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THE END OF THE DINOSAURS
Everyone loves the dinosaur. Whether they appear in skeletal, in fossil, or even in molded-plastic form, both young and old are captivated. Kids love these creatures from the past—building models and memorizing names that most adults can barely pronounce. A museum anywhere with a dinosaur display will exhibit heavy foot traffic—from both kids and their more senior counterparts. Curators at natural history museums are well aware of the attraction of these bizarre ancient reptiles. The American Museum of Natural History in New York counts among its chief attractions the enormous skeletons of a Tyrannosaurus rex (which translates as “king of the lizards”) and an apatosaurus, as well as related models that greet you in the entryway.
Further evidence of their popularity is the starring role that dinosaurs play in popular culture—from Dino in The Flintstones (no, terrestrial dinosaurs didn’t coexist with people) to the regenerated dinosaurs of Jurassic Park (no, they probably won’t in the future either). Even the filmmakers of King Kong weren’t satisfied with a giant ape that could scale the Empire State Building. They had to include a completely superfluous (opinions expressed represent those of the author only) scene with dinosaurs.
Why? Because dinosaurs were amazing. They looked enough like current animals to seem familiar, but were different enough to be exotic and weird in ways that activate our imaginations. They had horns and crests and bony armor and spines. Some were big and slow and others were small and swift. Some walked on the ground—some on two legs and some on four—while some of them flew in the air.
Yet for many people, the first thing that comes to mind when they think about dinosaurs is that those magnificent animals no longer walk the face of the Earth. Though dinosaurs did evolve into birds that survive today, the dinosaurs that had dominated the land for millions of years went extinct about 66 million years ago. Some people even regard the dinosaurs’ demise with a touch of superiority—how could it be that such strong and agile creatures were foolish enough to disappear? Yet the fact is the dinosaurs were the major players on the planet for far longer than humans or apes are likely to survive. When they disappeared, it was through no fault of their own.
The question of what caused the land-dwelling dinosaurs to depart the planet was for a long time a tremendous mystery that mesmerized both scientists and the public alike. Why would this diverse, vigorous group that seemed to have mastered its environment suddenly disappear at the end of the Cretaceous period? This topic might seem a distant remove from physics—especially that associated with dark matter. But this chapter presents the many pieces of evidence that have demonstrated that a meteoroid impact was almost certainly the culprit—connecting this extinction to an extraterrestrial object in the Solar System. And, if the more speculative work that I did with my collaborators turns out to be correct, a disk of dark matter in the plane of the Milky Way was responsible for triggering the meteoroid’s fatal trajectory. Whatever the role that dark matter had, the impact of an object from outer space that wiped out at least half the species on the planet certainly occurred—binding this extinction to our solar environment. The tale of how geologists, physicists, chemists, and paleontologists came to this conclusion is one of the best stories in modern science.
DINOSAUR TIME
The dinosaurs—apart from their range of sizes and their coolness—were striking as a category in their longevity, dominating the planet for more than 100 million years. Yet despite their apparent robustness and the proliferation of flora and fauna that accompanied them, a great deal of life ended abruptly 66 million years ago. The questions that persisted until late in the twentieth century were why this happened, and how.
Before answering these questions, let’s first reflect on the age of the dinosaurs, and how different the Earth was back then. Dinosaurs lived in the Mesozoic era, which ranged from 252 to 66 million years ago. (See Figure 29.) The name Mesozoic comes from the Greek term for “middle life” and indeed this era lies in the middle of the three geological eras of the Phanerozoic eon. The Mesozoic era is wedged between the Paleozoic, with its name meaning “ancient life,” and the Cenozoic whose name refers to “new life.” This bracketing reflects the most devastating mass extinction we know of—the Permian-Triassic extinction event, which defines the first boundary, and the Cretaceous-Paleogene extinction—formerly known as the K-T extinction—which defines the second one, in which the (non-avian) dinosaurs and many other species disappeared.
The K in K-T comes from the German word Kreide, which means chalk. Similarly, the word “Cretaceous” to which it refers comes from the Latin word creta, which is literally Cretan earth—also meaning chalk. The T on the other hand comes from Tertiary—a relic from a now defunct naming scheme that divided the Earth’s history into four parts, of which the Tertiary was the third.* Even so, like many others, I occasionally fall back on the colloquial term K-T for the extinction, though I will usually use the more correct term—K-Pg—from now on.
Eras are divided into periods that are further divided into epochs and stages. The Mesozoic era is subdivided into three periods: the Triassic period—lasting from 252 to 201 million years ago, the Jurassic period—from about 201 to 145 million years ago, and the Cretaceous period—from 145 to 66 million years past. “Mesozoic Park” might even have been a more accurate name for the Michael Crichton–Steven Spielberg movie, which features two Jurassic dinosaurs but also several that did not emerge until the Cretaceous period. I will nonetheless concede that “Jurassic Park” has a better ring to it, so I won’t question the wisdom of the choice.
A lot changed on Earth during the Mesozoic era. Warming and cooling as well as significant tectonic activity transformed the atmosphere and the shape of the landmasses. The supercontinent Pangaea split in the Mesozoic era into the continents we see today, and resulted in extensive land movement over time.
Even though tectonic movement in the late Cretaceous brought the planet closer to its current state, the continents and oceans were not yet in their current positions. India had yet to collide with Asia and the Atlantic Ocean was much narrower. As tectonic plates have moved since then, oceans have changed size at the rate of several centimeters each year.
This effect alone tells us that 66 million years ago, most shores were several thousand kilometers away from their current locations—so, for example, the Americas and Europe were much closer. Moreover, sea levels were probably a hundred meters higher than they are today. Temperatures—especially in regions far from the ocean—were higher then as well. These factors turned out to be critical to deciphering some of the clues revealed at the K-Pg boundary. Although we now know that at the time of its formation, Italian sediment containing the clay that the Berkeley geologist Walter Alvarez, had decided to study was part of a continental shelf that lay under hundreds of meters of water, researchers initially didn’t know this to be the case.
