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Where does beach sand come from? – Sly M, age 6, Cambridge, Massachusetts, US
David R Montgomery, professor of earth and space sciences, University of Washington, Seattle, US:
“Mountains end their lives as the sand on beaches. Over time, mountains erode. The mud, sand, gravel, cobbles and boulders they shed are washed into streams, which come together to form rivers.
As they flow down to the sea, all this sediment is ground up and worn down in nature’s version of a rock tumbler.
Big rocks break down into smaller pieces, so most of what reaches the sea is mud. These silt and clay particles are too small to see with the naked eye. But you can see individual grains of sand, which are just bigger bits of rock.
Next time you’re at the beach, pick up a handful of sand, look closely and examine the colour. Some beaches in Hawaii in the US have black sand because the islands were formed by erupting volcanoes. Many volcanic minerals are dark in colour.
Different colours of sand come from different minerals, such as khaki feldspar, smoky white quartz, green olivine or black basalt. The mix of colours in beach sand tells you what kinds of rocks produced it.
The shape of sand grains also provides clues about where they come from. Angular grains of the same type of sand have not travelled as far as smooth round grains, which have been more worn down. And weak rocks break down to mud faster than hard rocks, so sand tends to be made of the harder types that break down slowly.
About a tenth of the supply of sediment that reaches the sea is sand. These particles are between about half a millimetre and 2mm in size – roughly as thick as a small coin. These particles are large enough that they don’t flow right out to the deep sea.
But the beach is just a temporary stop for sand. Big waves pull it offshore and smaller waves push it along the coast, so keeping a beach nourished with sand is essential for keeping it sandy. Many beach towns spend millions of rands on rebuilding eroded beaches with new sand.
Yet, today, many beaches are starving. Many dams trap the sand that flows down rivers, piling it up in reservoirs. All in all, human activity has cut off about half the sand that would otherwise end up on the world’s beaches.
But humans haven’t turned the waves off. So, as beach sand washes away and isn’t replenished, the shoreline erodes. This means that many beaches around the world are shrinking, slowly but surely.
Next time you dig your toes into beach sand, think about the epic journey it took to arrive beneath your feet.”
To read more of Montgomery’s answer, go to theconversation.com
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