Home Page Science Frontiers

No. 86: Mar-Apr 1993

Issue Contents

Other pages

Other Interesting Sites











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

Science Frontiers Sourcebook Project Reviewed in:


  • "A sourcebook of unexplained phenomena is therefore a valuable addition to a collection of scientific literature. William R. Corliss has provided this in the past with his source books of scientific anomalies in several subjects, and now he has provided it for astronomy. He has done an excellent job of collecting and editing a large amount of material, taken in part from scientific journals and in part from scientific reporting in the popular or semi-scientific press." -- "The Mysterious Universe: A Handbook of Astronomical Anomalies", reviwed by Thomas Gold, Cornell University, in Icarus, vol.41, 1980

  • "An interesting, systematic presentation of unusual weather [..] This book is recommended for a general audience" --"Corliss, William R., Tornados, Dark Days, Anomalous Precipitation, and Related Weather Phenomena, Sourcebook Project, 1983.", revieweed in Choice, September 1983
  • "..the science is necessarily somewhat speculative, but Corliss's symthesis is based on reputable sources." -- "Corliss, William R. (Compiler). Lightning, Auroras, Nocturnal Lights, and Related Luminous Phenomena" reviwed by Joseph M. Moran, Univ. of Wisconsin in Science Books and Films, Sep/Oct 1983

  • "Before opening the book, I set certain standards that a volume which treads into dangerous grounds grounds like this must meet. The author scrupulously met, or even exceeded those standards. Each phenomenon is exhaustively documented, with references to scientific journals [..] and extensive quotations" -- "Book Review: The moon and planets: a catalog of astronomical anomalies", The Sourcebook Project, 1985., Corliss, W. R., Journal of the Royal Astronomical Society of Canada, Vol. 81, no. 1 (1987), p. 24., 02/1987