Let’s dive a bit deeper into the dirt with the help of Darrel Long, sedimentologist at the Department of Earth Sciences at Laurentian University in Sudbury, Ontario. As Long describes at Scientific American, quicksand can form when individual grains of sand are ovular rather than spherical. Spherically grained sand is packed together pretty tightly, with about 25% to 30% of it being empty space. This is the kind of sand on a beach. Your foot smooshes it, and because of the space between its grains, the sand rises around your feet and between your toes. The same displacement happens when you drop an ice cube into a glass of water and the water level rises.
Quicksand, however, forms from longer grains of sand. Such grains stack in a different arrangement that leaves more empty space — from 30% to 70%. This kind of sand, like the aforementioned stack of oranges, or even a stack of cards, loses its form more easily. When vibrations, pressure, or moving water overcome the friction holding the grains together, the whole thing collapses.
And for the record, it’s possible for quicksand to form in a desert, but it’s very rare. In this case, the space between grains is filled with air, not water. This makes the sand very loosely compacted. If something disturbs the sand, like wind, its grains can slip into a more densely compacted shape. Usually, though, this kind of quicksand only sinks a few centimeters.
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