Extinction of the non-avian dinosaurs

The story

A mystery 150 years old

What happened to the dinosaurs? Ever since the 19th century, when people understood that they had existed and then vanished, scientists have suggested explanations of every kind: a climate too hot or too cold, diseases, mammals eating their eggs, even boredom, some said as a joke. None was really convincing. The breakthrough came from an unexpected place: a mountain gorge near Gubbio, in Umbria.

The clay of Gubbio

At the end of the 1970s the American geologist Walter Alvarez was working in Italy, in the Bottaccione Gorge, near Gubbio. The rocks there are pinkish limestones formed on the floor of an ancient sea, layer after layer, over millions of years. In the older limestones there were plenty of fossils of tiny sea organisms, the foraminifera; in the more recent ones very few were left. Between the two groups there was a thin layer of clay, about a centimetre thick. Alvarez wanted to know how long it had taken to be laid down.

His father, the physicist Luis Alvarez, a Nobel Prize winner, suggested measuring in that clay a metal that is very rare on the Earth's surface, iridium, which reaches the Earth mainly with cosmic dust at a steady rate. The more iridium there is, the more time has passed. Together with the chemists Frank Asaro and Helen Michel, the researchers found a surprise: the clay held much more iridium than normal, tens of times more than the rock around it. In 1980 they published their idea in the journal Science: that iridium could not have come from dust building up slowly, but from a big asteroid that had struck the Earth about 66 million years ago, bringing with it, and scattering in the air, an enormous quantity of that metal.

The idea was met with scepticism. Then, over the years, the same layer of iridium was found in dozens of other places around the world, always of the same age. There was other evidence: small grains of quartz with marks of violent impact, and tiny spheres of molten rock that had solidified in flight.

The crater

If the asteroid had fallen, it must have left a crater. It was found in Mexico, on the Yucatán Peninsula, near the village of Chicxulub. Already between the late 1970s and the early 1980s two geologists from the Mexican oil company, Antonio Camargo and Glen Penfield, had seen in geophysical data a gigantic buried circular structure, but at the time it was not linked to the dinosaurs. In the early 1990s the geologist Alan Hildebrand and his colleagues identified it as the crater being sought. It measures about 180 kilometres across, and is largely under the sea and under sediments. In 2016 an international drilling expedition collected rock samples from inside it, confirming that it was formed by an impact.

The asteroid that made it was about 10 to 15 kilometres wide, like a mountain, and arrived at a speed of tens of thousands of kilometres an hour. The energy released was greater than that of many billions of atomic bombs.

What happened next

The effects were of two kinds. In the first minutes and days there were earthquakes, very high tsunami waves and a rain of molten rock falling over much of the planet, and probably fires near the place of impact. But the real catastrophe was what followed. Dust, soot and droplets of sulphur rose into the upper atmosphere and cut down the sunlight for months or years. Without light, plants could not carry out photosynthesis; the seas cooled and became more acidic; food chains collapsed, starting with plankton and plants.

About three quarters of living species disappeared. Gone were the great marine reptiles, the pterosaurs, the ammonites and all the non-avian dinosaurs, that is, all except the birds. Survivors were small animals, able to hide, to burrow or to feed on seeds, insects and scraps: small mammals, birds, crocodiles, turtles, frogs. Some scientists recall another factor: at the same time in India the volcanoes of the Deccan Traps were erupting, and their gases may have made the crisis worse. The weight of the different causes is still debated, but most researchers consider the impact the main cause.