“All these fossils occur in a layer no more than 10cm thick,” said palaeontologist Ken Lacovara of the Chicxulub impact that ended the dinosaur epoch. “They died suddenly and were buried quickly. It tells us this is a moment in geological time. That’s days, weeks, maybe months. But this is not thousands of years; it’s not hundreds of thousands of years. This is essentially an instantaneous event.”
Scientists drilled into the Chicxulub crater buried underneath the Yucatán Peninsula recovering rocks from under the Gulf of Mexico that were hit by an asteroid 66 million years ago creating the niche that made the rise of homo sapiens possible. The 15 km-wide asteroid could not have hit a worse place on Earth. Scientists have put together a detailed picture of the minutes following the giant impact.
“This is where we get to the great irony of the story – because in the end it wasn’t the size of the asteroid, the scale of blast, or even its global reach that made dinosaurs extinct – it was where the impact happened,” said Ben Garrod, in the BBC documentary The Day The Dinosaurs Died.
It is hard to imagine billions of tons of rock suddenly starting to splash about like a liquid, reports the BBC, but that is what happened when the asteroid struck. Analysis of rocks drilled in 2016 from the leftover crater show they underwent a process of fluidization, where the pulverized material literally began to behave as if it were a substance like water.
The impact description – scientists call it the dynamic collapse model of crater formation – is only possible if the hammered rocks can, for a short period, lose their strength and flow in a frictionless way. And it is the evidence for this fluidization process from something called the “peak ring” – essentially, a circle of hills in the center of the remnant Chicxulub depression.
“What we found in the drill core is that the rock got fragmented. It was smashed to tiny little pieces that initially are millimeter sized; and that basically causes this fluid-like behavior that produces in the end the flat crater floor, which characterizes Chicxulub and all such large impact structures, including those we also see on the Moon,” explains Ulrich Riller, from the University of Hamburg, Germany.
This is not rock being melted; rather, it is rock being broken apart by immense vibrational forces, says Sean Gulick, the drill team co-lead at the Institute for Geophysics at the the University of Texas told The Daily Galaxy. “It is a pressure effect; it’s mechanical damage. The amount of energy moving through these rocks is equivalent to something like 10 billion Hiroshima bombs.”
“The Chicxulub impact causes a range of extreme effects including generating seismic energy that traveled through the Earth’s crust equivalent to a magnitude 10-11 earthquake” noted Gulick. “Plate tectonics can only generate a magnitude 9.5 or so as no earthquake generating fault zone on Earth is long and wide enough to even get to a magnitude 10.
Chicxulub Induced Seismic Waves?
“It is unclear what effects a magnitude 10-11 equivalent earthquake energy caused by the Chicxulub impact would have had at greater distances,” said Gulick, “however within the Gulf of Mexico the shaking caused large landslides resulting in the single largest event deposit ever discovered.
“Suggestions that the Chicxulub induced seismic waves propagating through the Earth would have induced volcanism is interesting,” Gulick concluded, “but has not been demonstrated in terms of the physics required and timing of suggested triggered volcanism remains uncertain.”
Ultimately, the rocks will regain their strength. They have to if they are to build the ring of hills. This return of rigidity, again, is witnessed in the drill core samples.
The outer rim of the crater lies under the Yucatan Peninsula itself, but the inner peak ring is best accessed offshore. The 12 km-wide object dug a hole in Earth’s crust 100km across and 30km deep. bowl then collapsed, leaving a crater 200 km across and a few km deep. The crater’s center rebounded and collapsed again, producing an inner ring. Today, much of the crater is buried offshore, under 600m of sediments. On land, it is covered by limestone, but it’s rim is traced by an arc of sinkholes.
“It is manifested in what we call shear fractures – planar discontinuities where rocks can slide past each other,” Prof Riller added. “We see these fractures over-printing the smashed rocks that formed beforehand. These planar structures are evidence that the rock must have regained strength towards the end of crater formation.”
“We are explaining a fundamental process that will occur on any rocky body,” says Gulick. “For the first time ever, we now have rocks that tell us the kind of deformation they’ve undergone to temporarily behave like a liquid and then become like a rock again at the end – without melting. It’s all done by overlapping deformation mechanisms. This will be a fundamental process that resurfaces planets, not just in our Solar System but presumably in all Solar Systems.”
Riller and Gulick were part of the Expedition 364 drilling project, which was conducted in April/May 2016 under the auspices of the European Consortium for Ocean Research Drilling (ECORD) and the International Ocean Discovery Program (IODP).
The Last Word –It’s a Fairy Tale
Gerta keller, a Princeton geologist has endured decades of ridicule for arguing that the fifth extinction was caused not by an asteroid but by a series of colossal volcanic eruptions, says of the Chicxulub event: “It’s like a fairy tale: ‘Big rock from sky hits the dinosaurs, and boom they go.’ And it has all the aspects of a really nice story. It’s just not true.”
The catastrophe has consumed her for the past 30 years: the annihilation of three-quarters of the Earth’s species—including, famously, the dinosaurs—during our planet’s most recent mass extinction, about 66 million years ago.
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