Modern Mind

many, however, this process wouldhave been too slow – plants and animals would have adjusted, whereas in fact about half the life forms on Earth suddenly disappeared between the Cretaceous and the Tertiary. After the study of so many craters on other moons and planets, some palaeontologists began to consider whether a similarly catastrophic event might not have caused the mass extinctions seen on earth 65 million years ago. In this way there began an amazing scientific detective story that was not fully resolved until 1991.
    For a meteorite or asteroid to cause such a devastating impact, it needed to have been a certain minimum size, so the crater it caused ought to have been difficult to overlook. 18 No immediate candidate suggested itself, but the first breakthrough came when scientists realised that meteorites have a different chemical structure to that of Earth, in particular with regard to the platinum group of elements. This is because these elements are absorbed by iron, and the earth has a huge iron core. Meteorite dust, on the other hand, would be rich in these elements, such as iridium. Sure enough, by testing rocky outcrops dating from the Cretaceous/Tertiary border, Luis and Walter Alvarez, from the University of California at Berkeley, discovered that iridium was present in quantities that were
ninety times
as rich as they should have been if no impact had taken place. 19 It was this discovery, in June 1978, that set off this father- and-son (and subsequently daughter-in-law) team on the quest that took them more than a decade. The second breakthrough came in 1981, in
Nature,
when Jan Smit, a Dutch scientist, reported his discoveries at a K/T boundary site at Caravaca in Spain. 20 He described some small round objects, the size of a sand grain, called spherules, which he said were common at these sites and on analysis were shown to have crystals of a ‘feathery’ shape, made of sanidine, a form of potassium feldspar. 21 These spherules, it was shown, had developed from earlier structures made of olivine – pyroxene and calcium-rich feldspar – and their significance lay in the fact that they are characteristic of basalt, the main rock that forms the earth crust under the oceans. In other words, the meteorite had slammed into the earth in the ocean and not on land.
    This was both good news and bad news. It was good news in that it confirmed there had been a massive impact 65 million years ago. It was bad news in the sense that it led scientists to look for a crater in the oceans, and also to look for evidence of the massive tsunami, or tidal wave, that must have followed. Calculations showed that such a wave would have been a kilometre high as it approached continental shorelines. Both of these searches proved fruitless, and although evidence for an impact began to accumulate throughout the 1980s, with more than 100 areas located that showed iridium anomalies, as they were called, the actual site of the impact still remained elusive. It was not until 1988, when Alan Hildebrand, a Canadian attached to the University of Arizona, first began studying the Brazos River in Texas, that the decade-long search moved into its final stage. 22 It had been known for some time that in one place near Waco the Brazos passes over some rapids associated with a hard sandy bed, and this bed, it was recognised, was the remnant of a tsunami inundation. Hildebrand looked hard at Brazos and then went in search of evidence that would link it, in a circular fashion, with other features in the area. By examiningmaps, and gravity anomalies, he finally found a circular structure, which might be an impact crater, on the floor of the Caribbean, north of Colombia, but also extending into the Yucatán Peninsula in Mexico. Other palaeontologists were sceptical at first, but when Hildebrand brought in help from geologists more familiar with Yucatán, they soon confirmed the area as the impact site. The reason everyone had been so confused was that the crater – known as Chicxulub – was buried under more recent rocks. 23 When Hildebrand and his colleagues published their paper in 1991, it caused a sensation, at least to geologists and palaeontologists, who now had to revise their whole attitude: catastrophic events
could
have an impact on evolution. 24
    The discovery of Chicxulub produced other surprises. First, it turned out that the crater was to an extent responsible for the distribution of cenotes, small, spring-fed lakes that