Life on Earth evolved in response to its changing environment. The many moving pieces of land separated by water allowed new species to emerge. During the Triassic period, arthropods, turtles, crocodilians, lizards, bony fishes, sea urchins, marine reptiles, and the first mammal-like reptiles arose. The Late Triassic is also when many distinct species of dinosaurs, including terrestrial dinosaurs, first appeared. They went on to become the dominant land vertebrates during the Jurassic period.
Birds too emerged during this time, evolving from a branch of theropod dinosaurs. Jurassic Park doesn’t necessarily get all the science right, but the film taught many that birds evolved from dinosaurs. Flying reptiles, marine reptiles, amphibians, lizards, crocodilians, and dinosaurs continued into the Cretaceous period, during which time snakes and early birds first appeared, as did flying reptiles and gingkoes, as well as modern plants such as cycads, conifers, redwoods, cypresses, and yews, whose forms we still observe today. Mammals also made an appearance, but they were small—generally between the size of a cat and a mouse. This changed only after the dinosaurs went extinct and left room and resources for them to evolve to bigger-bodied animals.
LOOKING FOR ANSWERS
Two fascinating books I read while working on this one were the geologist Walter Alvarez’s T. rex and the Crater of Doom and the science writer Charles Frankel’s The End of the Dinosaurs. Walter Alvarez was in large part responsible for the meteoroid hypothesis and his book was very entertaining. I confess that one of the reasons Frankel’s book seemed so special to me is that when I bought it on Amazon it was already out of print, so the copy I received was from the Rockport Public Library with a big stamp labeling it DISCARD. Had the book not been mailed to my house—a far more suitable habitat—it apparently would have gone extinct too.
Both books tell the truly remarkable tale of how geologists, chemists, and physicists established that an enormous meteoroid (remember I’m using “meteoroid” for large objects too) was the most likely cause of the extinction that wiped out the dinosaurs—along with a good deal of the other species that were alive at the time. Evidence abounds that this meteoroid precipitated the dramatic shift in fossil record during the K-Pg transition. All the features that characterize impact craters, including spherules, tektites, and shocked quartz, were found in the vicinity of a boundary iridium layer, which separates relics of abundant life underneath it from the much sparser fossil record above.
The books also relate the incredible and inspiring detective story of how scientists actually found the crater that corresponded to that meteoroid hit, though consultation with experts taught me that some of the literature is a little misleading. I will do my best to get it right here. It’s a great story.
Although the idea of meteoroids causing extinctions took hold only in the late twentieth century, people had speculated about their potentially dire consequences for centuries. When people first noticed comets, they considered them life threatening—but for superstitious and unsubstantiated reasons. In 1694, Edmond Halley boldly suggested a comet was the source of the biblical deluge. About 50 years later, in 1742, the French scientist and philosopher Pierre-Louis de Maupertuis put the potential threat from comets on stronger scientific footing when he recognized that the disturbances to the ocean and atmosphere that a comet strike could create had the potential to wipe out many forms of life. Another Frenchman, the great scientist Pierre-Simon Laplace, whose work on the Solar System’s formation survives to today—also suggested that meteoroids could precipitate extinctions.
But their ideas were largely ignored, since they couldn’t be tested and furthermore seemed a little crazy. Neglected too were the ideas of the American paleontologist M. W. de Laubenfels, who in 1956 recognized the potential import of the meteoroid that hit Siberia in 1908 and devastated a vast swath of forest—identifying the damage such as fires and heat that even a fragment of a comet could cause. In an amazingly prescient analysis, he also recognized that these environmental impacts would affect various species differently, so that burrowing mammals might survive—as indeed turned out to be the case following the K-Pg event.
Even as late as 1973, most scientists ignored the geochemist Harold Urey when he suggested, based on the glassy tektites of molten rock, that a meteoroid impact was responsible for the K-Pg extinction. Urey was a little overly enthusiastic, however, in suggesting that not only the K-Pg extinction, but all other mass extinctions, were due to comet impacts. Even so, he anticipated future studies and helped turn earlier proposals into real science by pointing out that detailed investigations could identify rocks whose shape or composition could be explained only by the heat and/or pressure of a meteoroid hit.
However, any such smart and prescient ideas were essentially ignored before Alvarez made his proposal. The notion of a cosmic impact causing an extinction was radical even in the 1980s, and might have sounded somewhat absurd on first hearing. It is reminiscent of some theories I hear from 12-year-olds who attend my public lectures, where they try to impress me by combining all the scientific terms they have ever heard. This can lead to contrived and usually pretty funny scenarios, such as when one youngster asked me about a theory he claimed he had always wondered about in which black holes from warped extra dimensions solved all the remaining problems of the Universe. Fortunately, he laughed when I suggested to him that he hadn’t actually always thought about this.
But as with any more radical theories that eventually take hold, the meteoroid proposal could explain observations that defied more conventional explanations. No terrestrial process could account for all the detailed phenomena that would be found that ultimately supported the hypothesis. The proposal gained credence because it made predictions, many of which have since been validated.
[FIGURE 30] With the Geoparque director, Asier Hilario, at the K-Pg boundary at Itzurun Beach in Zumaia, Spain. (Jon Urrestilla)
HOW INSPIRATION STRUCK
The tale of Walter Alvarez’s scientific sleuthing began in Italy. The hills of Umbria near Gubbio, a couple of hundred kilometers north of Rome, reveal a marine sediment that dates from the Late Cretaceous to the Early Tertiary (now the Paleogene). The Scaglia Rossa, as it is known due to its pink color, is a sedimentary rock consisting of a very unusual deep-water limestone—calcite or calcium carbonate, which is what most seashells are made of and is what bone supplements sometimes contain—that was formed on the seafloor and later pushed upward so it is now exposed. This meant that the evidence of the extinction—a thin layer of clay separating the lower, whiter rock layer from a red layer above—would have been visible to an attentive passerby. The fossils in the lower, whiter rock are mostly the remnants of foraminifera, single-celled protozoa that live in the deep oceans and are extremely useful to us for deducing the ages of sedimentary rocks. But only the very smallest of the foraminifera are present in the upper, darker layer. The “forams” almost went extinct along with the dinosaurs, making the extinction boundary very clear.
