Red-hot glass balls rain down
40,000 cubic kilometres of molten rock is flung into the atmosphere, much of it in the shape of glass droplets, which harden into balls. Ejected at speeds up to 36,000km/h, they rain down on the planet’s animals and plants.
Monster waves bury river bank
Back then, 66 million years ago, DePalma’s Tanis was a subtropical river delta with swamps, cypresses and ginkgo trees. A few kilometres to the east, the river was linked with a large inland sea that cut the US in two, stretching from the Gulf of Mexico to the northernmost part of the modern US, with Tanis located at the northern end, 3000km from the asteroid strike.
The site includes several layers of rock. The layer that includes the newly-discovered fossils is around 1.3m-thick, while underneath are sloping rocks that once made up the banks of the river. Above the fossil layer a few centimetres of reddish clay is rich with the iridium that landed during the days, weeks, perhaps years after the impact.
The fossil layer consists of hardened mud or fine sand, and is divided in two. The bottom section was washed onto the river bank in one single violent event. This part shows no signs of drying out, indicating that the top part ‘landed’ to cover it more quickly than would occur under natural processes. The fossils of the layer include many marine animals, so DePalma concluded that the layer must have been produced by two monster waves from the sea that flowed several kilometres up-river, leaving their contents on the river banks. The sloping river banks rose at least 10 metres above the river’s normal level – and are completely covered in wave contents. So the waves must have been at least 10 metres high.
The glass tektites from the strike have been discovered throughout the layer. Scientists calculate that the first of these balls would have reached the sky above Tanis 13-25 minutes after the strike, and that they would subsequently have rained down for about two hours. The entire layer must have been deposited during this limited time span.
A seiche, not a tsunami
The Tanis death scene reminds us of the entangled wreckage we see in news reports following modern tsunamis. But DePalma is confident that Tanis did not experience a tsunami. The Mexico asteroid certainly did cause monster tsunamis; they spread across the world, but not to Tanis. First of all, the inland sea to which the Tanis river linked was shallow, so that a tsunami would have lost its power en route. Secondly, a tsunami from Mexico would have taken 18 hours to reach Tanis, yet the tektites indicate that the location was hit no more than two hours after the impact.
DePalma and his colleagues believe instead that Tanis was hit by another type of wave, a seiche, which originates in an enclosed water mass, with the motion more like the waves created if you push a bowl of water. A seiche can be triggered by an earthquake thousands of kilometres away. In 2011, for example, a Norwegian fjord-side community experienced waves almost two metres high only 30 minutes after an earthquake 8000km away in Japan, a category 9.2 on the moment magnitude scale (the modern version of the Richter scale). The asteroid strike 66 million years ago triggered earthquakes of 10-11.5 on the same scale, which is 2800 times more forceful than the 2011 earthquake. Such intense earthquakes could have caused seiches of up to 100 metres throughout the world.
DePalma and his colleagues have been able to establish the first accurate picture of the earliest minutes and hours following the strike. But much work remains to be done. Only a few of the site’s fossils have yet been studied, yet according to DePalma, Tanis includes a treasure trove of extraordinary finds, including the bones of a catalogue of dinosaurs, pterosaurs and mammals, plus large, well-preserved feathers, and eggs with embryos. If that is true, it indicates that the dinosaurs – at least in North America – were doing well up until the impact, supporting the idea that it was the asteroid, not the Indian volcanic eruptions, that ended dinosaur dominance in this region.