The impact of impacts
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By David Morrison
Approximately 66 million years ago a gigantic meteorite slammed into the Earth at incredible speed, wiping out most non-avian dinosaurs and 60 percent of the planet’s other flora and fauna. It left behind a ~ 200 km diameter crater beneath and off the coast of the Mexican state of Yucatán. The crater is called “Chicxulub”, named for the small town situated almost exactly over its geographic centre.
Belgian geologist, planetary scientist and geochemist, Professor Philippe Claeys, has been studying the Chicxulub crater for almost 30 years. Claeys has come to the University of British Columbia as a Peter Wall International Visiting Research Scholar, a program that brings exceptional academics to the Point Grey campus. Dominique Weis, a Peter Wall Faculty Associate and professor of the Department of Earth, Ocean and Atmospheric Sciences, is hosting Prof Claeys. When his research began, there were many questions to be answered about the crater, and the topic was highly controversial.
“In 1988-99, I started working on my PhD on this topic,” Prof. Claeys begins. “It was an exciting time because people were coming up with plenty of arguments to support the meteorite impacting at the Cretaceous-Tertiary Interval (K-T), but the big mystery remained. Where was the ‘murder weapon’? As in any good mystery novel, no weapon, no conviction. The resulting crater was very difficult to find; it could have been anywhere on Earth, but despite its size it was invisible to us. It was purely by chance that the crater was discovered.”
At ~ 200 km in diameter, mostly under the sea and beneath over 600 metres of sediment, the Chicxulub crater is dizzyingly bewildering in scale, firing the imaginations of scientists like Prof. Claeys who study it, and ordinary people astonished by the incredible facts and figures relating to it. For example, the energy released by the collision was more than five billion Hiroshima bombs. So, it is exciting and valuable that a small trace of the crater’s rim on land can actually be visited.
Throughout the Yucatan peninsula, around the rim of the crater, there are beautiful cenotes filled with fresh water. Photo: Jose Ignacio Soto/Thinkstock
“Although the crater is buried under Yucatan, so you can’t see it, it’s there, deep under your feet,” says Claeys. “The only thing that you can see, is a ring of cenotes (sinkholes), with very clear, beautiful blue water, located precisely at the rim of the crater onshore. Cenotes are dissolution pits you can swim in. They’re really beautiful. I prefer the ones without crocodiles.”
Hydrogeologists in Yucatan think the cenotes are still related to enhanced water percolation generated by a deep network of fractures generated by instabilities at the rim of the crater.
It is where this material eventually fell to Earth that ultimately revealed the whereabouts of the Chicxulub crater.
Locating the crater
The circular morphology was recognized in 1978 during an airborne magnetic survey of the Gulf of Mexico, conducted by geophysicists Glen Penfield and Antonio Camargo for potential oil drilling locations for Pemex, Mexican state-owned oil company.
“At the Lunar and Planetary Science Conference in 1989, people were talking about Central America as a potential place where the crater could be located,” Claeys says. “The reason was if you look at K-T in North America—in the Western Interior, going from New Mexico all the way to Alberta—this boundary was a little thicker than, I would say, in Italy or in New Zealand. So, we knew that it was all over the world, no discussion, and it looked like a two cm clay layer everywhere, but in North America in the Western Interior, we could distinguish two distinct layers. Only in North America could you see these double layers, so this was taken as evidence that we might be close to the impact site.”
It has taken until 2016 to recover the first core samples from the peak ring of the Chicxulub crater, taken from a depth of 1,335 metres by the International Ocean Discovery Program, in the process confirming that it is categorically the crater linked to the extinction event.
“Chicxulub is definitely the crater for several reasons,” explains Claeys. “First of all, if you obtain an isotopic (radiometric) dating on rocks within the crater, it is almost precisely the same age as the extinction; the two are indistinguishable. Also, if you look at the rocks within the crater that were molten by the energy and shock waves generated by the impact, they are chemically and isotopically very similar to small pieces of glass we find in the Cretaceous-Tertiary Boundary, all around the Gulf of Mexico."
An elongated two millimetre long spherule of glass that was aerodynamically shaped when it was expelled from the crater.
The chemistry of glass reflects precisely the composition of the rock in Yucatan, which was made of three km of carbonate and evaporite, and on top of granite. Of that dual layer, from New Mexico to Alberta, the lowermost one contains spherules composed of material from deep within the Chicxulub crater that were expelled on low angle trajectories, to land within a radius of 4,000 to 5,000 km.
"The spherules are elongated and have been shaped by travelling through the atmosphere and back, so it’s obvious from their aerodynamic shape they were cooling, and then solidified in flight," Claeys says. "Moreover, minerals within this layer contain evidence of high pressure generated by the collision. How do you get highly shocked deep crustal material a couple of kilometres under Yucatan spread all around the Gulf of Mexico? It’s only possible from ejecta during a cratering event.”
Sixty-six million years ago the Chicxulub impactor stuck the Earth, disrupting the climate worldwide and causing mass extinction of plant and animal species.
Hello meteorite, goodbye dinosaur
When the meteorite impacted with the shallow water and sediment of Yucatan, a large volume of water vapour, CO2 and deadly sulphur-rich components were propelled into the atmosphere to create a dense global cloud, impenetrable to sunlight. This would have interrupted plant photosynthesis and affected the planet’s food web, triggering mass extinction.
“When you have such a large meteorite arriving at a velocity of at least 20-25 kilometres per second you are going to excavate down to probably two projectiles in diameter,” Claeys says. “When the transient cavity, which has a size of 80-90 kilometres, forms within the crust of the Earth, it vaporizes a huge volume of material, maybe down to around 20 km deep, and an enormous amount of fine pulverized dust is released.”
The Chicxulub impact doomed the dinosaurs but, as sometimes is the case with tragedy, there was a silver lining. The mammals that had eked out an existence up to that point were able to flourish and become the dominant species on land. After about 64 million years, large brained hominids appeared—but that’s a story for another time.