The Flysch Geoparque that I visited during my recent university visit to Bilbao contains a piece of the K-Pg boundary—which appears as a thin dark line near the limestone cliff’s base. Like everywhere else on the globe where this clay layer exists, the boundary dates from the time of extinction. I considered myself very lucky when my physicist colleague and his geologist cousin helped arrange a visit to the incredibly beautiful site at Itzurun beach, where I could run in at low tide to view the boundary up close. Getting to touch this 66-million-year-old piece of history was almost surreal. (See Figure 30.) Though the cliff derives from the distant past, its trove of information is still with us as part of our world.
AT THE K-PG BOUNDARY
In the 1970s, Walter Alvarez studied a similar boundary layer in the Scaglia Rossa, where he focused his attention on the clay that separated the lighter-colored limestone below that was full of fossils from the darker limestone without them above. This clay that Alvarez made the target of his studies was critical to unraveling the cause of the devastation that took place 66 million years in the past. The thickness of the clay depended on the length of time that passed between the deposition of the lighter and darker rock and could therefore help him determine if the extinction event was rapid or slow.
When Alvarez first started thinking about the K-Pg layer in the 1970s, geology was dominated by the uniformitarian, gradualist viewpoint that had recently been vindicated by the theory of plate tectonics developed over the previous two decades. Entire continents could gradually move apart, mountain ranges could form over time, and canyons as deep as the Grand Canyon could emerge through gradual effects—including a river such as the Colorado River cutting through the ground, water- and ice-induced erosion, land plate movements, or magma eruptions—any of which could dramatically alter terrain over time. No catastrophe was needed to account for these seemingly very dramatic changes.
The limestone formation seemed mysterious in that the difference between the upper and lower levels of limestone indicated a very abrupt transition, inconsistent with the gradualist point of view. Had he been there, Charles Lyell would have interpreted the thinness of the K-Pg layer as simply misleading and concluded that despite appearances, it had taken many years to create. Darwin might have thought the formation was simply an illusion created by an inadequate fossil record.
The only way to truly know if the transition was sudden—and not simply a clay deposit that had washed in over a few days—was to measure how long it would have taken to deposit the clay that separated the two differently colored limestone layers. And that was the task Alvarez, who had a long-standing interest in dating geological events, set for himself. Alvarez hoped to study geomagnetic reversals to learn more about the timing of the deposition of the K-Pg boundary, which he knew could be an important clue to the triggering event. (Andy Knoll, a professor of natural history and Earth and planetary sciences at Harvard, mentioned that Alvarez and his wife might have been even more interested in looking at Medieval art and architecture. I suspect both interests played a role.)
But the better method for measuring how long it took for the clay to be deposited turned out to be measuring its iridium content. Iridium is a rare metal and, next to osmium, is the densest element. Its corrosion-resistant properties make it useful for spark plug electrodes and fountain pen nibs, among other things. It also turns out to be good for science. The iridium spike that Walter Alvarez and his collaborators discovered turned out to be the key to nailing down the origin of the extinction event.
Although I had known for a while about the iridium spike, I was astonished when I more recently learned that the original intention of Walter and his physicist father, Luis Alvarez, had been to measure iridium levels in the clay based on reasoning exactly opposite to what they soon realized was true. Luis Alvarez knew that meteoroids had a much higher level of iridium than the Earth’s surface. Although the iridium level on Earth should be the same as that of a meteoroid, most of the Earth’s original iridium long ago dissolved into molten iron and sank with it to the Earth’s core. So any iridium on the surface should have an extraterrestrial origin.
Luis Alvarez assumed that meteorite dust should be deposited at a fairly steady rate. (Actually he had originally suggested using beryllium-10, but the half-life turned out to be too short to be a practical tool for this problem.) Iridium levels on the surface should be quite low were it not for the deposition from this steady extraterrestrial “rain.” The two Alvarezes had the clever idea that by studying iridium levels on Earth, they could access this cosmic hourglass that would help determine how long it took to deposit the K-Pg boundary clay. What they expected was a smooth distribution over time indicative of a steady, nearly constant deposition that could be used to deduce the length of time it took for the clay layer to form.
What Walter and his collaborators found when they examined the actual rock was completely different. The surprise that convinced Alvarez that something odd was afoot (in his case quite literally) came from much higher than expected iridium levels in the clay. In 1980, a team of scientists at the University of California, Berkeley—the father-son team of Luis and Walter Alvarez, partnering with the nuclear chemists Frank Asaro and Helen Michel, who could measure iridium abundances at extremely low levels—found a decisive elevation in iridium—30 times higher in the Scaglia Rossa than in the surrounding limestone. And that number was later corrected to 90.
This type of formation was found not only in Italy (sadly so many people have sampled the Scaglia Rossa since Walter Alvarez’s time that the K-Pg boundary clay is now hard to reach), but all over the globe, and iridium levels in those locations noticeably spiked too. In a similar clay layer in Stevns Klint—a coastal cliff with well-preserved K-Pg evidence in Denmark—the increase was by a factor of 160. Other laboratories confirmed the elevated levels of iridium in similar boundary layers elsewhere.
If the original hypothesis (and incentive for the measurement) was correct and meteoritic dust rained down at a constant rate, the K-Pg clay would have required more than three million years to form. But that was far too long a time for the thin clay layer that represented the K-Pg boundary. Alternatively, if the iridium level was similarly elevated all around the globe, then 500,000 tons of iridium—thought to be a rare metal on Earth—had suddenly descended on our planet at the time of the K-Pg extinction. The only explanation for this enormous deposition could be a cosmic origin. The Earth is so intrinsically low in iridium on the surface that without some extraterrestrial phenomenon the high iridium level would be essentially impossible to explain.
