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For a very long time, there has been consensus among paleontologists that a mass extinction event, sometimes called The Great Dying, occurred around the boundary between the Cretaceous and the Paleogene or Tertiary periods, a boundary also marking the end of the Mesozoic era and the beginning of the Cenozoic, about 65.5 million years ago. The fossil record stands as clear evidence of a mass extinction around that time, with the fossil remains of dinosaurs being the best known and most illustrative examples of species extinction during that period. However, scientists have not always been in consensus regarding just what caused this mass extinction. Some have theorized that gradual climate change may have made the planet uninhabitable for them, or volcanic activity, or falling sea levels. The idea that an asteroid impacted the earth was something of an outlier, considered dubious by many, for it seemed so very unlikely that in the vast, howling void of space a meteor or a comet would be on just the right trajectory to collide with our pulsing globe of life, let alone that its impact could have such a massive destructive effect on species the world over. But two physicists out of Berkeley, Luis and Walter Alvarez, would change this notion forever, and through the scientific detective work of various disparate specialists and investigators, evidence supporting the Alvarez Hypothesis would eventually convince the world that some massive impact resulted in the Cretaceous-Paleogene extinction. But is this known with any certainty, and what doubts or alternative theories persist, making the cause of the extinction of the dinosaurs a prehistoric blind spot?

In 1977, a young geologist named Walter Alvarez was collecting limestone samples in a little Italian village called Gubbio. What he discovered was that between two layers of limestone marking the Cretaceous-Peleogene boundary, a thin layer of red clay was present. In the Cretaceous layer below, there were a great variety of different species of tiny, fossilized marine creatures called “foram,” while in the red clay there were none. Then in the Paleogene layer above, there remained only one species of foram. Returning to Berkeley with his samples, Walter consulted his father, Nobel Prize winning physicist Luis Alvarez, an impressive man with a storied career that included designing the detonator of the bomb dropped on Nagasaki, inventing a radar system commonly used by air traffic controllers, discovering an isotope, and pioneering the use of bubble chambers in particle physics and cosmic ray detection for the purposes of searching for hidden chambers in Egyptian pyramids. Luis Alvarez suggested using neutron activation analysis on the Gubbio samples to determine the length of time it took for the red clay layer to form, but to their surprise, the most interesting thing this analysis turned up was the immense quantity of iridium in the clay. Of course, as we know from the scientific study of the Tunguska Event, iridium is an element common in asteroids and other extraterrestrial objects, but not typically present in the crust of the Earth. With further discoveries of this anomaly at other sites, such as New Zealand and Denmark, evidence mounted that some extraterrestrial material had been dispersed across the world at the end of the Mesozoic Era, just when the mass extinction was known to have occurred. And what’s more, beyond iridium and some other elements, the Cretaceous-Paleogene boundary layer contained a great deal of soot, indicating some kind of mass conflagration.

Luis Alvarez winning the Nobel Prize, via Wikimedia Commons

As the Alvarez Hypothesis took shape and gained momentum, a terrifying vision of apocalyptic doom began to form. The idea of an asteroid striking the Earth and causing a tsunami was comprehensible, but that would not account for global devastation of species far inland. But Luis Alvarez’s experience with nuclear explosions gave him some insight, for he know that a blast of great magnitude would throw an immense amount of dust into the stratosphere, theorizing that it could possibly blot out the sun for years, which would result in plummeting temperatures and the death of plant life and thereafter of most animal life. And based on the iridium levels they had observed, Alvarez inferred that the asteroid must have been massive indeed, at 300 billion tons, and thus its impact would have resulted in an explosion equivalent to 100 billion megatons of TNT. In one second, this world killer would have torn a hole in our atmosphere, superheating to hotter than the surface of the sun, and when it struck, it would have thrown matter half way to the moon, much of which would have come falling back down as separate meteor strikes. The blast wave would have utterly destroyed all life for a few hundred miles, and creatures beyond that radius faced fires, earthquakes, landslides, and acid rain as the skies filled with dark clouds far worse than any of the so-called Dark Days I discussed in the latest patron-exclusive Blindside episode. These were dark months, perhaps even benighted for as long as a year as dust clouds filled the atmosphere. 70-75 percent of all life would have died as photosynthesis failed, starving herbivores and thereby reducing the food supply of carnivores. The Alvarez Hypothesis was not only horrifying, it had geological evidence to support it, but it remained largely contested by those who subscribed to the theory that the extinction was precipitated by massive volcanic activity in the Deccan Traps region of India. They and others challenged the Alvarez Hypothesis, refusing to believe in an impact with no crater as proof.

