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GEOSCIENCES BY PETER J. SMITH Department of Earth Sciences Open University Milton Keynes, U.K. " To avoid the semantic confusion inherent in the use of the term “catastrophe” to describe mass extinctions, a scale has been devised to express the magnitude of such rare events. By partial analogy with the Richter earthquake-magnitude scale, extinction magnitude (Me) is defined as the natural logarithm of the ratio of accelerated extinction rate to background extinction rate over a

Peter Smith

GEOSCIENCES

BY PETER J. SMITH
Department of Earth Sciences
Open University
Milton Keynes, U.K.

" To avoid the semantic confusion inherent in the use of the term “catastrophe” to describe mass extinctions, a scale has been devised to express the magnitude of such rare events. By partial analogy with the Richter earthquake-magnitude scale, extinction magnitude (Me) is defined as the natural logarithm of the ratio of accelerated extinction rate to background extinction rate over a specific period.

K.J. Hsti, “Catastrophic extinctions and the inevitability of the improbable,” Journal of the Geological Society, 146, 749-54, September 1989. (Geologicshes Institut ETH- Zentrum, Zurich, Switzerland)

" Application of the U-Pb dating method to zircons and baddeleyite (ZrO2) has given the first precise date of 615 million years for the Long Range dikes in Southeastern Labrador. Because mafic dike swarms are an early product of tensional episodes, this age may represent the start...

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