The Science of Discworld Revised Edition

series of climatic changes at the end of the Cretaceous, with very drastic changes of sea level as ice caps grew or melted. There is also good evidence that some seas, perhaps all, lost their oxygen-based ecology to become vast, stinking, black, anaerobic sinks. The fossil evidence for this consists of black iron-and sulphur-rich lines in sediments. The most dramatic terrestrial events were undoubtedly associated with the vulcanism which resulted in the so-called Deccan Traps, huge geological deposits of lava. The whole of Asia seems to have been covered with volcanoes, and they produced enough lava that it would have formed a layer 50 yards (45 m) thick if it had been spread over the whole continent. Such extensive vulcanism would have had enormous effects on the atmosphere: carbon dioxide emissions that warmed the atmosphere by the greenhouse effect, sulphur compounds resulting in terrible acid rain and freshwater pollution over the entire planet, and tiny rock particles blocking sunlight and causing ‘nuclear winters’ for decades at a time. Could the volcanoes that formed the Deccan Traps have killed the dinosaurs, instead of a meteorite? Much depends on the timing.
    Our preferred theory, not because there is good independent evidence for it but because it would explain so much, and because it has a moral, is that the two causes are linked. The Chicxulub crater is very nearly opposite the Deccan Traps, on the other side of the planet. Perhaps volcanic activity in Asia began some millions of years before the K/T boundary, causing occasional ecological crises for the larger animals but nothing really final. Then the meteorite hit, causing shockwaves which passed right through the Earth and converged, focused as if by a lens on just that fragile region of the planet’s crust. (A similar effect happened on Mercury, where a gigantic impact crater called the Caloris Basin is directly opposite ‘weird terrain’ caused by focused shockwaves.)
    There would then have been a gigantic, synchronized burst of vulcanism – on top of all the events of the collision, which would have been pretty bad on their own. The combination could have polished off innumerable animal species. In support of this idea, it should be said that another geological deposit, the Siberian Traps, contains ten times as much lava as the Deccan system, and it so happens that the Siberian Traps were laid down at the time of another mass extinction, the great Permian extinction, which we mentioned earlier. To pile on further evidence: some geologists believe they have found another meteorite impact site in modern Australia, which in Permian times was opposite to Siberia.
    Is there any evidence for this suggestion? In 2000, Dallas Abbott and Ann Isley analysed impact events to see whether they coincided with volcanic ‘superplumes’, where large amounts of liquid rock well up from the Earth’s mantle. A superplume is thought to be responsible for the Hawaiian island chain, for instance, whose relics stretch across half the Pacific Ocean. The likely explanation is that as the continents drifted, the Pacific Plate moved, so that the superplume welled up in what now seem to be different places. However, very recent observations suggest that superplumes can move, too. The Deccan and Siberian Traps were also probably caused by superplumes.
    The result of the analysis is that impacts and superplumes occur together far more often than they would if their association is due to chance. That suggests that impacts can make superplumes bigger, or more likely. (It’s hard to see how superplumes could make impacts more likely, except by attracting invading aliens who are heavily into vulcanism.) However, thanks to continental drift, it is not certain that the Deccan Traps were in fact on the exact far side of the Earth from Chicxulub at the time when the K/T meteorite hit. So we can’t yet be sure that the meteorite really did trigger massive volcanic activity.
    The moral of this tale is that we should not look for ‘the’ cause of the dinosaur extinction. It is very rare for there to be just one cause of a natural event, unlike scientific experiments which are specially set up to reveal unique explanations.
    On Discworld, not only does Death come for humans, scythe in hand, but diminutive sub-Deaths come for other animals – for example the Death of Rats in
Soul Music
, from whom a single, typical quote will suffice: ‘S QUEAK.’
    The Death of Dinosaurs