One of the first craters studied as possible proof of the asteroid impact theory was located in Manson, Iowa. This was one of the largest craters ever discovered on the planet at 22 miles in diameter. But Luis Alvarez had estimated the crater of their world killer would have to be more than a hundred miles across. Moreover, new evidence had appeared in Haiti, where Florentin Maurrasse of Florida International University discovered an even thicker layer of iridium. When Glen Izett of the U.S. Geological Survey examined samples from the Haitian layer, he found tektites, little pieces of glass naturally created by meteorite impacts. Drill samples from the Manson, Iowa, crater were then compared with the Haitian tektites, settling once and for all through chemical analysis that the Manson impact could not have been the culprit. And this further narrowed the search field, for those who had examined the Cretaceous-Paleogene boundary sediments at Haiti and the tektite glass therein, including a graduate student named Alan Hildebrand, agreed that the impact site must have been close by, somewhere in the Caribbean. Little did Hildebrand and others searching for the crater realize that their crater had already been found. Leave it to a newspaperman to make that connection. In 1981, Carlos Byars, writing for the Houston Chronicle, suggested that a ring formation recently surveyed on the Yucatan Peninsula was not a volcano as previously believed, but may in fact have been an impact crater. When Byars contacted Alan Hildebrand and shared the idea, Hildebrand followed up, reaching out to the geophysicists who had been studying the formation, not realizing he was about to enter the final chapter of a story that had been unfolding for decades.

Example of tektite glass, via Wikimedia Commons, licensed under the Creative Commons Attribution-Share Alike 3.0 Unported, Attribution: I, Brocken Inaglory — Example of tektite glass, via Wikimedia Commons, licensed under the Creative Commons Attribution-Share Alike 3.0 Unported, **Attribution: I,** **Brocken Inaglory**

In 1947, PEMEX, the national Mexican oil company, performed a gravity survey of the Yucatan peninsula in search of fossil fuel deposits, and they picked up a semicircular formation that looked promising. However, when they drilled the area in the 1950s, they came up empty handed. Drilling near the pueblo of Chicxulub, a village whose name meant “the devil’s tail” in Mayan, they turned up no oil, but rather some odd samples of volcanic rock. Being that there were no known volcanoes in the region, it was theorized that the circular formations their survey had perceived were from a long extinct and buried volcano. More than a decade later, after capping their failed wellheads, PEMEX contracted Robert Baltosser to reconsider their gravity survey data, and Baltosser was the first to recognize the presence of an ancient crater, as he had previously surveyed a crater site in Tennessee. So the discovery was made, but Baltosser had signed a confidentiality agreement, and PEMEX was not in the business of publicizing data that might prove to be valuable. Thus the existence of the massive crater remained a secret.

In 1978, Glen Penfield, another contractor, was mapping magnetic fields for PEMEX using survey flights over the sea off the Yucatan coast. He discovered a magnetic anomaly and mapped out a huge semicircular ridge beneath the water, and when he compared it with the company’s old gravity map, he found to his astonishment that they matched, forming a perfect circle. He realized he had made a momentous discovery, but like Baltosser before him, he was bound by his agreement with PEMEX not to publish his findings. However, he knew that a journalist learning of the discovery would certainly be able to publish. So Penfield arranged for a reporter acquaintance, Carlos Byars of the Houston Chronicle, to be present at a meeting of the Society of Exploration Geophysicists when he and his colleague would be presenting a report of their findings. The plan worked; Byars published the story and thereafter put Penfield in touch with Hildebrand, the grad student who was becoming a major player in the search for the impact site that would prove the Alvarez Hypothesis.

