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Quicksand forms in saturated loose sand when the sand is suddenly agitated. When water in the sand cannot escape, it creates a liquefied soil that loses strength and cannot support weight. Quicksand can form in standing water or in upwards flowing water (as from an artesian spring). In the case of upwards flowing water, seepage forces oppose the force of gravity and suspend the soil particles.
The saturated sediment may appear quite solid until a sudden change in pressure or shock initiates liquefaction. This causes the sand to form a suspension and lose strength. The cushioning of water gives quicksand, and other liquefied sediments, a spongy, fluidlike texture. Objects in liquefied sand sink to the level at which the weight of the object is equal to the weight of the displaced soil/water mix and the submerged object floats due to its buoyancy.
Liquefaction is a special case of quicksand. In this case, sudden earthquake forces immediately increase the pore pressure of shallow groundwater. The saturated liquefied soil loses strength, causing buildings or other objects on that surface to sink or fall.
Quicksand is a shear thinning 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 decrease 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 0.01 m/s would require the same amount of force as "that needed to lift a medium-sized car."
Contrary to popular belief, quicksand itself is harmless: a human or animal is unlikely to sink entirely into quicksand and drown at all due to the higher density of the fluid (assuming the quicksand is on dry ground and not under water, but even if underwater, sinking is still impractical). Quicksand has a density of about 2 grams per milliliter, whereas the density of the human body is only about 1 gram per milliliter. At that level of density, sinking in quicksand is impossible. Descending about up to the waist is possible, but not any 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.
Continued or panicked movement, however, may cause a person in quicksand to sink deeper, leading to belief that quicksand is dangerous. Since it increasingly impairs movement, this, then, can lead to a situation where other factors such as weather exposure, dehydration, hypothermia, tides or carnivores may harm a trapped person.
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People falling into (and, unrealistically, being submerged in) quicksand or a similar substance is a trope of adventure fiction, notably in movies. According to Slate, this gimmick had its heyday in the 1960s, when almost 3% of all films showed someone sinking in mud, sand, or clay. For instance, T.E. Lawrence's servant boy Daud dies in quicksand in a scene in the 1962 movie Lawrence of Arabia. Actor Al Mulock's character dies by falling into a quicksand pit in the 1959 adventure Tarzan's Greatest Adventure. In 1960's Tarzan the Magnificent Gordon Scott and Jock Mahoney hide from John Carradine in a pit of African Black Quicksand. The proliferation of quicksand scenes in movies has given rise to an Internet subculture scene dedicated to the topic.
Several episodes of Gilligan's Island involved the Castaways nearly coming to grief in quicksand. In one episode Thurston Howell III temporarily faked his own death by placing his hat on top of a patch of quicksand.
- Khaldoun, A., E. Eiser, G. H. Wegdam, and Daniel Bonn. 2005. "Rheology: Liquefaction of quicksand under stress." Nature 437 (29 Sept.): 635. doi:10.1038/437635a
- Bakalar, Nicholas (September 28, 2005). "Quicksand Science: Why It Traps, How to Escape". National Geographic News. Retrieved October 9, 2011.
- Discovery Channel. MythBusters. Season 2. "Killer Quicksand." October 20, 2004.
- Engber, Daniel (23 August 2010). "Terra Infirma: The rise and fall of quicksand.". Slate. Retrieved 23 August 2010.
- Seeger, Pete. "How Waist Deep in the Big Muddy Finally Got on Network Television in 1968". Pete Seeger Appreciation. Jim Capaldi. Archived from the original on August 5, 2013. Retrieved 2 August 2013.
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