How Quicksand Works
by Kevin Bonsor
How many times have you watched a movie where the hero is sucked down into a pit of quicksand, only to be saved at the last minute by grabbing a nearby tree branch and pulling himself out? If you believed what you saw in movies, you might think that quicksand is a living creature that can suck you down into a bottomless pit, never to be heard from again. But no -- the actual properties of quicksand are not quite those portrayed in the movies.
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With quicksand, the more you struggle in it the faster you will sink. If you just relax, your body will float in it because your body is less dense than the quicksand.
Quicksand is not quite the fearsome force of nature that you sometimes see on the big screen. In fact, quicksand is rarely deeper than a few feet. It can occur almost anywhere if the right conditions are present. Quicksand is basically just ordinary sand that has been so saturated with water that the friction between sand particles is reduced. The resulting sand is a mushy mixture of sand and water that can no longer support any weight.
If you step into quicksand, it won't suck you down. However, your movements will cause you to dig yourself deeper into it. In this article, you will learn just how quicksand forms, where it's found and how you can escape its clutches if you find yourself hip-deep in it.
Main > Science > Earth Science
How Quicksand Works
by Kevin Bonsor
What's Quicksand?
Quicksand is an interesting natural phenomenon -- it is actually solid ground that has been liquefied by an oversaturation of water. The "quick" refers to how easily the sand shifts when in this semiliquid state.
Quicksand is not a unique type of soil; it is usually just sand or another type of grainy soil. Quicksand is nothing more than a soupy mixture of sand and water -- in essence, the sand is floating on water. It can occur anywhere under the right conditions, according to Denise Dumouchelle, geologist with the United States Geological Survey (USGS).
Quicksand forms when uprising water reduces the friction between sand particles, causing the sand to become "quick."
Quicksand is created when water saturates an area of loose sand and the ordinary sand is agitated. When the water trapped in the batch of sand can't escape, it creates liquefied soil that can no longer support weight. There are two ways in which sand can become agitated enough to create quicksand:
* Flowing underground water - The force of the upward water flow opposes the force of gravity, causing the granules of sand to be more buoyant.
* Earthquakes - The force of the shaking ground can increase the pressure of shallow groundwater, which liquefies sand and silt deposits. The liquefied surface loses strength, causing buildings or other objects on that surface to sink or fall over.
Vibration tends to enhance the quickness, so what is reasonably solid initially may become soft and then quick, according to Dr. Larry Barron of the New South Wales Geological Survey.
The vibration plus the water barrier reduces the friction between the sand particles and causes the sand to behave like a liquid. To understand quicksand, you have to understand the process of liquefaction. When soil liquefies, as with quicksand, it loses strength and behaves like a viscous liquid rather than a solid, according to the Utah Geological Survey. Liquefaction can cause buildings to sink significantly during earthquakes.
While quicksand can occur in almost any location where water is present, there are certain locations where it's more prevalent. Places where quicksand is most likely to occur include:
* Riverbanks
* Beaches
* Lake shorelines
* Near underground springs
* Marshes
The next time you're at the beach, notice the difference in the sand as you stand on different parts of the beach that have varying levels of moisture. If you stand on the driest part of the beach, the sand holds you up just fine. The friction between the sand particles creates a stable surface to stand on.
If you move closer to the water, you'll notice that the sand that is moderately wet is even more tightly packed than the dry sand. A moderate amount of water works to increase the friction between sand particles. This is what allows you to build sand castles.
But beach sand could easily become quicksand if enough water were thrust up through it. If an excessive amount of water flows through the sand, it forces the sand particles apart. This separation of particles causes the ground to loosen, and any mass on the sand will begin to sink through it.
How to Escape
If you ever find yourself in a pit of quicksand, don't worry -- it's not going to swallow you whole, and it's not as hard to escape from as you might think.
The human body has a density of 62.4 pounds per cubic foot (1 g/cm3) and is able to float on water. Quicksand is denser than water -- it has a density of about 125 pounds per cubic foot (2 g/cm3) -- which means you can float more easily on quicksand than on water. The key is to not panic. Most people who drown in quicksand, or any liquid for that matter, are usually those who panic and begin flailing their arms and legs.
It may be possible to drown in quicksand if you were to fall in over your head and couldn't get your head back above the surface, although it's rare for quicksand to be that deep. Most likely, if you fall in, you will float to the surface. However, the sand-to-water ratio of quicksand can vary, causing some quicksand to be less buoyant.
"By the same token, if the quicksand were deep, as in up to your waist, it would be very difficult to extract yourself from a dense slurry, not unlike very wet concrete," said Rick Wooten, senior geologist for Engineering Geology and Geohazards for the North Carolina Geological Survey. "The weight of the quicksand would certainly make it difficult to move if you were in above your knees."
If you can't see this animation, download the free Flash 5 player.
With quicksand, the more you struggle in it the faster you will sink. If you just relax, your body will float in it because your body is less dense than the quicksand.
The worst thing to do is to thrash around in the sand and move your arms and legs through the mixture. You will only succeed in forcing yourself farther down into the liquid sandpit. The best thing to do is to make slow movements and bring yourself to the surface, then just lie back. You'll float to a safe level.
"When someone steps in the quicksand, their weight causes them to sink, just as they would if they stepped in a pond," Dumouchelle said. "If they struggle, they'll tend to sink. But, if they relax and try to lay on their back, they can usually float and paddle to safety."
When you try pulling your leg out of quicksand, you are working against a vacuum left behind by the movement, according to The Worst-Case Scenario Survival Handbook. The authors of the book advise you to move as slowly as possible in order to reduce viscosity. Also, try spreading your arms and legs far apart and leaning over to increase your surface area, which should allow you to float.
While quicksand remains the hackneyed convention of bad adventure movies, there's very little to be afraid of in real life. As long as you keep a cool head in the situation, the worst result will be a shoe full of wet sand.
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