Quicksand
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Quicksand and warning sign at a gravel extraction site. |
This article is about the geological feature. For other meanings, see Quicksand (disambiguation)Quicksand is a
hydrocolloid gel consisting of fine
granular matter (such as
sand or
silt),
clay, and
water. When unperturbed, it often appears to be
solid; however, even a minor (less than 1%) change in the
stress on the quicksand will cause a sudden decrease in its
viscosity. After the initial perturbation - 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 suddenly increase. In order 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 one
centimeter per
second would require the same amount of force as "that needed to lift a medium-sized car."
["A. Khaldoun, E. Eiser, G. H. Wegdam and Daniel Bonn Rheology: Liquifaction of quicksand under stress" 'Nature' Vol. 437, Pg. 635, 29 September 2005 doi:10.1038/437635a]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. As of September,
2005 it has been 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.
Quicksand is not as dangerous as depicted in many
movies. As quicksand is rarely more than a few feet deep, there is usually little danger of sinking below the surface. Furthermore, even when the quicksand is deep enough, deliberate effort is required to sink below the surface. Quicksand is typically
denser than the
human body, meaning that a body is much more
buoyant in quicksand than in water. Thus, the body will float quite easily in quicksand.
This was demonstrated in an episode of the TV series
MythBusters and in an article in
Nature.
The sand's higher density will gradually push a human body upward, eventually allowing one to paddle toward more solid footing. Typically, the greatest danger of getting stuck in quicksand comes from
exposure,
starvation,
flash flooding, or
tidal flooding.
Quicksand can be found inland (on riverbanks, near
lakes, or in
marshes) or near the coast. It can also form when an
earthquake increases
groundwater pressure, forcing the water to the surface and causing
soil liquefaction.
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 quite rapidly.
*
Dry quicksand*
Liquid limit*
Plastic limit*
Thixotropy*
Howstuffworks.com - How quicksand works
