Some beach sand has passed through the gut of a fish
Near coral reefs, fish that graze algae off rocks and coral swallow grit along with their food, digest the organic part and excrete the rest. Over a lifetime, a single fish can turn out a substantial amount of sand. It is one of many surprising ways beaches are built, shaped and constantly rebuilt.
A beach is a strip of loose particles beside water, made of rock fragments like sand, gravel and pebbles or of biological material like shells and coralline algae. Most sit on coasts, though lakes and large rivers have them too. Sandy beaches line roughly a third of the world's coastlines. The part the waves work on is the berm, with a crest and a face sloping to the water; offshore, long underwater bars form where waves first break, and behind the beach wind can pile sand into dunes.
Beaches change with the seasons. In temperate summers, gentle waves with long gaps between crests carry sand up the beach and leave it there, raising the profile. Winter storms bring powerful waves in rapid succession that keep sediment suspended and drag it along the shore or out to sandbars, lowering the beach. Waves that allow sand to settle between breakers are called constructive; those that arrive too fast are destructive. Shingle beaches stay steep because water drains quickly through the large stones, weakening the backwash, while sheltered coasts collect mud and grow mangroves.
Colour reveals geology. White sand is mostly quartz or limestone that scatters light without absorbing it; tropical white sand is calcium carbonate from coral and shells, and Bermuda's pink sand gets its tint from coral fragments. Black sand comes from volcanic rock such as basalt, red sand from oxidised iron, as on Santorini's Kokkini Beach, and Hawaii's Papakolea Beach is green with the mineral olivine. Olivine beaches may absorb carbon well enough that artificial green-sand beaches are being tested for the purpose. In hot, calm weather, evaporating seawater can even glue surface grains into a salty crust that resists wind.
The outlook is uncertain. Some estimates suggest sea level rise could remove up to half of the planet's sandy beaches by 2100, and construction, pollution and erosion add pressure. Undeveloped beaches serve as nesting grounds for sea turtles and seabirds, and dunes shield inland areas from storms.
Source: Beach
