The Cretaceous‐Tertiary boundary events: External or internal causes?

Alvarez [1986] recently summarized in Eos the evidence for the hypothesis that an impact was actually responsible for the terminal Cretaceous events. Alvarez reviewed the physical and chemical evidence for an impact; at the same time, he provided an interesting account of how quickly ideas evolved after the original discovery, by himself and his colleagues [Alvarez et al., 1980], of a conspicuous iridium anomaly at the Cretaceous‐Tertiary boundary (also known as the KTB). An alternate hypothesis is a volcanic (either quiet basaltic or violent siliceous) origin for the KTB [e.g, Officer and Drake, 1983, 1985]. Alvarez summarized his observations with a table [Alvarez, 1986, Table 1, p. 649] in which each hypothesis is tested against the following evidence: iridium, spherules, shocked quartz, soot, and worldwide distribution. The “yes” and “no” answers to these tests are, of course, a function of present data and knowledge. The purpose of this article is to stress that answers have changed rapidly in recent years and may still evolve, and also that Alvarez's declaration that the case for a volcanic cause is extinct may be somewhat premature.