The Berkeley team also determined the relative abundances of other rare elements so they could further narrow down the possibilities. For example, maybe the extraterrestrial source was a supernova. In that case, the supernova would have led to plutonium-244 in the clay too. And the initial analysis made it appear that indeed, this element was present as well. But in a responsible exercise in scientific standards, Asaro and Michel repeated their analysis the following day and discovered there was no plutonium to be found. The initial finding was simply contamination in the sample.
After racking their brains for alternatives, the Berkeley scientists were left with basically one plausible explanation for the high levels of iridium—a large impact from an extraterrestrial object that occurred roughly 65 million years before. In 1980, the group led by Walter and Luis Alvarez proposed that a big meteoroid had collided with the Earth and rained down rare metals, including iridium. Such a meteoroid impact—either by an asteroid or from a comet—was the only event that could account for both the total amount of iridium and the elemental ratios, which matched those that are characteristic of the Solar System.
Based on the measured iridium and the average meteorite iridium content, the researchers could furthermore guess the size of the object that had hit. They concluded that it had to have been an incredible 10–15 kilometers in diameter.
STRIKING EVIDENCE
Given the many fatal mechanisms that a huge meteoroid provides, as well as the paucity of adequate explanations for the geological evidence associated with the K-Pg extinction, an extraterrestrial explanation seemed a plausible and reasonable alternative to more conventional suggestions such as geologically or climate-induced processes. Yet despite the compelling nature of the hypothesis, any scientist—no matter how daring—has to exercise caution when introducing a new idea. Sometimes radical theories are correct, but more often than not, a more conventional explanation has been overlooked or not properly evaluated. Only when existing scientific ideas fail where more daring ones succeed do new ideas get firmly established.
For this reason controversy can be a good thing for science when considering a (literally) outlandish theory. Although those who simply avoid examining the evidence won’t facilitate scientific progress, strong adherents to the reigning viewpoint who raise reasonable objections elevate the standards for introducing a new idea into the scientific pantheon. Forcing those with new hypotheses—especially radical ones—to confront their opponents prevents crazy or simply wrong ideas from taking hold. Resistance encourages the proposers to up their game to show why the objections aren’t valid and to find as much support as possible for their ideas. Walter Alvarez even wrote that he was pleased it took a while for the meteoroid idea to find conclusive support since it allowed time for all the secondary evidence that strengthened the case to be found.
The meteoroid hypothesis was indeed met by resistance from those who thought it an extravagant theory—many of whom preferred the gradualist point of view. Confusingly, plate tectonics lent support to this viewpoint at about the same time that the Moon missions, with their close-up views of many craters, were making a strong case for the possible catastrophic effects of impacts. Perhaps these two different advances were why—taken as a group—geologists tended toward the gradualist viewpoint whereas physicists went for the catastrophic.
Of course the Moon’s craters could have all been created in the early stages of its formation—and in fact most of them were—so their existence was not in itself an argument for the significance of meteoroid impacts in later evolution. Still, their prevalence should have made less surprising an assumption that not only gradual, but also catastrophic, processes played a role in our Solar System and in the development of its life. Craters were clear and palpable evidence of lunar impacts. The Earth is both bigger and very near the Moon so clearly meteoroids would have hit here too.
But at the time of the Alvarez proposal, many paleontologists favored gradualist explanations. Some took the perspective that dinosaurs had simply died away in the late Cretaceous period due to some form of adverse environmental conditions—such as climate change or bad diet. Many others thought volcanic activity was the culprit. Support for this point of view came from the Deccan Traps in India, which were formed by an enormous amount of volcanic activity that occurred around the time that the dinosaurs went extinct. The Deccan Traps cover a region bigger than half a million square kilometers—comparable to the size of France—and they are about two kilometers thick. That’s a lot of lava. To further confuse the situation, the traps can be dated to very near to the Late Cretaceous–Early Tertiary boundary.
Indeed, groups of dinosaurs such as sauropods, a group that includes the apatosaurus—the original and perhaps temporarily preferred name for the brontosaurus (a debate rivaling that over planet Pluto)—had already gone extinct by the end of the era. But support for the gradual decline idea stemmed in part from the incomplete fossil record that existed at the beginning of the investigation, which became less compelling as additional regions were studied and more fossils were found. Fossils discovered in Montana revealed at least between 10 and 15 dinosaur species that survived up to the end of the Cretaceous period. Recent digs in France uncovered evidence of dinosaurs within a meter of the K-Pg boundary and those in India showed dinosaur evidence below the boundary as well. Other species, such as ammonites, did show a decline in diversity at first. But closer and broader inspection again revealed that at least a third of the dinosaur species survived to the boundary—though some had indeed gone extinct earlier.
On top of that, although people initially thought that the traps were created very quickly, later work showed that their formation took a few million years, and that the K-Pg event corresponds to a layer in the middle, which oddly enough seemed to be a time of suppressed volcanic activity. Probably the most convincing evidence that the volcanoes were not solely responsible for the dinosaurs’ extinction is that Indian geologists have found dinosaur bones and fragments of their eggs right in the sediment up to the region that constitutes the K-Pg boundary. Dinosaurs were not only alive—they were living in the region of the traps themselves.
Even so, more recent developments place the formation of the traps closer to the extinction time than people had formerly thought, supporting a place for some volcanic activity—even if not for all the destruction. Some speculate that the volcanic activity was in fact a result of the meteoroid impact, in which case whatever volcanic effects took place were indirectly attributable to the meteoroid too. Whatever their role, volcanoes don’t explain the many other coincidences in geological features that persuasively argue for the significance of the meteoroid.
Indeed, once people started to look in earnest, evidence for the meteoroid hypothesis rapidly accumulated. Details matter and they can help resolve many a controversy. After the 1980 Berkeley suggestion, the K-Pg clay layer was meticulously studied in Italy, Denmark, Spain, Tunisia, New Zealand, and the Americas. By 1982, almost 40 locations around the globe had been carefully scrutinized. The Dutch paleontologist Jan Smit observed high iridium levels in Spain, and other paleontologists measured them in Stevns Klint. Smit also measured other rare metal levels, such as those of gold and palladium. He found osmium and palladium levels a thousand times higher than seen elsewhere on Earth. And again, the relative metal abundances corresponded to those expected in meteoroids.
