Sand . . . we play in it, we stroll on it, we make castles out of it, but what do we really know about it?
Most people use the term “sand” to refer to loose material on a beach, but sand is actually a grain size measurement used by geologists to describe sediments varying in size from about 1/16 mm to 2 mm in diameter. Sand can be found not only along ocean beaches, but along flowing rivers and beside land-locked lakes.
Mineral and volcanic sands are terrigenous, meaning that they come from the land. Mineral sands start as mountains and boulders and are gradually broken into smaller and smaller particles by weathering and erosion. Being alternately heated and cooled through the seasons, rolling and tumbling around in mountain streams and rivers, rocks are broken apart by the water’s movement. The black sand beaches of some islands (e.g. Hawai’i) are volcanic in origin. Lava rocks, chunks of cooled molten lava, are broken down into sand over time by physical and chemical weathering.
Some sand is biogenous, meaning that it comes from living things, such as mollusk shells and corals. The energy from water breaks shells down into tiny pieces that eventually are washed onto beaches. Parrotfish in the tropics use their teeth to scrape off and grind down bits of coral reefs; the undigested particles pass through the fishes’ digestive systems and end up as sand on tropical beaches!
If you examine the beauty of beach sand with a magnifying glass, the shape, size, and color of the sand grains can tell many stories.
Shape indicates the age of sand grains. Different materials break down at different rates. For example, quartz is very sturdy and takes longer to become sand than the calcium carbonate material of mollusk shells. Younger sand has sharp and angled edges; it needs time and wave action to become rounded. Sand particles that are smooth and well-rounded are evidence that they have been rolled around for awhile.
Wave action is the story told with grain size. The story is simple: small waves move small sand grains around; large waves move large grains. In addition, Aeolian transport, or sand movement driven by winds, shifts the sand around on the beach, shaping and reshaping, covering and uncovering.
Color offers evidence for source materials – sand particles that are clear, tan, gray, or brown might be quartz or feldspar, black or gray particles could be ilmenite (titanium oxide) or magnetite (iron oxide); garnets are red, mica is silvery black or gray, and shells are purple, white, black, or brown.
The chorus from the song “Mountain in my Hand” (on the Only One Ocean CD by the Banana Slug String Band) explains it best:
“I’ve got a mountain in my hand from the rain-washed land; down by the sea now is where I stand, with this mountain in my hand trying to understand; Oh, oh, wonderful sand.”