Geology Where is quicksand

Physics: Treacherous quicksand

Physicists in the Netherlands investigated the question of whether people can actually drown in the desert sand

The young servant Daud only ventured a few steps up a dune when the soft sand of Sinai suddenly swallowed him - as shown in a scene in the film "Lawrence of Arabia". So far, scientists have dismissed the filmmakers' idea as nonsense: It was said that quicksand could only arise where the subsoil was saturated with water - that is, in no way in deserts. In the moist environment, the grains of sand floated next to each other, their surfaces hardly touched and could not develop any holding forces; the sand is therefore not stable. In the dry sand, on the other hand, the grains formed a dense network - even houses could safely stand on them.

Ball sinks in the sand

Nevertheless, the servant was able to sink into a woe in the desert sand, researchers at the University of Twente in Enschede have now discovered. They pressed air into a vessel filled with sand and thus whirled up the grains. After these had sat down again, the researchers hung a table tennis ball filled with bronze particles directly on the surface. No sooner had they cut the holding thread than the ball sank into the sand within milliseconds - almost like in water.

Shortly afterwards, a fountain of sand shot up, and then a small air bubble escaped. To recover the ball, the researchers had to dig deep: it was 22.4 centimeters deep - more than five times its diameter. The more mass the ball has, the deeper it sinks. The fact that the "dry quicksand" behaves like a liquid is due to its low density. The choppy grains of sand arrange themselves in a particularly "airy" way by being whirled up. Instead of 55 to 60 percent of the volume, the proportion of sand is only 40 percent - the rest is air.

All GEOSKOPE from magazine No. 2/05

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