Some scientists in favor of the volcano explanation suggested that large amounts of iridium were pumped up by volcanoes from the Earth’s mantle and core, where levels are known to be higher. But known volcanoes don’t emit nearly enough iridium to account for the 500,000 tons worldwide that Alvarez and others calculated to be present at the K-Pg boundary, even allowing for other potential concentrating effects such as precipitation in the ocean. In any case, iridium isn’t the only heavy element in meteorites and the abundances of other elements didn’t match those in volcanic emissions either.
Supplementary support from other observations in and around the K-Pg layer provided further evidence of the meteoroid proposal. The discovery in diverse locations of rock droplets such as microkrystites—smaller versions of tektites, the glassy rocks with rounded shapes that emerge from impact melts that have spun and solidified in the atmosphere before dropping back to Earth—lent support to the meteoroid idea.
But these glassy spherules, as they used to be known, also presented a red herring at first. The chemical composition resembled that from ocean crust, which it turns out was very likely representative of the impactor and not the target. Had the initial misleading conclusion been correct and the landing been in the ocean and not on land, it would have meant that despite all the mounting evidence for an impact, the impact site would probably have remained concealed.
This misplaced concern abated when geologists found evidence indicating that the meteoroid had landed on a (potentially accessible) continental shelf. This was the discovery of shocked quartz, which indicates a high-pressure origin that could come only from collisions on quartz-containing rock. Rocks that don’t melt are shattered, so the minerals they contain can move to form crisscrossing bonds. (See Figure 24.) The only known sources for these bonds are meteoroid impacts and nuclear explosions. Presumably nuclear tests didn’t happen 66 million years ago—although a researcher told me a radio interviewer actually asked him about this possibility—which left a meteoroid impact as the only potential surviving explanation.
In 1984, when shocked quartz was found in Montana, and later in New Mexico and Russia, the new discoveries argued strongly for a meteoroid impact too. That this evidence was a type of quartz argued furthermore that the crater, assuming there was one, should be located on land, since quartz is rare in rocks from the ocean.
Further evidence continued to accumulate in the meteoroid hypothesis’ favor. Canadian scientists found microscopic diamonds in the K-Pg layer in Alberta. These could have come from meteoroids that simply couriered the diamonds from outer space, or they could have been formed on impact. Detailed studies of size and carbon isotope ratio favored the latter interpretation. In Canada, as well as in Denmark, amino acids that are not known to exist anywhere else on Earth were discovered in the layers. This evidence had the interesting feature that it favored a comet interpretation, since these amino acids were found in the bordering limestone as well—as would be the case if comet dust were around at the time the layer was formed.
One other important geological feature that argued for high-pressure impacts were crystals called spinels. These are metal oxides that contain iron, magnesium, aluminum, titanium, nickel, and chromium and which have bizarre snowflake, octahedral, or other shapes that indicate rapid solidification after high-temperature melting. Spinels occur in volcanic magma too, but the spinels that were found contained the elements nickel and magnesium—unlike volcanic spinels, which contain more iron, titanium, and chromium. Better yet, the amount of oxygen helps determine where the spinels were formed. The oxidated spinels of the K-Pg layer indicated a low-altitude origin—below 20 kilometers. The crystals also were found only in a thin layer, confirming the hypothesis that the catastrophic event that occurred at the K-Pg boundary was very brief.
Volcanoes couldn’t account for the shock-induced materials. Although volcanoes do produce deformations, existing volcanic regions don’t produce the shocked quartz that would be necessary to match observations from the time of extinction. The dislocations in volcanic shocked quartz run along a single plane, rather than two or more intersecting ones, which is a phenomenon that occurs only at high shock pressures. These details are important since these phenomena are found in precisely the location that demarcates the K-Pg extinction boundary.
Even so, having firmly established the meteoroid’s destructive influence, we shouldn’t entirely dismiss the gradualist viewpoint. Very likely, conditions were changing around the time of the K-Pg extinction in a way that increased the fragility of the ecosystem so that when a meteoroid did strike, it did more damage than it would otherwise have. Evidence shows that some significant fraction of species had gone extinct even before the more dramatic extinction event took place. Recent more precise measurements of the timing of the Deccan Traps lend further support to volcanic activity playing some role. Though unlikely to be responsible for the extinction event that eventually happened, volcanoes and other phenomena very likely abetted it—both before and after the meteoroid struck.
But to do colossal damage, the impact didn’t need any assistance.
HOW LIFE STRUCK OUT
It’s difficult to fathom just how huge and devastating the meteoroid must have been. The impactor had a span that was about three times the width of Manhattan. And it wasn’t just big. It was also moving very quickly—at least 20 kilometers per second and if a comet, perhaps three times faster. The object’s speed was at least 700 times faster than a car on a freeway moving at at 100 kilometers per hour. This impactor would have been an object the size of a major city moving 500 times faster than a vehicle on an autobahn. Since the energy an object carries increases with its mass and with the square of its speed, such a fast, big object hitting the Earth would have had a devastatingly enormous impact.
To put it in some perspective, an object of its size and speed would have released an energy equivalent of up to 100 trillion tons of TNT, more than a billion times greater than that of the atom bombs that destroyed Hiroshima and Nagasaki. The comparison is not accidental. Luis Alvarez worked on the Manhattan Project and he made similar observations. More broadly speaking, the Cold War preoccupation with the effects of nuclear explosions enhanced people’s interest in the crater. Research on both benefited from increasing knowledge about the K-Pg impactor’s long-term environmental effects.
