| MadSci Network: Earth Sciences |
A very fun question to answer, Angie!
Quicksand forms when sand accumulates in hollows and holes and becomes totally saturated with water and is not allowed to drain. Most of the time, the water cannot drain because the hollow or hole has an impervious layer of clay or other dense material underneath the sand. There may be lots of mud and vegetation mixed into the sand, making it into a thick soupy consistency that can be very sticky. With certain types of clays mixed into the sand and water, quicksand looks and feels solid (like a gel). However, when a disturbance occurs (for example, if you were to step into it), the solid becomes liquified. After a short period of time of non-disturbance, the sand-clay-water mixture can re-solidify. This reversible behavior of liquifying and resetting is called thixotropy (thix-ot-ro-py). Quicksands are often found along beaches, marshes, river deltas and stream beds.
Quicksand does not "suck" things down. Actually, because the density of the human body is less than that of the sand-water mixture, you cannot sink below the surface (much like being in a swimming pool where you can just float by not moving around). Indeed, the way you get out of quicksand is to stop struggling and you will just float on the surface. Struggling is what usually causes a person to lose their footing and subsequently drown in quicksand.
I hope that this answers your questions.
R.Ted Jeo
Biological Science Tech
USDA-ARS