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No. 86: Mar-Apr 1993

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Precariously Balanced Rocks As Earthquake Detectors

Precariously balanced rocks (PBR)
PBRs, such a this "rocking stone" near Peekskill, NY, signify a lack of recent quakes in the area
Precariously balanced rocks (PBRs) are rather common where earthquakes have never occurred. In this sense, balanced rocks are measures of seismic stability. For example, says J. Brune, you won't find PBRs within 10 miles of spots where quakes have shaken the ground over the past few thousand years. To illustrate:

"Rocks stacked in piles and balanced on their narrow ends on Yucca Mountain near the Nevada border with California, he said, have not moved in at least 10,000 years and perhaps as many as 100,000 years, judging from the depth of "rock varnish," or weathering, on their exposed surfaces."

Looking for PBRs is not really as useless as it sounds, for they are indicators of stability to construction engineers planning nuclear waste disposal sites and similar projects requiring long-term seismic quiet.

(Petit, Charles; "Seismologist Studies Precariously Balanced Rocks," San Francisco Chronicle, December 8, 1992. Cr. J. Covey)

Comment. How do rocks become "precariously balanced" in the first place? Melting glaciers and snow packs are known to ease their cargos of rocky debris gently down into unstable configurations.

From Science Frontiers #86, MAR-APR 1993. 1993-2000 William R. Corliss