IMPACT-INDUCED VOLCANISM: A GEOLOGIC MYTH

The idea that the impact of a large comet or meteorite may cause a volcanic eruption is becoming entrenched in the geologic literature. In spite of the apparent appeal of this idea, there is no evidence that it has ever actually happened, either on the Earth or on any other planet in the solar system. Impacts produce melt directly from the target rocks by the process of shock wave compression and release. This process is well understood thermodynamically and does not lead to volcanoes or lava-like deposits. It has been suggested that pressure relief melting, the effective source of mid-ocean ridge basalts and plume-related volcanism, might occur beneath the uplifted floors of large craters. Quantitative analysis of this process, however, shows that the amount of uplift is only about 1/10 of the crater diameter and, except for impacts that produce craters of continental size, cannot produce significant quantities of basaltic melt. A currently popular idea is that seismic energy from an impact might be sharply focused below the surface at the antipode of the impact. However, this energy is at best only a small fraction of the total impact energy and again quantitative analysis shows that impacts the size of the Chicxulub crater (100 km transient crater diameter) could have raised the temperature at its antipode by only a fraction of a degree. The total energy required to melt the basalts in many flood basalt provinces greatly exceeds the total energy available from the impacts that have been proposed to form them. Furthermore, study of planets such as the Moon and Mercury that do show signs of antipodal focusing from large impact basins show no trace of volcanism at the antipodes. There is thus neither evidence for impact-induced volcanism nor a plausible mechanism for creating it. Proponents of an impact origin for flood basalt provinces should probably look for more effective causes.