Sink or Swim in Quicksand?
|By Kathy Wollard|
|Illustration by Debra Solomon
What causes quicksand, and where is it found? asks Edward McClarty, of San Francisco.
Unless you're on Gilligan's Island, roiling pits of dangerous quicksand aren't found around every bend in the trail. There are a lot of myths about quicksand, among them:
Quicksand sucks you down like a vacuum cleaner. In fact, quicksand doesn't pull you down any more than a swimming pool does. Quicksand is more buoyant than water, so it's actually easier to float in quicksand than in a swimming pool.
Quicksand is a bottomless pit. Most patches of quicksand are a few inches to a few feet deep.
Quicksand is always made of sand. In addition to sand, clay, swamps, and silt can all become what scientists call "quick."
By now you might have guessed that what we call "quicksand" is more of a phenomenon than a thing. The phenomenon is "quickness," the way water flowing through sand, clay, or other material lifts and separates its small grains.
Take sand. In ordinary sand, whether wet or dry, sand particles are shoved up against each other. But when sand becomes quick, an invisible cushion of water holds sand grains a bit apart. So what looks like a solid surface is really liquid, a very thick soup of water and sand.
In a typical patch of quicksand, ordinary sand is sitting above a body of water, such as a bubbling spring. The water is trying to push upward; the sand is weighing the water down. The sand becomes "quick" when the water pressure underneath balances or exceeds the weight of the sand above. As each grain is surrounded by a thin film of water, the sand grains lose contact and friction. Toss a rock onto what looks like solid sand, and you'll see it disappear below the surface, just as if you'd thrown it into a lake.
(Another, less common way quicksand can form is when a disturbing vibration, such as from an earthquake, loosens grains and causes them to quickly shift from side-to-side. Instead of resting quietly against their neighbors, grains of sand sift and shift until the whole patch is behaving like a liquid.)
Any kind of sand--rough or smooth, mixed with pebbles or not--can become quick. But heavier grains need a more powerfully surging spring to lift them, while the finest-grain, roundest sand can become quick even from weakly flowing water.
Quicksand is mostly found everywhere water and sand or clay live side-by-side--creek beds and ocean coasts, prairies and mountains. One good place to find it is in hilly country, with lots of caves and underground springs; such springs breed quicksand above.
If you stumble into a quicksand patch, it probably won't reach above your knees. But even if it's deeper, unless you're wearing a ton of camping equipment, you'll float quite nicely. To extricate yourself, squirm out of anything heavy, such as a backpack, and swim or dog-paddle to solid ground, just as you would if you fell in a lake.
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