Life's Little Mysteries

How Does Quicksand Work?

An intrepid explorer in khaki uniform trudges through the jungle, until, with a whoosh and a plop, he falls chest deep into a pool of sand. Struggling, he sinks inexorably until only his pith helmet floats above the surface. This is the story that probably comes to mind when we think about quicksand, but according to scientific tests, it's only half correct.

Quicksand is a sloshy mixture of sand and water that appears solid when viewed from above, but will collapse into a more liquid form if a heavy object, like a human or animal disturbs it. After liquefying, the mix tends to solidify, cementing said animal in gluey, thick wet sand.

But, while it will trap you, the fear that quicksand will swallow you whole is a myth, according to some basic physics. The density of quicksand is about twice that of a human body, so a human should sink only half way in, unless that person struggles, which could cause them to submerge, according to research by Dutch and French scientists published in the journal Nature. [Read: Why Do Stars Twinkle? ]

Quicksand's "quick" nature is a result of a particular mixture of sand and water, often with clay or other material added. Mildly soaked, sand remains firm and solid like a wet shoreline. But when churned up with a larger proportion of water, sand grains form a structure like a house of cards, balanced on each other with open, watery space between, Darrel G. F. Long, of Laurentian University, Ontario, writes in a Scientific American article.

In this precarious arrangement, a weight placed on the sand, like a human body, will disturb the mixture of sand and water, which suddenly becomes more liquid, allowing the body to sink in. After liquefying, the sand and water begin to separate from each other, forming a more solid mixture and trapping the body in place.

Waterless, dry quicksand can also exist, according to Long. Lab experiments at the University of Twente in The Netherlands have shown that man-made sand can suck an object deep under it's surface, but researchers do not report that this type of deep, dry quicksand has ever been observed in nature.

Buried in watery quicksand, a body must use a lot of force to loosen the mix enough to escape, according to the Nature article. Spreading limbs out helps the body float to the surface. And if stuck solid, the authors advise a quicksand victim to "take solace" in the knowledge that they can't sink in over their head.

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