The Tunguska object and the meteorite that created the Meteor Crater in Arizona carried a fraction of this energy—the equivalent of perhaps 10 megatons of TNT. The diameter of the impactor in each case was more like 50 meters, rather 10 to 15 kilometers for the impactor creating the K-Pg event. Krakatau’s energy was only a few times more than that of these smaller meteoroids, which was comparable to the most powerful nuclear weapons ever made (about 50 times what exists now). A kilometer-wide meteoroid would already be sufficiently big to do global damage. The object Alvarez suggested was at least 10 times bigger than that—bigger than the height of Mount Everest, which stands nine kilometers above sea level.
The impact (in both senses of the word) of this enormous speeding object was devastating. As described in Chapter 11, many disasters follow such a huge heavy rock hurling itself into Earth. Near to the blast—within about a thousand kilometers—extreme winds and waves raged, and huge tsunamis radiated from the blast site. These tidal waves would have been enormously powerful, though of limited range since as it turned out the water depth back at the impact location was only about a hundred meters. Tidal waves would also have appeared on the opposite side of the globe, triggered by perhaps the most massive earthquake the Earth has ever experienced. Extreme winds would have blown outward from the impact location, and then rushed back in. This would have carried a cloud of superheated dust, ash, and steam that was thrown up when the meteoroid initially drove into the ground. This wind and water would have used up about one percent of the impact energy. The rest would have gone into melting, vaporizing, and sending seismic waves throughout the Earth—the equivalent of 10 on the Richter scale.
Trillions of tons of material would have been ejected from the site of the crater and distributed everywhere. Afterward, when the hot solid particles descended through the atmosphere, they would have been heated to incandescence and raised temperatures around the globe As a consequence, fires would have raged everywhere and the Earth’s surface would have literally been cooked. In fact, in 1985, the chemist Wendy Wolbach and her collaborators found evidence of fires in the K-Pg layer in the form of charcoal and soot. The abundance and shape of the carbon flecks they found confirmed that the fires happened—and destroyed then-existing plant and animal life. The researchers concluded that more than half the world’s biomass was incinerated within months of the impact.
And that’s not all. The water, air, and soil were all poisoned. Perhaps people weren’t just superstitious in their fear of comets, which turn out to contain poisonous materials such as cyanide and heavy metals including nickel and lead. Although some chemicals would have vaporized before they could do any damage, very likely heavy metals rained down from the sky.
Probably even more damaging would have been the nitrous oxide created in the atmosphere, which would have descended to the ground in acid rain. Sulfur would have been released into the atmosphere too, creating sulfuric acid that could have remained there and blocked sunlight, creating global cooling that followed the global heating immediately after the catastrophe occurred and lasting perhaps for years. The loss of photosynthesis would have reverberated throughout the food chain. Global warming and dust particles blanketing the Earth could have played some role too—extending the deviant heating and cooling for many more years.
Indeed, fossil records demonstrate that the destruction’s legacy persisted well after the initial impact. Even the species that did survive had their ranks severely diminished. The oceans didn’t recover for hundreds of thousands of years and most likely saw destructive influences for at least a half million to a million years afterward. The fossil record demonstrates the absence of plankton and other fossils in the darkness of the limestone that contains little or no carbonates. There is instead evidence primarily of detrital particles—the small fragments of weathered and eroded rock that remained. The normal color doesn’t resume for at least centimeters and sometimes meters in these layers, depending where on the globe one looks.
The many disasters provided abundant opportunities for plants and animals to go extinct. It seems no living creature survived that was heavier than about 25 kilograms—about the weight of a midsize dog. In order to make it through, a means of hiding from the disaster—through hibernation or otherwise—would have been essential. Depending on their reproduction methods (seeds had a better chance of surviving than other means of procreation) and on their source of food (species that fed on waste fared better) some species did survive. Animals that could escape into the skies had better chances too. But most plants and animals died. A 10- or 15-kilometer-wide meteor was enormously devastating—to the environment and to life.
STRIKING PAYDIRT: REDISCOVERING THE CRATER
Nonetheless, researchers at the time knew that even with all the evidence discovered in the 1980s and the increased understanding of the implications that a huge meteoroid could have on life on the planet, finding a tangible 66-million-year-old crater of the right size would clearly strengthen the case for the impact suggestion. A crater would not only substantiate the hypothesis, but would permit more detailed investigations that could better pinpoint the size and time of the meteoroid hit, along with other features that could help confirm an impact.
The crater’s size—along with its age—was a critical prediction. Based on the amount of iridium that had been measured, Walter Alvarez deduced that the meteoroid should have been at least 10 kilometers across, so the crater should be about 200 kilometers wide, since craters are usually about 20 times the size of the impacting object. Alvarez wasn’t the only one to estimate a crater of this magnitude. Another paleontologist independently predicted a size of 180 kilometers based on the assumption that the clay contained seven percent meteorite material, with the rest just pulverized rock from the target.
A crater of the right scale from the correct date would be the smoking (technically no longer smoking) gun for the Alvarez proposal. Yet it took more than a decade for the crater to be discovered—spawning one of the best detective stories in modern science. In fact, the odds of finding the impact site didn’t seem all that favorable when people first began to look. Although some large craters have been discovered over the years, many more go missing. Even if we are “lucky” enough that a meteoroid hits land and not the ocean, erosion, burial by sedimentation, or tectonic destruction can eliminate any sign that a crater had been formed.
For the meteoroid responsible for the K-Pg event, the discovery challenge was exacerbated by the apparent lack of clues to the strike’s location. The very ubiquity of the iridium and other geological evidence, distributed more or less uniformly around the globe, confirmed the meteoroid’s worldwide impact but didn’t single out any particular region. When people first began to search, it seemed a daunting, if not impossible, task to determine where on Earth one particular meteoroid had hit more than 65 million years ago.
However, in the crater hunter’s favor, the shocked quartz that had been found suggested its origin was continental—or on a continental shelf—so that searches on land stood a chance of successfully identifying the remains of the culprit. Several seemingly promising candidate craters emerged but were soon ruled out on further investigation—disagreeing with more precise measurements of their time of impact, determinations of their size, or mineralogical studies.
