T. rex and the Crater of Doom
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T. rex and the Crater of Doom

Walter Alvarez

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T. rex and the Crater of Doom

Walter Alvarez

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Sixty-five million years ago, a comet or asteroid larger than Mount Everest slammed into the Earth, inducing an explosion equivalent to the detonation of a hundred million hydrogen bombs. Vaporized detritus blasted through the atmosphere upon impact, falling back to Earth around the globe. Disastrous environmental consequences ensued: a giant tsunami, continent-scale wildfires, darkness, and cold, followed by sweltering greenhouse heat. When conditions returned to normal, half the plant and animal genera on Earth had perished.This horrific chain of events is now widely accepted as the solution to a great scientific mystery: what caused the extinction of the dinosaurs? Walter Alvarez, one of the Berkeley scientists who discovered evidence of the impact, tells the story behind the development of the initially controversial theory. It is a saga of high adventure in remote locations, of arduous data collection and intellectual struggle, of long periods of frustration ended by sudden breakthroughs, of friendships made and lost, and of the exhilaration of discovery that forever altered our understanding of Earth's geological history.

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But it was too late. At that moment the rock quivered and trembled beneath them. The great rumbling noise, louder than ever before, rolled in the ground and echoed in the mountains. Then with searing suddenness there came a great red flash. Far beyond the eastern mountains it leapt into the sky and splashed the lowering clouds with crimson. In that valley of shadow and cold deathly light it seemed unbearably violent and fierce. Peaks of stone and ridges like notched knives sprang out in staring black against the uprushing flame in Golgoroth. Then came a great crack of thunder.
—J.R.R. TOLKIEN, The Lord of the Rings
Try to imagine a different world—different from the one we live in. Not wildly different, like the settings of science fiction stories which take place on airless planets or in giant spaceships. We are looking for a world much like our own, but different in subtle ways. J.R.R. Tolkien described such a world in The Lord of the Rings—with mountains, swamps, and plains like ours, but with a slightly different geography—much like Europe, but not quite the same. Tolkien’s “Third Age of Middle Earth” has familiar inhabitants like humans and horses, but other creatures that we know well, like dogs and cats, are missing. Middle Earth also has unfamiliar inhabitants—dwarves, elves, wizards, and hobbits. It is terrorized by the merciless, sharp-clawed goblins called orcs. Tolkien’s world seems ancestral, or perhaps alternative to ours.
The world we seek is reminiscent of Tolkien’s Middle Earth. It has mountains, deserts, forests, and oceans, arranged in a geography that is something like our Earth, yet noticeably different. It has rivers and canyons, plateaus and sand dunes. It has cloudbursts in the mountains, and glowing sunsets in the clear air after a thunderstorm. Some of the inhabitants seem familiar, though not exactly like the ones we know. Evergreen trees and deciduous trees shade the landscape, and the streams are full of fish. But the ground is bare of grass, and the animals look different. Little furry ones are recognizable as mammals. But there are also giant creatures, some placidly grazing while others hunt, with claws as terrifying as those of any orc in Middle Earth.
This world is different from ours, but it is familiar through museum reconstructions, paintings, and films. For this is not the Third Age of Middle Earth, but rather the Third Period of Middle Life. Geologists use the term Mesozoic, or Middle Life, for the Age of Dinosaurs. The third period of the Mesozoic was the Cretaceous, following the Triassic and the Jurassic periods.
More precisely, the world we are imagining was the very end of the Cretaceous, 65 million years ago. It was ancestral to our modern world, with a geography that was different but still familiar, because continental drift since then has moved the Earth’s land masses around but has not completely rearranged them. India had not yet collided with Asia to thrust up the Himalayas, but there were already mountains in western North America. Sea level was higher than today, and part of the interior of North America was covered by a shallow sea.
