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USGS Geology in the Parks

Calcareous Sand

What is Sand Anyway?

SCUBA diver holding sand grainsCalcareous sand, sand which is composed primarily of calcium carbonate, present in the Dry Tortugas National Park and elsewhere is composed of debris from once living marine organisms. When these plants and animals used calcium carbonate, or CaCO3, (the same material limestone and your skeleton is made of) to form their skeletons and shells. When the organisms died, these pieces became part of the beach sand. Let's take a closer look at what these organisms look like with the naked eye and also under a microscope or a powerful device called a Scanning Electron Microscope (SEM). Below are some photographs of the skeletons that make up sand.

Some of the most common sources of calcareous sand include:

  Staghorn Coral Cross-section of coral  

Staghorn coral and a coral cross section, showing the skeletons that coral grow. These large pieces get broken into sand-sized grains by both natural and disturbed processes, such as storms and human interaction. Coral needs warm, clear water and lots of sunlight to grow, which is why they primarily live in the tropical regions, generally 30 degrees latitude towards the equator. Within these parameters coral can be a large proportion of the sand.

Green Algae Thin section of Green algae  

A living plant of the green algae Halimeda, next to a thin section. This particular species of algae produces flakes of calcium carbonate. These flakes become a component of sand when the leaves die.

Molluscs Scanning electron microscope image of mollusc shell  

Molluscs are present at all latitudes, and in particularly productive regions their shells can represent a large proportion of beach sediment. The circular image at right is a Scanning Electron Microscope image of a mollusc shell at x2000 magnification

echinoderm or sea urchin   Cross-section of  a sea urchin  

Above is a type of echinoderm, a sea urchin, next to a cross section of one. Echinoderms have a hard, calcareous shell and spines. When the animal dies these skeletal portions remain and are broken up by waves and animals.

Foraminifera   Thin-section of foraminifera  

Foraminifera are tiny, single-celled animals that live in bodies of water. They may live within the water column, on the bottom, or attached to plants, as the circled one is at left. They grow a shell of calcium carbonate. Each species produces a distinct style of shell, which can be quite complex. At right is a thin section of a foraminifera shell, at left is an example of a living 'foram'.

Text and images adapted from the "What is Sand Anyway?" poster prepared for the Dry Tortugas National Park by Christopher D. Reich, U.S. Geological Survey.

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