Physics Photo of the Week

October 17, 2008

Deadly Trap
Don't fall into this pit, you will never get out. There lurks a vicious "monster" hidden in the bottom.  The sides of the pit consist of loose "rocks" that are constructed at the Angle of Repose - the steepest angle that dry aggregates can maintain under the influence of gravity.  The rubble at the angle of repose is highly unstable.  Any animal that crawls onto the slope of the pit may trigger an "avalanche"; the avalanche will carry the victim to the base of the pit where the hungry "monster" will quickly devour it.

Actually the "monster" is an insect larva called an antlion
- ants are its common prey that fall into the trap.  This trap was about 3 cm diameter underneath a spruce tree.  This trap is deadly only if you are an insect or arthropod walking around on the ground.  The antlion living in the bottom of the trap builds the trap by burrowing through the sandy soil and flipping the grains of dirt or sand out of the pit while burrowing deeper in a circular fashion.  As the grains fall on the sides of the area they distribute themselves in a pile that slopes at the angle of repose.  Sand banks at the angle of repose are very unstable.  An unsuspecting ant walking on the slopes at such an angle can trigger the avalanche - the ant falls to the bottom of the pit, gets trapped by sand falling on top of it, making easy prey for the jaws of the antlion larva lurking in the bottom.

The same angle of repose is seen is piles of sand or
gravel when the sand/gravel spills out of the end of a conveyor in a sand mining operation.  In road construction where engineers build berms of fill to elevate the road over the surrounding terrain, the sides of the banks must be less steep than the angle of repose, or the road bed will be highly unstable leading to disastrous results. 

at the base of mountains and cliffs - formed from the accumulation of rocks and boulders falling off the top of the cliffs during thousands of years - forms a slope just smaller than the angle of repose.  The photo at left shows the talus at Lake Louise, Banff National Park, Canada.  (Look for future physics photos featuring Lake Louise).

There will be no Physics Photo of the Week next week (October 24) due to Warren Wilson College fall vacation.  The next Physics Photo of the Week will be published on October 31, 2008

Physics Photo of the Week is published weekly during the academic year on Fridays by the Warren Wilson College Physics Department.  These photos feature an interesting phenomena in the world around us.  Students, faculty, and others are invited to submit digital (or film) photographs for publication and explanation.  Atmospheric phenomena are especially welcome.  Please send any photos to 

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