What if two people stumbled into quicksand: Would the heavier person sink faster?

Quicksand isn't as scary as Hollywood makes it out to be in the movies.
Quicksand isn't as scary as Hollywood makes it out to be in the movies.
Photographer: Disorderly I Agency: Dreamstime.com

Before we delve into this question, we need to take a look at how quicksand works. When you see quicksand in the movies, a doom-destined hero or heroine is typically being sucked into a huge mass of moving, wet sand. Fortunately, quicksand isn't quite the fearsome force of nature that you sometimes see on the big screen.

Quicksand isn't a unique type of soil; it's usually just sand or another type of grainy soil and can occur anywhere under the right conditions, according to Denise Dumouchelle, a geologist with the United States Geological Survey. A good example is sand at the beach. Think about the wet sand just at the edge of the surf. It looks solid, but when you tap it with your big toe, it jiggles. Quicksand is basically just ordinary sand with reduced friction between sand particles. The "quick" in "quicksand" refers to how easily the sand shifts.

Two things can make sand become quick:

  • Flowing underground water: The water seeps up into the sand, creating a tiny pocket between each sand granule. As the sand becomes saturated with water, the friction between the sand particles is reduced. The sand takes on a more fluid-like quality.
  • Earthquakes: The immense force of the earthquake agitates the sand or dirt, sifting it apart, so that friction between the particles is reduced. The affected area becomes unstable, causing buildings or other objects on that surface to sink or fall over.

OK, so now we know how it works -- but what happens when two people stumble into some quicksand? The average human body has a density of 62.4 pounds per cubic foot (1 g/cm³) and is able to float on water. If you have a low percentage of body fat, you're going to be denser, but the air trapped in your lungs will still let you float. 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 not to panic. Most people who drown in quicksand, or any liquid for that matter, usually panic and begin flailing their arms and legs.