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.

Continued on Next Page >>


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