Not only was that world ancestral to ours, it was in some sense alternative as well. For it was a stable world. Despite the violent hunting of the carnivorous dinosaurs and the oftdepicted dramatic battles between Tyrannosaurus rex and Tri ceratops, life patterns and the inhabitants themselves had changed only slowly during the previous 150 million years. The dinosaurs were very successful large animals and shared their world with equally successful small animals and with plants of all kinds. There is every reason to believe that if it had remained undisturbed, the Mesozoic world could have continued indefinitely, with the slightly evolved descendants of the dinosaurs dominating a world in which humans never appeared.
But the Mesozoic world did not remain undisturbed. It ended abruptly, and with no warning, 65 million years ago. Vast numbers of highly successful animal and plant species suddenly disappeared in a mass extinction, leaving no descendants. This break in the history of life is so impressive that geologists use it to define the boundary between the Cretaceous, or last period of the Mesozoic, and the Tertiary, or first period of the Cenozoic. Today’s world is populated with descendants of the survivors of the mass extinction that ended the Cretaceous world.
Looking back across the abyss of time which separates us from the Cretaceous, we can somehow feel nostalgia for a long-lost world, one which had its own rhythm and harmony. We feel a special sadness when we think about its plants and animals, fish and birds—for most of the Cretaceous animals and plants are irretrievably lost. We can even feel some sorrow as we imagine the sun setting over a western ocean, painting the clouds with orange and red and yellow and gold, on the last evening of that world. For the Cretaceous world is gone forever, and its ending was sudden and horrible.
Doom was coming out of the sky, in the form of an enormous comet or asteroid—we are still not sure which it was. Probably ten kilometers across, traveling tens of kilometers a second, its energy of motion had the destructive capability of a hundred million hydrogen bombs. If an asteroid, it was an inert, crater-scarred rock, dark and sinister, invisible until the last moment before it struck. If a comet, it was a ball of dirty ice, spewing out gases boiled off by the heat of the Sun, and it announced impending doom with a shimmering head and a brilliant tail splashed across half the sky, illuminating the night, and finally visible even in the daytime as Armageddon approached. Let us think of it as a comet, remembering that perhaps it was an asteroid instead. Comets have been mistakenly interpreted by humans in times past as harbingers of doom, foretelling famine, plague, and destruction. Although no humans were there to witness the giant comet of 65 million years ago, in this case it really did portend disaster.
The Solar System abounds in comets and asteroids, some even bigger than the one which was nearing Earth on that day 65 million years ago. Most asteroids remain in a belt between Mars and Jupiter, and most comets orbit the Sun far beyond distant Pluto. Occasionally, however, an asteroid has its orbit deflected by Jupiter’s gravitational pull, or a comet orbit is altered by the gravitational tug of a passing star. A few of these asteroids and comets are diverted into orbits which cross that of the Earth. An impact occurs when such an object intersects the Earth’s orbit just as Earth happens to be at the crossing point. This is what is going on every time you see a shooting star flashing across the night sky. Those streaks of light are due to tiny fragments of comets or asteroids burning up through friction in the Earth’s atmosphere. Somewhat larger objects, the size of a fist, are too big to burn up completely in the atmosphere, but are slowed down enough to survive their impact on the Earth’s surface. These objects are the meteorites displayed in museums and studied by geologists interested in extraterrestrial rocks.1
Large impacts can also happen, and they were frequent in the early history of the solar system, as witnessed by the ancient, crater-scarred face of the Moon. But large impacts are rare nowadays, because the debris that was abundant in the early solar system has been swept up by the planets, large Earth-crossing comets and asteroids are now rare, and Earth is a very small target. To see how small, look at Venus just after sunset, when it is the “evening star.” Venus is the size of the Earth, and from our distance it is a tiny, although brilliant, dot in the sky—a very difficult target to hit.
Earth is protected, therefore, by the fact that large comets and asteroids rarely come into the inner Solar System, and those that do are unlikely to hit something as small as our planet. So we can imagine the giant comet of 65 million years ago coming close to the Earth again and again, over a period of centuries or millenia, as it orbited the Sun—sometimes far from Earth, sometimes close enough to put on a spectacular display in the night sky. A set of near misses like this must take place every now and then in Earth history, but usually the comet hits the Sun or another planet, or is deflected out of the inner Solar System. In this particular case, however, there came a time when the invader’s orbit intersected that of Earth just as both were approaching the intersection point. This time there would be no escape. The comet was aimed toward the southern part of North America—toward the shallow seas and coastal plains which are now the Yucatán Peninsula of Mexico.