But one very important independent observation had been largely overlooked for quite some time. As early as the 1950s, industrial geologists had identified a buried circular structure 180 kilometers in diameter that extended half onshore, under the Yucatán limestone plains, and half offshore, where it was buried under water and sediment in the Gulf of Mexico. Geologists from Petróleos Mexicanos, or Pemex, as the Mexican oil company is known, drilled wells into this feature. They hit crystalline rock at a depth of about 1,500 meters, leading them to think they had found evidence of a volcano rather than what for them would have been a far more interesting oil trap.
But in the late 1960s, the geologist Robert Baltosser—who was involved in a second round of exploratory drilling, just in case the first investigators had missed an oil cache—suggested that it could in fact be an impact crater. His suggestion was based on measurements of the shape of the feature’s gravitational potential—how the force of gravity varied over the circular structure. But it still wasn’t oil and Pemex didn’t allow him to release his observations. The consequence was that most of the people who knew about the structure worked for the oil industry, which for obvious reasons did detailed surveys of the ocean floor but wanted to protect their results.
But Pemex was persistent in its oil search and in the 1970s did further geological studies, including an aerial magnetic survey over the entire Yucatán Peninsula. The American consultant Glen Penfield noticed a strong magnetic anomaly about 50 kilometers across bordered by an outer ring with uncommonly low magnetism that was about 180 kilometers in diameter. This is precisely the pattern expected for a large impact crater, with the central region associated with the impact melt and the outer region containing hardened target debris. This correspondence wasn’t lost on Penfield. Aerial gravity data lent further support to this interpretation. The heightened and depressed gravity field correlated with the variations in the magnetic signals.
So as early as 1978, Penfield had a reasonably strong indication of an impact crater. He recognized that evidence for a previously unknown impact event could be a pretty big deal so he got permission from Pemex to release what was normally considered to be proprietary data. Along with Pemex geologist Antonio Camargo, Penfield presented his results in 1981 at the Society of Exploration Geophysicists convention in Los Angeles. But the discovery didn’t get a lot of attention. Most people who were listening were still unaware of the impact hypothesis for the K-Pg extinction so no one at the time envisioned such a connection.
In fact, most people interested in locating the impact crater responsible for the K-Pg extinction didn’t get around to studying this particular crater until 1990. But how they arrived at it is also an incredible story. Those in search of a specific 66-million-year-old crater of about 200 kilometers in diameter to verify the Alvarez proposal had gone about the search from an entirely different perspective than the Pemex geologists. They studied the K-Pg layer searching for hints of the location of an impact. Despite the uniformity of the iridium deposits across the globe, they knew of one clue that—if found—promised to be more location specific. If the meteoroid had hit the ocean but landed close to shore, it would have created a tsunami powerful enough to leave its trace in the continental platform. This might have seemed like wishful thinking, given the evidence of a terrestrial hit, but geologists kept their eyes peeled and were rewarded for their efforts.
In 1985, Jan Smit and a collaborator studied an outcropping of disturbed sediments in the K-Pg sediment in the Brazos River bed in Texas near the Gulf of Mexico, which they were convinced had been shaped by the proposed tsunami. The geologist Joanne Bourgeois of the University of Washington carefully followed up their work, finding unusually coarse sandstone containing fragments of shells, fossilized wood, fish teeth, and clay matching the local seafloor—and fixing the location’s depth 66 million years earlier to be 100 meters below the level of the sea at that time. She was able to use the size of the sandstone blocks to estimate that the current ran faster than a meter a second, corresponding to a wave height of at least 100 meters, and furthermore found clay with patterns indicative of a current that moved both to and from shore. By assuming the maximum possible size wave as being the same as the full sea depth of 5,000 meters, Bourgeois deduced that the impact must have been less than 5,000 kilometers away from her site, which meant in the Gulf of Mexico, the Caribbean, or the western Atlantic.
The other hint of the location came from the geologists Bruce Bohor and Glen Izett, who in 1987 found that the largest and most abundant deposit of shocked quartz was found in the western interior of North America, suggesting impact near the continent. This was consistent with the analyses of Smit and Bourgeois, which had suggested that the impact was near the southern end of the continent.
The impact site was narrowed down even further when the Haitian geologist Florentin Maurrasse identified some interesting debris at the K-Pg boundary layer in his native country. His description of the unusual sediments attracted the attention of the University of Arizona graduate student Alan Hildebrand, his thesis advisor Bill Boynton, and the researcher David Kring. Although Maurrasse had described the debris as volcanic in origin, the Arizona group were aware of how readily confused volcanic and impact debris can be. Once they saw the Haitian samples, they recognized the tektites and decided to visit Haiti themselves. There, in 1990, they found a half-meter-thick sediment outcrop that seemed to contain tektites—and shocked quartz and iridium clay as well. This had the appearance of a region that was very likely associated with the meteoroid impact. From the thickness of the layer, they concluded that the crater should have been no farther than about a thousand kilometers away at the time of impact.
Though Hildebrand initially favored a possible Caribbean candidate that he later rejected, the Arizona team ultimately zeroed in on the Yucatán feature that had been identified a decade before. Yet it was not scientists, but a reporter—Carlos Byars of the Houston Chronicle—who first made the connection. After listening to Hildebrand present the Arizona group’s research at a scientific meeting, Byars told him about Penfield’s earlier discovery of a potential impact crater—helping the scientists bring the mystery of the missing crater to its remarkably satisfying conclusion.
The Pemex-discovered crater was in the right location. And it also had the right size. This agreement was a major argument in favor of the crater’s connection to the K-Pg extinction. Even so, when in 1990 Hildebrand submitted two abstracts suggesting the connection to a scientific journal, they went unpublished—in part because the initial evidence wasn’t sufficiently convincing. However, opinions changed when the Arizona team identified shocked quartz from the crater.
