Sand Beaches: How Black and White Sand Beaches Are Formed

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How Are Sand Beaches Formed?

Sand is defined by geologists as a granular mineral between 1/16 mm and 2 mm in diameter. Particles that have a larger diameter are called gravel. Particles with a smaller diameter are called silt, which resembles flour in its consistency. There are subcategories of coarseness within the sand range, as set by the International Organization for Standardization (ISO): fine, medium and coarse. Sand accounts for 2 percent of the Earth’s crust. Most of that sand is on the ocean floor, in rivers, or in desert dunes, not on coastal beaches.

Beaches are formed when ocean currents bring sand with them toward the shore. Small waves bring sand and other sediment from the shallow ocean floor and deposit it on the coast. Medium waves bring in sand on the up-rush and take some back, usually less, on their way back to the ocean. Both of these types of waves cause beaches to grow. During a storm, very strong waves erode the beach, taking more sand than they contribute. Because the strength of waves is always changing, the amount of sand on a beach fluctuates as well.

In addition to size, sand is categorized by color. Sand can be found around the world in a variety of colors, depending on the source material that is broken down to form the sand. Most sand is made from eroded rocks of many kinds or calcium carbonate from marine organisms. Sand in deserts and nontropical beaches is commonly made of the chemical compound silicon dioxide in the form of quartz. Black sand beaches are composed of volcanic rock. Black sand beaches retain the heat from the sun – something to take into consideration before planning to walk barefoot on the beach during the sunny part of the day. White sand beaches are made of limestone, which primarily contains calcium carbonate. There is even a green sand beach in Hawaii where the sand is composed of olivine, a green crystal that forms in lava and pink sand that is formed from the broken up red shells of the sea creatures that inhabit the ocean in certain beaches in the Bahamas and Bermuda.

The source material could be local or it could have been transported a long distance along rivers and ocean currents. Waves, wind and other natural processes erode the source material into grains of sand. Some beaches have alternating layers of darker and lighter sand. The darker sand is denser and brought in by stronger waves. The stronger waves alternate with the more gentle waves that bring in the lighter sand and cause the layers. Sand can also be colored by natural minerals, chemicals or pollutants.

How are White Sand Beaches Formed?

White sand beaches usually form in tropical or subtropical climates near coral reefs. Coral, crushed seashells, and the skeletal material of marine organisms that live and die near the reef provide the calcium carbonate that becomes the white sand. Some tiny marine organisms that contribute to white sand beaches are halimeda, echinoderms and foraminifera.

Coral reefs can be eroded into sand in two ways. The first is bioerosion, which is caused by animals such as fish, worms, urchins and sponges. These animals break down coral into sediment as an indirect result of their grazing or boring activities. For example, the parrot fish eats coral and the calcium carbonate from the coral passes through the digestive system of the parrot fish. The waste that the parrot fish expels is one of the major sources of fine white sand. The second way that coral reefs can be eroded into sand is mechanical erosion. This type of erosion is caused by the force of waves hitting the shore or abrasion by pebbles in the water.

In deserts, wind forces the grains of sand to rub against each other, making desert sand particles very smooth and round. The strong desert winds cause waves in the sand much like the waves in the ocean. The White Sands National Monument is not a beach, but a huge desert full of large, white dunes in the Tularosa Basin in the southern part of New Mexico. The dunes are composed of a unique type of white sand made of gypsum. Gypsum is not found on beaches because it is water-soluble. Rain dissolves the gypsum, but there is no outlet to the ocean from the Tularosa Basin. The water instead drains into the ground or evaporates, leaving the gypsum in crystallized form. The crystals are then eroded into sand-sized particles. Unlike the sand of most beaches, which is made of quartz, the gypsum sand does not heat up in the sun. People are able to walk barefoot on it without burning their feet.

How are Black Sand Beaches Formed?

Black sand beaches, found on volcanic islands in the South Pacific and Caribbean, are created when hot lava flows into the sea. The extreme temperature difference between the cold ocean water and the hot lava breaks the lava into small particles of black glass that eventually collect on shore to form black sand beaches. Lava flows also harden to form solid structures such as cliffs at the edge of the ocean. Wave action throughout time erodes this solid lava, breaking it into small granular material that forms part of the black beach sand.

Black sand is often deposited as a layer resting on top of lighter colored sands in areas that have high wave energy. Large waves can sort the sand grains by weight and deposit heavy minerals on the surface. The component elements that make up black sand rocks such as basalt, gabbro and other dark-colored mafic rocks are also dark. The mafic material that the black sand is composed of has high concentrations of magnesium and iron; mafic lava has lower silica content than felsic lava. Mafic lava eruptions tend to be less explosive than felsic lava eruptions. Mafic lava volcanoes are typically oceanic, like in Hawaii.

Some of the dark colored minerals that make up black sand, such as magnetite, are magnetic; they will stick to a magnet if you run it through the sand. Miners and prospectors use black sand to find placer deposits. Placer deposits are accumulations of valuable minerals such as gold, platinum group metals, titanium, tungsten and zirconium. This technique was the predominant mining method used during the California Gold Rush. Placer deposits are formed by dense particles collecting at a site where the flowing water is not strong enough to carry them further downstream. Alluvial placer deposits are typically found in the insides of river and creek bends, in natural hollows, in places where the slope of a stream levels out, at the base of a waterfall, in sand dunes, in gravel beds and on ocean beaches. Placer materials, such as black sand, must be dense and weather resistant. Placer mining produces black sand concentrates that can contain, in addition to precious metals, other valuable materials such as gemstones. Garnets, rubies, topaz, sapphire and diamonds are found in the black sand deposits during placer mining.

Resources about Beaches

Ask the Earth Science Teacher answer “I (teach) 3rd grade; part of the 3rd grade curriculum deals with geology. I read somewhere that the sands of the beaches of the earth are made always (mostly) of quartz. Is this true and if so why? ”

California’s 1,100 miles of coast include some of the world’s most spectacular beaches.

Smithsonian Marine Station at Fort Pierce beach habitats

EPA is to protect human health and to safeguard the natural environment air, water and land upon which life depends.

Resource by B.Matthews

I am a fun outgoing girl who loves to go to concerts and sit on the grass and read. My favorite books are those that make me think about life, love and how the world spins around no matter the troubles that you're going through.

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