Quicksand is a colloid mixture, typically consisting of fine granular material like sand or silt, that has become supersaturated with water. This combination creates a shear-thinning non-Newtonian fluid, meaning its viscosity changes dramatically when stress or force is applied. When left undisturbed, quicksand may appear solid, but the slightest disturbance, such as a footstep, causes it to lose its structural integrity and behave like a liquid.
The Historical Meaning of Quick
The name “quicksand” does not refer to the speed at which a person sinks, which is often quite slow. The word “quick” in this context is derived from the Old English word cwic, which meant “living,” “alive,” or “lively.” This older meaning is still preserved in phrases like “the quick and the dead.” In the case of quicksand, the term was used because the ground itself seems to move and yield easily, behaving as if it were unstable or “alive”. The name therefore describes the unstable, shifting nature of the sand rather than the velocity of sinking.
How Quicksand Loses Its Strength
The transformation of solid ground into quicksand is governed by a process known as liquefaction. Quicksand forms when a volume of loose, saturated sand experiences a sudden increase in internal water pressure. Sand is a granular material with pore spaces between the individual grains that are normally filled with water or air.
Under normal conditions, the friction between the sand grains, known as effective stress, allows the material to bear weight. When water saturates the sand and cannot drain away quickly, the pore spaces become entirely filled with water. An external disturbance, such as the vibration from an earthquake or a person stepping onto the surface, introduces a sudden stress.
This stress causes the sand grains to momentarily try to compact, but the trapped water resists compression, leading to a rapid rise in interstitial water pressure. The elevated water pressure pushes the sand grains apart, effectively floating them and eliminating the friction holding them together. The effective stress drops to nearly zero, and the sand-water mixture behaves like a dense fluid that cannot support the weight of an object.
The material’s loss of strength means the sand particles are temporarily suspended in the water, giving quicksand its spongy, fluid-like texture. This liquefied state allows objects to sink into the material until buoyancy takes over. This mechanism explains why quicksand is a specific geological condition where water pressure overcomes the gravitational forces of the granular material.
Why You Cannot Sink Completely
The common portrayal of a person completely disappearing into quicksand is scientifically inaccurate because the density of the quicksand mixture is far greater than the density of the human body. The density of quicksand is approximately 2 grams per cubic centimeter, as it is a combination of water and heavy mineral grains. In contrast, the average density of the human body is typically around 1 gram per cubic centimeter.
According to Archimedes’ principle, an object floats in a fluid if the object’s density is less than the fluid’s density. Since quicksand is twice as dense as a human, a person will naturally float on its surface. Initial sinking occurs because the quicksand temporarily loses its yield strength, but the body will stop sinking once about half of its volume is submerged.
The human body stabilizes at a depth roughly equivalent to the waist or chest. The real danger is not sinking completely, but rather becoming trapped and unable to exert the massive force required to pull a limb out of the highly viscous mixture.