Quicksand

This article is about the geological feature. For other uses, see Quicksand (disambiguation).

Quicksand and warning sign at a gravel mine.

Quicksand is a colloid hydrogel consisting of fine granular matter (such as sand or silt), clay, and salt water.

Water circulation underground can focus in an area with the optimal mixture of fine sands and other materials such as clay. The water moves up and then down slowly in a convection-like manner throughout a column of sand, and the sand remains a generally solid mass. The water lubricates the sand particles and renders them unable to support significant weight. Since water does not usually go up to the surface of the sand, the sand on top appears solid and can support leaves and other small debris, making quicksand difficult to distinguish from the surrounding environment.

The danger to hikers and other persons is relatively small, because there are few quicksand areas. Additionally, sand is twice as dense as water, making it easy to float to the surface. Fiction writers, however, have dramatized the peril of people being swallowed into the depths.

Properties

Quicksand is a non-Newtonian fluid: when undisturbed it often appears to be solid ("gel" form), but a minor (less than 1%) change in the stress on the quicksand will cause a sudden decrease in its viscosity ("sol" form). After an initial disturbance—such as a person attempting to walk on it—the water and sand in the quicksand separate and dense regions of sand sediment form; it is because of the formation of these high volume fraction regions that the viscosity of the quicksand seems to increase suddenly. Someone stepping on it will start to sink. To move within the quicksand, a person or object must apply sufficient pressure on the compacted sand to re-introduce enough water to liquefy it. The forces required to do this are quite large: to remove a foot from quicksand at a speed of .01 m/s would require the same amount of force as "that needed to lift a medium-sized car."

Because of the higher density of the quicksand, it would be impossible for a human or animal to completely sink in the quicksand, though natural hazards present around the quicksand would lead people to believe that quicksand is dangerous. In actuality the quicksand itself is harmless on its own, but because it greatly impedes human locomotion, the quicksand would allow harsher elements like solar radiation, dehydration, or tides to harm a trapped person.

Recent research findings

It was commonly believed that the behavior of quicksand was due solely to saturated or supersaturated suspensions of granules in water. Pressure from underground sources of water would separate and suspend the granular particles, reducing the friction between them. In September 2005, it was shown that it is the presence of salt that is largely responsible. The stability of the colloidal quicksand is compromised by the presence of salt, increasing the likelihood of sand flocculation and the formation of the high viscosity regions of sediment responsible for quicksand's "trapping" power.

Prevalence

Quicksand may be found inland (on riverbanks, near lakes, or in marshes), or near the coast.

One region notorious for its quicksands is Morecambe Bay, England. As the bay is very broad and shallow, a person trapped by the quicksand would be exposed to the danger of the returning tide, which can come in rapidly. The tidal flats around Mont Saint-Michel in Brittany are also notorious for quicksand. The medieval Bayeux Tapestry depicts future English King Harold Godwinson rescuing two Norman knights from the quicksand at Mont Saint-Michel.

See also

• Dry quicksand
• Fech fech
• Liquid limit
• Plastic limit
• Soil liquefaction
• Thixotropy

External links

• Howstuffworks.com - How quicksand works
• "What is quicksand?". Scientific American. {{cite news}}: Cite has empty unknown parameter: |coauthors= (help)
• Quicksand in Fiction
• Popular Quicksand Movies