National Geographic News: NATIONALGEOGRAPHIC.COM/NEWS
 

 

Quicksand Science: Why It Traps, How to Escape

Nicholas Bakalar
for National Geographic News
September 28, 2005
 
If stumbling into quicksand ranks on your list of worries, don't panic. You won't sink in—at least not all the way. Real quicksand is certainly hard to get out of, but it doesn't suck people under the way it always seems to in the movies.

According to a study published in the current issue of the journal Nature, it is impossible for a person immersed in quicksand to be drawn completely under. The fact is, humans float in the stuff.

Researchers in the Netherlands and France studied quicksand, a combination of fine sand, clay, and salt water. At rest, quicksand thickens with time, but it remains very sensitive to small variations in stress.

At higher stresses, quicksand liquefies very quickly, and the higher the stress the more fluid it becomes. This causes a trapped body to sink when it starts to move.

But a person moving around in quicksand will never go all the way under. The reason is that humans just aren't dense enough.

Floating in Quicksand

Quicksand has a density of about 2 grams per milliliter. But human density is only about 1 gram per milliliter. At that level of density, sinking in quicksand is impossible. You would descend about up to your waist, but you'd go no further.

Even objects with a higher density than quicksand will float on it—until they move. Aluminum, for example, has a density of about 2.7 grams per milliliter. But a piece of aluminum will float on top of quicksand until motion causes the sand to liquefy.

During their study, researchers placed an aluminum bead on top of a container of laboratory-created quicksand. At rest, the bead remained on the surface, despite aluminum's higher density.

But then scientists started shaking the container. When they shook it only a little, the bead stayed floating on top. But when they shook the container a bit harder, the ball descended to the bottom.

Difficult to Get Out Of

But if quicksand becomes less viscous as you struggle, why is it so difficult to escape? The reason, explain the study's authors, is that after its initial liquefaction, quicksand's apparent viscosity (thickness or flow resistance) increases.

The increase is due to the formation of sand sediment, which has a very high viscosity. It's the difficulty of moving this dense sand that causes the problem.

Water has to be introduced into the sand sediment to loosen it, and this requires considerable amounts of force. The authors estimate that the force needed for someone to pull their foot out of quicksand at a speed of a centimeter a second would be the equivalent of that required to lift a medium-size car.

What to Do When You're Stuck

If you do step into quicksand, says study co-author Daniel Bonn, you'll only sink in a little deeper than your waist. "I would say there would be some pressure on the chest, but not enough to cause serious trouble."

So how do you get out? Don't ask your friends to tug on you; they're likely to pull you "into two pieces if [they] try hard to pull [you] out," said Bonn, a physics professor at the Van der Waals-Zeeman Institute at the University of Amsterdam.

"The way to do it is to wriggle your legs around. This creates a space between the legs and the quicksand through which water can flow down to dilate [loosen] the sand," he explained. "You can get out using this technique, if you do it slowly and progressively."

Origins of the Myth

A person will gradually begin to sink in quicksand, and movement will make the victim sink faster. This may be the origin of the advice to "never struggle if you're caught in quicksand."

But no amount of struggling will send you in over your head. Bonn suggests that it isn't struggling that can get you into trouble, but getting caught in quicksand near the sea, which is generally where quicksand is found.

When the high tide comes in, you could drown.

Free E-Mail News Updates
Sign up for our Inside National Geographic newsletter. Every two weeks we'll send you our top stories and pictures (see sample).

 

© 1996-2008 National Geographic Society. All rights reserved.