One of the most devastating events in the history of life on Earth is believed to have happened 65.5 million years ago, when an asteroid approximately 15 km across impacted our planet in what is now the Yucatán Peninsula of Mexico, killing off the dinosaurs and setting the stage for our mammalian domination of the planet. This is the main conclusion presented in a recent review article in the journal Science through the collaborative efforts of 41 international scientists.
The Chicxulub asteroid — named for the town located near the center of the crater — impacted the Earth at a velocity 20 times faster than a speeding bullet — upwards of 100,000 km/h — according to Gareth Collins, an author on the paper. The force of the impact was so strong that it is comparable to the energy that would be released from one billion Hiroshima-sized atomic bombs, and resulted in a crater measuring nearly 200 km in diameter.
The aftermath of such a bombardment would have been dire. The initial impact is thought to have triggered earthquakes with magnitudes greater than 10 on the Richter scale and massive tsunamis which would have swept across coastal areas and surrounding oceans. Following that, the release of environmentally damaging gases contained within the rocks at the impact site would have caused catastrophic environmental effects including acid rain, extended darkness and global cooling.
The impact on animal and plant populations at the time was severe. It is estimated that over 50 per cent of all life on Earth went extinct as a result of the asteroid impact. It’s no wonder the researchers refer to it as a mass extinction event; this thing was devastating. A number of major animal groups disappeared completely, including non-avian dinosaurs, and flying reptiles. On the plant side of things, some groups came close to checking out completely, including all land plants and a type of plankton known as calcareous nannofossils.
One of the main pieces of evidence that allows researchers to link the mass extinctions to the Chicxulub asteroid impact is the existence of an impact fallout (ejecta) layer the world over.
In the early 1970s, Walter Alvarez discovered a thin band of reddish clay that divided the Cretaceous period from the Paleogene. Curious as to whether this could have accounted for the dramatic drop in fossil evidence of so many species above this thin band, he had the chemical composition of the clay analyzed. The sample of reddish clay found between the Cretaceous-Paleogene (K-Pg, formerly K-T) boundary had more than 300 times the amount of normal levels of the chemical element iridium found on Earth. He concluded that it must have come from space.
Since then, samples from over 350 K-Pg boundary sites around the world have been analyzed, and beyond showing characteristically high levels of iridium, they also have a specific distribution pattern, with an increase in distance from the Chicxulub crater producing a decrease in the thickness of the ejecta-layer.
Further evidence for the Chucxulub asteroid impact 65.5 million years ago resulting in the mass extinction event is the existence of “shocked” quartz in the geological column. When quartz is hit by a very massive force, like those found at atomic bomb or meteorite impact sites, the pressure causes a change to the quartz’s crystalline structure. This structurally-altered quartz has been identified the world over in rock layers at the various K-Pg boundary sites.
Previously offered as a challenge to this single asteroid impact theory was the interpretation of data from the crater site, which indicated that the crater was made many hundreds of thousands of years before the K-Pg boundary. However, this was discounted by modern analysis of the geological processes that take place at an impact site.
One other main challenge offered against the explanation championed in the current study was the existence of volcanoes that produced massive eruptions in India spanning over 1.5 million years. The large amounts of lava — estimated at over one million cubic kilometres — were thought to have caused an environmental catastrophe, which could have resulted in the observed mass extinctions. However, with the volcanoes erupting upwards of 500,000 years before the K-Pg boundary with relatively little effect on the environment, this alternative was also dismissed.
I reached Jason Anderson, a vertebrate palaeontologist from the University of Calgary for his thoughts on the recent review article. He pointed out that the article almost completely neglected the ongoing debate over “whether non-avian dinosaurs were already on a long decline in the late Cretaceous” before the asteroid hit. He notes that there wasn’t a single vertebrate palaeontologist among the authors, which could explain the absence of this debate regarding the extinction of non-avian dinosaurs. But despite this oversight, the evidence collected from paleontologists, sedimentologists, geochemists and geophysicists points to one main conclusion.
An asteroid impacted the Earth 65.5 million years ago, and this lead to a mass extinction so devastating that over half the plants and animals disappeared from the fossil record in a relative instant. Although perhaps not such a good thing for those creatures that went extinct, and the untold billions that suffered horrible deaths at the hands of fireballs, earthquakes and tsunamis, it worked out in the long run. Without such a cataclysmic event, the dinosaurs’ reign would likely not have ended when it did, and our mammalian ancestors wouldn’t have come out of their burrows and evolved into the most incredible species on Earth: cheetahs.
