The most important calcium-carbonate grain type formed as a precipitate is the ooid. Ooids are important today along the margins of many carbonate banks most notably in the Bahamas, Arabian Sea and Red Sea. The city of Miami and part of the Florida Keys are built on a ridge of a fossil ooid bank. Ooids are formed of concentric growth layers of precipitation made round by the constant movement on high-energy shoals where they form. Next time you are at the beach, take a moment to examine that sand between your toes. Take along a small magnifying glass and see what the handful of sand is trying to tell you. The biographies of the grains are waiting to be told to the careful observer.
Dr. Lenore P. Tedesco is the Executive Director of The Wetlands Institute in Stone Harbor. Tedesco has a Ph.D. in marine geology and geophysics from the University of Miami, and a bachelor’s degree in geology from Boston University. She joined The Wetlands Institute in October 2011. Prior to then, she had been a professor in the Department of Earth Sciences at Indiana-Purdue University for 21 years. She has focused her research activities on coastal and wetland ecosystem dynamics and restoration.
TICKET PRICES Non-member: $10 adult, $8 child, $30 family pack of 4 Member: $8 adult, $6 child, $25 family pack of 4 Come enjoy a pancake breakfast for the whole family. Hunt for terrapin eggs, play games and earn prizes. Build arts & crafts and get up close and personal with live turtles, and more. Stop by the aquarium and watch the animals have breakfast, too! Call to make a reservation time for breakfast (gluten-free pancakes available upon request). Seatings at 9, 10, 10:45 and 11:30 am.
Saturday and Sunday May 16 and 17 TWO-DAY TICKET PRICES Non-member: $15 adult, $10 child Member: $13 adult, $8 child ONE-DAY TICKET PRICES Non-member: $10 adult, $8 child, $30 family pack of 4 Member: $8 adult, $6 child, $25 family pack of 4 Join The Wetlands Institute and the Delaware Bay Shorebird Initiative for a festival that celebrates an amazing spectacle of nature – the shorebird migration and horseshoe crab spawning season. Children and guests of all ages can enjoy guided shorebird walks along our local beaches and salt marshes, guided viewings of shorebird and horseshoe crab interactions along the Delaware Bay, naturalist-led horseshoe crab night walks, opportunities to view and participate in shorebird tagging with the International Shorebird Research Team, live horseshoe crab demonstrations, face painting, and hands-on education and conservation-based activities and games.