No. 60: Nov-Dec 1988
An increasingly popular scenario is: (1) Every 34 million years the solar system bobs up and down through the thickly populated disk of our galaxy; (2) The resulting encounters lead to showers of comets and/or asteroids on earth; (3) The mechanic trauma leads to basalt flooding; (4) Great biological extinctions occur in consequence; and (5) The terrestrial magnetic field reverses in step.
Now, if scientists could show that all of these phenomena occur at the same frequency and are roughly in phase, it would constitute one of science's most important syntheses. The stratigraphic record and the estimated ages of meteor craters certainly hint at such synchrony. Recently, two more papers have appeared which support the above scenario. First, M.R. Rampino and R.B. Stothers show that during the past 250 million years, eleven episodes of basalt flooding have occurred with an average cycle time of 32 million years. Second, J. Negi maintains that the earth's magnetic record boasts a similar string of disturbances, with an average period of 33 million years.
(Anonymous; "Regular Reversals in Earth's Magnetic Field A Fluke?" New Scientist, p. 32, August 25, 1988.)