The end of the Cretaceous and the onset of the Tertiary period 65 million years ago—the K-T boundary marking the end of the dinosaurs—was the result of a massive asteroid/comet impact. How, though, did we figure that out?
There’s a romanticized notion of scientific progress where groundbreaking ideas arrive as world-altering flashes of insight. The ring-structure of benzene, for example, supposedly came to Kekulé in a dream of a snake eating its own tail. But leaps forward aren’t often leaps so much as they are fitful and incremental progress—which is exactly how to describe the hypothesis, discovery, and evidence gathering for the “Crater of Doom” that left behind that dinosaur-annihilating asteroid. It’s a story of insights, unraveled clues, false leads and academic pissing contests, but it’s also the biography of an idea, and how it changes over time.
Thomas Kuhn described paradigms in scientific thought. A paradigm is an intellectual framework of an era: what is researched, how it is researched, and how things are thought about; not the theories themselves so much as the underlying beliefs about the world that compel those theories. Consider, for example, early 19th-century scientists, who discovered fossils of animals that no longer existed. It’s obvious now that those animals went extinct, but if your basic assumption is that god is divine and infallible and therefore would not allow one of his creations to die off, “extinction” doesn’t make any sense. Paradigms are, in some sense, constraints: if you reason forward assuming an infallible divine creator, extinction is not an option; if man is the center of the universe, heliocentricity is nonsense; if germs don’t cause disease, antibiotics are inconceivable (“Inconceivable!”).
Uniformitarianism, which sounds like a cult but isn’t, was the dominant geological paradigm of the 19th and 20th centuries. The main principle is gradual change, which for geologists suggests that past and present are similar, and we can draw conclusions about the past by investigating the present. From the uniformitarian perspective, the idea of a catastrophic, planet-altering events—like, say, a comet strike—were simply inconceivable (“Inconceivable!”).
Plate tectonics helped upend the belief in gradual change. Continental drift (and mega-continent Pangaea) were proposed all the way back in the 1920s, but uniformitarian thought held sway, and so most geologists continued to believe that continental positions were fixed. It wasn’t until the 1960s that plate tectonics was universally accepted (am I the only one shocked we were landing on the moon before agreeing on that?). When that revolution hit, it destroyed a lot of geological sacred cows, gradualism among them. Geologists began to accept that catastrophe and rapid change aren’t just real, but perhaps even frequent, on geological time scales.
From the uniformitarian perspective, the mass extinction of dinosaurs wasn’t a concern: maybe climate change or sea level changes did them in, but whatever it was, the process was gradual. But with the plate tectonics paradigm shift in the rearview, the idea of catastrophic extinction-level events becomes more compelling. Especially when you look at the K-T boundary, which can be plainly seen in rock outcroppings throughout the word. It’s visually jarring:
That abrupt line of demarcation almost demands an assumption that the cause was also abrupt, right? It doesn’t just look abrupt, either: amoeboid fossils are found by the thousands prior to the K-T line, and then become vanishingly rare immediately above it. This abrupt change is a big chink in the armor for claims of a gradual change leading to gradual extinction.
But what happened at that K-T line? The first step to proving a catastrophic event there was to determine how quickly that line—a thin layer of clay separating K from T—was laid down. If the event was catastrophic, it should have happened fast. To know how fast, researchers needed to find something that either grew or decayed at a constant rate, so they could measure it at the boundary and use the rate as a geological clock. Their choice was a stroke of good luck.
Unseen to the naked eye, meteorite dust is always falling. The dust contains iridium, so researchers could figure out how long that K-T boundary took to lay down by measuring how much iridium was deposited. But what they found was a lot of iridium, far more than that constant sprinkling of meteorite dust; anomalously high iridium levels found at K-T outcroppings all over the world (there’s something cool about isolating a 65 million year-old event by comparing rocks from thousands of miles apart).
Several years earlier, the first “catastrophe” theory of dinosaur extinction suggested a massive supernova. The supernova would cause high iridium levels, so it seemed initially like the iridium anomaly suggested that a supernova killed the dinosaurs. Searching for corroboration, they also tested the level of plutonium-244 at the K-T layer. There was none—meaning that it couldn’t have been a supernova.
What else causes high iridium levels? A big-ass comet. And any comet big enough to wipe out the dinosaurs should leave a huge impact site, which would be a critical piece of evidence for the extinction level event killed the dinosaurs hypothesis. But the earth is big and they had no idea where to look for the crater. They couldn’t find one of the right size on land, and after they noticed that K-T boundary rocks were similar in composition to ocean crusts, they spent several fruitless years searching for an ocean crater.
After about a decade of searching, they finally identified Chicxulub as the likely impact crater, just off the Yucatan peninsula. Petroleum engineers for Mexican oil companies had known of the crater for decades—the engineers had no reason to think the crater was geologically important—and the company’s archival surveys helped geologists to isolate the scope of the crater. That private-industry scientists were onto something long before the greater research community has been repeated multiple times in geologic history; it was also true of plate tectonics and continental drift. Only after identifying the crater did geologists realize that the impact was coastal—thus leaving behind evidence of both water and land impacts, and complicating their search for it.
In sum, a brief timeline: plate tectonics was generally accepted in the late 1960s. The first “catastrophe” theory of the K-T extinction event was in 1971. By the early 1980s, the iridium anomaly had been identified and replicated, and the lack of plutonium-244 had ruled out a supernova as the cause. By 1991, the probable impact crater had been identified.
• • •
Bonus side note: Mass extinctions are periodic, and occur every 27 million years, like geological-timescale clockwork. The research team that first identified the effect essentially threw up their hands and said: we have no idea why this would happen. One theory holds that our sun has an orbiting companion star that occasionally causes gravitational disturbances. This causes comets from the Oort cloud to be pulled into the inner solar system and increases the likelihood of impact events (known as the “Nemesis” or “Death Star” hypothesis; Han Solo once made the Oort Run in under 12 parsecs). No evidence for the companion star has yet been found, but that K-T extinction is the only one currently believed to be caused by an asteroid impact; most others have been explained as the result of flood basalts (literally, floods of lava), and sea level changes.