AN INTRODUCTION TO SAND COMPONENTS
Marine organisms and mineral components destined to become sand. Dog Beach, Hollywood, Florida.
Sand, the gritty stuff of river, lake and ocean beaches! It is also an essential component for concrete, abrasive products, paint, filtration, pigments, glass and on and on, and it is everywhere. Around the world, sand composition reflects the regional geology and aquatic ecosystem. The individual grains hold a story of parent rock, geologic processes and aquatic creatures.
Sand grains vary in shape from angular to rounded. The rounder the grains, the longer those grains have been free of their parent bedrock—cascading down rivers, rolling in the surf or blowing around in the desert. Shells and other animal pieces go through the same “angular to rounded” process.
Common Minerals in Sand
Quartz is the most common grain in sand around the world. Why so much quart sand you might ask? Quartz is one of the primary components of granite, a rock comprised primarily of quartz, feldspar and mica. Granite makes up the bedrock of the major continents and ancient mountains.
When granite breaks down, it is quartz that endures the longest because of its hardness and resistance to physical and chemical weathering. Feldspar is softer, and grains eventually break down into clay-size particles and disappear from sand.
“Younger sand” is more angular and shows more of the minerals of the parent granite. Ancient sand is often well rounded and consists only of quartz grains.
Some dark sands contain “heavy minerals” such as pink and red garnets, black magnetite and even peridot, sapphires and gold. These high-density mineral grains are often seen as wavy bands on the shore.
In some cases, strong storms concentrate large amounts of heavy minerals, and wave action removes the less dense minerals such as quartz. The result is deep layers of heavy minerals which can give the sand a dirty black, purple or red appearance.
Volcanic rock contributes to black sand beaches in Hawai‘i, Costa Rica, Iceland, Italy and many other areas including the Pacific Northwest.
Biogenic Components of Sand
Biogenic grains include both marine plants and animals that use calcium carbonate minerals as part of their body. Whole or broken up, their hard body parts contribute to sand. They are the ocean’s storytellers.
Familiar members of the Molluscs are the bivalves (like clams) and gastropods (snails). Their shells can be found whole or fragmented in sand.
Barnacle fragments are chalky looking and show distinctive parallel openings.
The spines and body parts of echinoids (sea urchins) are colorful and common.
Forams are single-celled organisms that make chambered “tests” that may be coiled, disc-shaped, elongated or star-shaped. They inhabit most marine habitats from mangrove swamps to coral atolls and the deep ocean.
Corals build large carbonate structures and take on many forms. Identifying a coral calyx, the “home” where the coral polyp lived, confirms the grain’s identification as coral.
Bryozoans, or moss animals, have minute oval-shaped zooid openings where the animal lived. They can resemble a window screen, be button-like, branched or encrusting.
Sponges are generally soft-bodied and only their hard spicules are found in sand.
Marine algae are another important source of biogenic sediments if they are calcified. Red coralline algae wear to smooth rods. Halimeda, a green alga, leaves chalky disc-shaped flakes.
Fossils from both plants and animals are found in sand too. Finding marine fossils, shark teeth or crinoids for example, is a clue that the area was once the site of an ocean.