Terrestrial mass extinctions



That galactic triggering or forcing of terrestrial biologic crises could arise as a result of collisions (or close encounters) of the solar system with intermediate-sized to large-sized interstellar clouds of gas and dust is a theory postulated by R. Rampino and B. Stothers of NASA's Goddard Institute for Space Studies. The idea is that episodes of major mass extinctions and impact cratering on the earth during the last 250 million years seem to have a dominant periodicity of 30 million years with essentially identical phasing. According to Rampino and Stothers, this terrestrial periodicity is found to be strongly correlated with the time needed for the solar system to oscillate vertically about the plane of the Milky Way galaxy, which is 33±3 million years based on current astronomical evidence (see Eos, February 28, 1984, p. 75).