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and reshaped their surfaces. It may have carved the moon from the earth. It produced the most energetic event in the last 600 million years of earth history, one that led directly to the K-T mass extinction. Even though it is counterintuitive, our intellect forces us to recognize that impact has happened thousands, indeed tens of thousands of times, since the earth cooled (though few impacts would have been the size of Chicxulub). Could the energy released by myriad impacts throughout geologic time be the grand unifier of geology?

rECOGNIZING iMPACT

Before we get too far out on a limb of speculation, let us ask first whether there is hard evidence for impact at any mass extinction horizon other than the K-T. Do any others show an iridium spike, shocked minerals, and spherules, not to mention spinel, diamonds, and soot? Do any others have an impact crater of corresponding age? If the answer to these questions is no, we would have to set aside the notion that impact, beyond its singular occurrence at the K-T boundary, has played an important role in earth history. Luis's prediction would have failed.

Of course, to be prepared to base a judgment on hard evidence presumes that the indicators of impact, if once present, would remain around to be discovered and that they could be detected. Are these fair presumptions? Not really. Recall that geologic boundaries were defined, well over a century ago, primarily because they were easy to spot in the field—they tend to be places where one rock type abruptly gives way to another. But these are the very places where erosion has done its work. Almost by definition then, geologic boundaries are apt to be the location of gaps in the rocks: levels at which erosion has removed whatever was present, including any thin impact ejecta layers.

Another difficulty is that subduction has removed oceanic crust older than about 125 million years. Any extinction older than that, which includes four of the Big Five, cannot be found preserved in cores drilled from the oceanic sedimentary layer, which offers the most continuous and least disturbed sections. Instead we must seek these older boundaries in continental rocks, where erosion is more likely to have removed them.

What about our old friend iridium? If found in high concentrations it is as good an indicator as ever, but the converse is not true: Low iridium levels do not necessarily rule out impact. First, comets, which travel through space at around 45 km/sec to 60 km/sec, cause a significant fraction of all impacts (see Table 1, page 51). Asteroids move at slower velocities, averaging about 20 km/sec. Since kinetic energy is proportional to velocity squared, this three-fold difference in speed means that a crater of a given size can be produced by a comet one-ninth the size of the asteroid required to produce that same crater. Thus an impact crater produced by a comet would leave no more than one-ninth the iridium to be found in a crater of the same size formed by an asteroid. But even this is an upper limit. Comets, being as much as 50 percent ice, carry much less iridium to start with (calculations that combine crater size and composition show that the amount of iridium left by a comet might be as low as 1 percent of that left by an asteroid, too little to be detected). As Table 1 shows, larger craters are successively more apt to have been formed by comets (one of the reason most specialists now believe that Chicxulub was formed by the impact of a comet). Thus it is ironic but true that the larger the crater, the less likely it is to leave iridium behind. To carry matters further, in the largest impacts, regardless of impactor type, almost all the ejecta is blasted back out into space, escaping the earth's gravity field altogether and leaving no trace behind.2 Still another complication is that in smaller impacts, extraterrestrial material composes only about 10 percent of the ejecta, so that if the impactor happened to be an asteroid relatively low in iridium, which some are, little would be left to find. Finally, even when iridium was present initially, reworking and bioturbation could have smeared it out, or acid leaching could have removed it. All in all, iridium is a kind of one-way indicator: Its presence is strong evidence of impact; its absence is not evidence of no impact.

Impact by either comets or asteroids, however, would leave behind shocked minerals and possibly spherules, maybe even spinel and diamond. Since these markers are less subject to alteration or removal by chemical and geologic processes, they make a better bet as indicators of impact than iridium. Most geologists continue to be most impressed by shocked quartz, the indicator that they discovered.

Finding a crater that dates to the time of a geologic boundary is fraught with the same difficulties that we encountered in the search for Chicxulub. Erosion will have erased most impact craters; others will have disappeared down subduction zones. The older the crater, the more likely one of these fates. Most crater ages are not known with precision, making it difficult to assign them to a given geologic boundary with much confidence. The Manson crater in Iowa is a good example. For years its age was known only roughly, then the first measurement gave 65 million years, and finally more precise methods yielded an age of 73.8 million years. And not only must we know the age of a candidate crater, we must know the age of the extinction boundary with which it might be correlated. But the ages of many boundaries, and even their positions, have yet to be pinned down.

When we consider all these uncertainties, the accidental finding of the iridium spike at Gubbio, and the diligent search that led to the discovery of the iridium-rich layer amid the lava flows and inter-trappean sediments of the Deccan, appear all the more remarkable. Even if impact has occurred at another geologic boundary, we could easily miss it. If after a diligent search, however, no evidence of impact has turned up, practical geologists, with limited time and resources, would move on to fields with more chance of results. While the absence of evidence may not be evidence of absence, it is discouraging. Being human, scientists tend to go where positive evidence and rewards can be found.

t HE b IG fIVE

The K-T mass extinction was one of five in which more than 70 percent of species died. If there is anything to the notion that impact has caused other mass extinctions, it is here, among the other four, that we should first look. Table 4 summarizes the ages of the Big Five plus the Eocene-Oligocene and Jurassic-Cretaceous extinction boundaries, and the evidence of impact that has so far been found associated with each. The three right-most columns give age and size information for craters that happen to have the same approximate age as the boundary. The table implicitly asks for each of these extinctions: Is there any evidence of impact, and is there a large crater of the same age?

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