What is Sand Anyway?
 Calcareous
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.
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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. |
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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. |
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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 |
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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. |
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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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