Experiencing Impacts: Probabilities, consequences, and possible human response to impacts during the Holocene
by
Peter H. Schultz
Department of Geological Sciences, Brown University, Providence, RI 02912-1846, USA

Impact cratering is a now recognized a natural geologic process that has affected all planetary surfaces. While we are able to assess the geologic record for the distribution of ejected materials and the structural effects, the consequences for humans during the Holocene would remain speculative if it were not for the Siberian Tunguska event. In 1908 an asteroid/comet disrupted 8 km above the surface and released energy equivalent to a 12megaton explosion. This type of an event should have occurred at least 35 times globally over the Holocene. At Tunguska around 80 million trees were leveled over an area of 2,000 square kilometers, even though a crater was not formed. The fallen trees at Tunguska provide a visible and distinctive record of the intense atmospheric blast. Such an event above the Argentine Pampas would not leave such clear evidence. There the consequences must be assessed using theory, laboratory experiments, computational modeling, and comparisons with preserved atmospheric collisions on other planets. These approaches indicate that such an event would generate intense scouring of surface sediments by winds exceeding 300m/sec. Such a blast would generate little impact melt, other than small, widely dispersed spherules. Slightly larger objects, however, can leave traces in the geologic record as meteorite strewnfields and small craters. The Campo del Cielo meteorite and crater field is one such example and is estimated to be about 10 ka BP. Nevertheless, even this event did not collide with sufficient velocities to generate melt. Indigenous peoples would not long forget such an event. If a larger asteroid were to reach the surface, then the porous nature of the sediments would enhance melting and generate vesicular slabs. Certain escorias found at certain horizons throughout Argentina Pampean sediments can be shown to be products of such collisions.

Date received: September 27, 2004