Igneous outbursts and mass extinctions

Publisher Website
Google Scholar
Abstract
Were dinosaurs and their friends done in by external causes, such as cometary impacts or by internal causes, such as massive volcanism? The popular imagination has been caught by the cometary impact hypothesis, perhaps partly because of the vicarious sense of danger it provides for all of us on planet Earth.One of the major differences between impact and volcanism scenarios in their simplest forms is in the duration of the events. Impacts, by their very nature are instantaneous, even on everyday time scales. By contrast, the volcanism in large igneous provinces such as flood basalts has often been thought to last for tens of millions of years. Recent work using 40Ar‐39Ar dating and paleomagnetic reversals, most notably on the Deccan [Courtillot and Cisowski, 1987; Duncan and Pyle, 1988; Courtillot et al., 1988], indicates that the great majority of the flood basalts in any particular province may be extruded very rapidly, within as little as 0.5–2 m.y. (Table 1). Detailed paleontological studies suggest that a gradual or a step‐wise extinction model lasting of the order of tens of thousands to 1 m.y. or more may describe the disappearance of species better than a single extinction event [Perch‐ Nielsen et al., 1982; Alvarez, 1986; Hut et al., 1987; Hallam, 1987; Keller, 1988; Sepkoski, 1989], although it is worth cautioning that sampling problems may make an abrupt extinction appear like a step‐wise change. With modification of the impact hypothesis to allow for multiple impacts over a period of up to a few million years, both impacts and volcanism provide viable mechanisms for explaining extinctions from the point of view of duration. It is worth commenting, however, that although there were rapid changes in the rate of species turnover throughout geologic time, with particularly marked and widespread extinctions near the Cretaceous‐Tertiary boundary, the reality of periodically repeated mass extinctions is, to say the least, not yet established [Hoffman, 1989].