Because the crater was in a submerged continental platform, sediment covered it, making the crater hard to find and study. But its burial was fortunate in some respects too, since the thousand meters of hardened mud above it shielded the crater from the erosion that would have occurred had it been on the surface. In order to investigate the buried and therefore initially inaccessible crater, the Arizona scientists contacted Penfield and Camargo to study the cores that had been drilled before. They obtained two thumb-sized samples stored in New Orleans. Sure enough, when the Arizona group studied these old Pemex drill cores, they found what they were looking for. They identified shocked quartz and impact melted rock that demonstrated the crater came from an impact and not a volcano. Kring announced the discovery at the NASA Johnson Space Center in March 1991.
The Arizona scientists combined their drill core studies with the geophysical data that Penfield and Camargo provided and the groups—along with a couple of other collaborators—accumulated strong evidence that the crater was the result of the impact that precipitated the K-Pg mass extinction. They presented this result, together with their size estimate of 180 kilometers, in the journal Geology in 1991. Confronted with the shocked quartz and other supporting evidence, many scientists began to pay attention.
The Arizona team named the crater after an unfortunately hard-to-pronounce small nearby fishing harbor, Chicxulub Puerto, which is located above the center of the structure. The term, which is pronounced CHICK-shuh-lube, is sometimes translated as the devil’s tail—appropriately enough for the imposing feature that Walter Alvarez dubbed the “crater of doom.”
Soon after the Arizona team’s publication, remote-sensing experts realized they could detect the crater’s circumference in satellite imagery, where small ponds were in a ring around the crater, 80 kilometers in radius. These were also most likely caused by the crater’s formation, which would cause groundwater to swell upward and cut through the Earth’s surface, and was therefore further evidence of the crater connection.
More support followed suit. The Arizona scientists could tell the material in the older cores was impact melt and that it contained features that resembled the microtektites of the K-Pg sediment in the surrounding gulf. Kring and Boynton also observed chemical similarities between the Chicxulub melt rock and glassy spherules deposited at the K-T boundary in Haiti—solid evidence that the Chicxulub crater was produced precisely at the K-T boundary where life gets extinguished. The evidence had become so strong by this point that the discovery made headlines and entered the public domain.
Geologists went on to find still more links between the Yucatán crater and the K-Pg extinction. Near to the crater in precisely the right boundary region, Jan Smit and Walter Alvarez identified precisely the sort of geographical outcrop—a tumble of breccias containing spherules and even glass. The glass discovery was important too. Glass forms only during a quick process like an impact and not during a relatively slow one like a volcano, where atoms and molecules have time to crystallize. The glass’s streakiness was further indication that it formed too quickly to homogenize.
Exploration and conversations with resident Mexican geologists exposed more regions that had been disrupted by the relatively nearby impact. Studies also showed that the thickness of the boundary ejecta in North America decreased with distance from the site exactly as predicted using Chicxulub as the source. And the geologist Susan Kieffer helped explain the relative distribution of iridium, impact melt, and shocked quartz in terms of successive ejections from the explosion.
By 1992, given all the accumulated evidence, most geologists were convinced that the Yucatán feature was indeed an impact crater. But they still weren’t sure about its relationship to the K-Pg extinction. Detailed dating, which would require studying the chemical composition of good-quality cores from the crater, would be the only way to firmly establish this connection.
Scientists managed to successfully determine the age of existing cores—in particular in three well-preserved beads of glass—by studying argon isotopes in the rock. They then dated the spherules from the Haitian K-Pg layer to check whether the impact and extinction times agreed. When the first measurement gave the age as 64.98+/−0.05 million years and the second gave 65.01+/−0.08, the results demonstrated that the events had occurred simultaneously (within the measurement uncertainty). This excellent agreement convinced many scientists that the meteoroid dinosaur extinction theory first put forth by Alvarez and collaborators was correct. The ejecta fell exactly on the paleontological boundary, confirming that the impact occurred at the
However, the initial dating of both the crater and the iridium layer—critical to establishing the causal relationship—turned out to be off by about a million years. The relative dates hadn’t changed, but the decay constants essential to assigning an age had initially been slightly incorrect. This is why we now think the K-Pg extinction occurred 66—not 65—million years ago.
Even stronger corroborating evidence for the meteoroid hypothesis is the recent significant improvement in the measurement of the alignment of dates. In February 2013, the researcher Paul Renne of the University of California, Berkeley and his colleagues showed that the Chicxulub impact and the mass extinction occurred less than 32,000 years apart, an incredibly accurate measurement for events that took place so long ago. The Berkeley team used argon-argon dating—the technique relying on radioactive argon isotopes that was mentioned in the previous chapter—to show that the impact and the extinction occurred within this very small time interval.
The proximity of dates that they found is almost certainly not mere coincidence and was a remarkable vindication of the impact hypothesis. Though the authors of the paper are careful to point out that the meteoroid event might have been the nail in the coffin of an extinction that had already been precipitated by volcanic activity or climactic change, it is now beyond reasonable doubt that the massive meteoroid strike that created the Chicxulub crater was the crucial trigger.
In March 2010, 41 experts on paleontology, geochemistry, climate models, geophysics, and sedimentology met to review the more than 20 years of evidence for the impact–mass extinction hypothesis that had accumulated by that time. They concluded that it had indeed been a meteoroid impact 66 million years ago that had both created the crater and given rise to the K-Pg extinction, with its most notable victim—the venerable dinosaur. A paper published in the journal Science in that year presented a consensus view of the meteoroid as the cause of the extinction. A few months later in the same journal, skeptical paleontologists signed off on a different paper in which they too agreed that a meteoroid had at least been a very significant contributing factor.
The Chicxulub crater is among the largest of the ones found on Earth. The story of its identification was an incredible example of science in action, involving clever inductions, testing and validating bold hypotheses, and explorations in regions as far-flung as Italy, Colorado, Haiti, Texas, and the Yucatán. The meteoroid that struck in the Yucatán had a profound influence on the planet and its life. Its origin and its consequences deftly illustrate the Earth’s abiding connections to the Universe.