In 1990, Hildebrand and Penfield began their joint search for evidence that could prove not only the existence of the Chicxulub crater but also its responsibility for the Great Dying. Evidence, however, would not be easy to come by. The data Penfield had turned over to PEMEX had gone through a normalization process that erased some of the magnetic anomalies that had been telltale signs of an extraterrestrial impact. Those anomalies remained on the original data records, but those tapes had been lost somehow. The only evidence of what Penfield had observed beneath the sea off of the Yucatan Peninsula now lay filed away somewhere at PEMEX headquarters, under lock and key, and PEMEX, already resentful that Penfield had spoken about his findings at a meeting of geophysicists, was not inclined to share them. So they went in search of the drill samples that had been taken in the ‘50s, which they discovered had been stored in a warehouse in Veracruz. Unfortunately, that warehouse had burned down, and when he arrived to sift through the ashes, he found that it had been bulldozed. Penfield’s only hope then was to search the towns were PEMEX’s failed wellheads had been capped, and eventually, he found that several of the core samples had been taken and distributed for study. Upon examining the melted sedimentary rock that many had previously believed to be the result of volcanism, they discovered the presence of “shocked quartz,” which is only ever formed at impact sites. Thus the Chicxulub formations were proven to be an impact crater, and further study has only further proven the crater’s existence. At 110 miles in diameter, it was the likeliest suspect to prove the Alvarez Hypothesis, which today is supported by the majority of the scientific community.

Gravity map of the Chicxulub Crater, via Wikimedia Commons

Scientific consensus does not make for scientific certainty, however, nor for objective truth. There had been many ridiculous hypotheses before the Alvarez Hypothesis—namely that dinosaurs had died off because of how very stupid they were, because they had eaten too much or eaten the wrong things and poisoned themselves, or because they had failed to procreate or just wasted away from some kind of primeval ennui—but like the dinosaurs, most of these theories died off with the coming of the asteroid theory. However, one old hypothesis has persisted. Some scientists still point to the Deccan Trap volcanic eruptions as the culprit. All four of the major extinction events before the Great Dying had been as a result of volcanic activity releasing carbon dioxide and sulphur into the atmosphere. This is generally agreed upon. And much evidence indicates that the fifth extinction event, at the Cretaceous-Paleogene boundary, coincided with the voluminous eruption of one of the largest volcanos on Earth in the Deccan Traps. It seems absurd to suggest that this volcanism had nothing to do with it, for certainly, if both were occurring, then both probably contributed to the poisoning of the world’s air. The two theories, after all, are not so very different. Both take a catastrophist view of prehistory, imagining very sudden and calamitous changes rather than a gradual transformation. Recently, though, some of the gradual views of dinosaur extinction have started to regain traction. In 2016, a phylogenetic study of dinosaur speciation published statistical evidence that dinosaur speciation, the ability to spawn new species as others went extinct, had been declining for tens of millions of years (Sakamoto et al. 5036). This, however, doesn’t diminish the significance of the effects that volcanism or a massive extraterrestrial impact would have. Rather, it only shows that dinosaurs would have been especially susceptible to extinction at the time.

One scientist, a Princeton micropaleontologist named Gerta Keller, has actually challenged the idea that an impact at Chicxulub could have caused the Great Dying. According to her research, the object that struck the Yucatan peninsula arrived 300,000 years before the Great Dying actually occurred and so could not be responsible for it. Moreover, she argues that since the layers at Chicxulub containing microtektites were lower than the layers containing iridium, then the iridium layer discovered around the world could not have been from the Chicxulub impact. While her claims have been contested, Keller remains steadfast, insisting that a sudden extinction likely required the coincidental combination of massive volcanism in the Deccan Traps and a massive extraterrestrial impact, but insisting that Chicxulub is not the one that did it. And now we have a new candidate, a hypothesized 310-mile-wide crater that just happens to be located offshore of India and the Deccan Traps volcanic plateau. This crater has been named Shiva after the Hindu god of destruction. Other, smaller impact sites dating to the Cretaceous-Paleogene boundary have started to turn up as well, like Manson, Iowa, and the 15-mile-wide crater in the Ukraine, at Boltysh. A picture begins to form of monstrous lifeforms already in decline and vulnerable to disaster, and a world reeling from colossal volcanic activity suddenly pummeled by a series of enormous asteroids, perhaps a swarm or perhaps castoff from one initial impact. But like all theories about the Great Dying, this is just conjecture—it is an informed opinion, supported by fact, but still only an opinion—and this, perhaps the most significant event in Earth’s past, which was directly responsible for the emergence of humanity as the dominant life form on the planet, may forever remain enigmatic.

A depiction of dinosaur extinction precipitated by Deccan Traps volcanism, via Wikimedia Commons

Blind Spot: The Great Dying and the Chicxulub Crater

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