It is very difficult to appreciate the impact that was about to occur, because such an extreme event is far beyond our range of experience—for which we can be most grateful! One can write down the measures of what happened—an object about 10 km in diameter2 slammed into the Earth at a velocity of perhaps 30 km/sec.3 But these measures only acquire meaning when we try to visualize them, or make analogies to help our understanding. How can we imagine a comet 10 km in diameter? Its cross section about matches the city of San Francisco. If it could be placed gently on the surface of the Earth it would stand higher than Mount Everest, which only reaches about 9 km above sea level. Its volume would be comparable to the volume of all the buildings in the entire United States. It was a big rock, or a big ice ball, but not of a scale beyond our comprehension.
What turned it into a cataclysmic weapon was its velocity. The estimated impact velocity of 30 km/sec is 1,000 times faster than the speed of a car on the highway and 150 times faster than a jet airliner. It is about 6 times faster than the speed of seismic waves in rock. When a collision takes place at velocities this high, our experience is not a useful guide, and rock materials do not behave in the ways we are used to. Instead, a shock wave is produced—a kind of sonic boom in the rock. The shock wave from such an impact crushes and compresses the impactor and target rock so intensely that after the shock passes, the decompressing rock will fly apart, or melt, or even vaporize. The concept of rocks instantaneously boiling away to vapor conveys a gut feeling for the extraordinary and violent conditions during an impact.
Scientists immediately ask about the energy of the approaching object, because energy is Nature’s currency, a measure of the ability to move things around and bring about changes.4 Nature runs a kind of automatic bookkeeping system for energy transfers, requiring that the energy of motion of the incoming comet be fully accounted for in all the kinds of damage done during the impact. When we do the bookkeeping, we find that the energy of motion of the comet just before impact was equivalent to the explosion of 100 million megatons of TNT, sufficient to vaporize the comet in about 1 second and to blow out a hole in the ground which was briefly 40 km deep but quickly collapsed into a broader, shallower crater 150–200 km across. To get a feeling for this quantity of energy, keep in mind that one large hydrogen bomb has a yield of about 1 megaton of TNT, and that the total nuclear arsenal of the world at the peak of the Cold War was about 10,000 such bombs. The 108-megaton impact of the comet which ended the Cretaceous was therefore equivalent to the explosion of 10,000 times the entire nuclear arsenal of the world (although the impact explosion was not nuclear).
Returning to the 10-km-wide comet as it approached the Earth at 30 km/sec, we can get a feeling for how fast the event happened. An airliner flies at an altitude of about 10 km, so imagine a plane unfortunate enough to be in the way of the incoming comet. In an instant the airplane would be smashed like a bug by the onrushing body. One-third of a second later the front of the comet, carrying the insignificant aircraft wreckage, would hit the ground, generating a blinding flash of light and initiating shock waves in the comet and the ground, and after another ⅓ second the back end would be passing below ground level. By one or two seconds after the loss of the airplane, there would be a huge, growing, incandescent hole in the ground and an expanding fireball of vaporized rock, and debris ejected by the explosion would be clearing the atmosphere on its way to points around the globe. Earth would suffer cataclysmic damage in less time than it takes to read this sentence.
Now that we have some sense of the scale of the impact that ended the Cretaceous world, let us look at our current, imperfect understanding of just what happened.
The comet approaching Earth 65 million years ago first encountered the tenuous air many kilometers above the surface. About 95 percent of the atmosphere lies below an altitude of 30 km, so depending on the velocity and the angle at which the impactor approached the surface, it would have taken only a second or two to penetrate most of the atmosphere. The air in front of the comet, unable to get out of the way, was violently compressed, generating one of the most colossal sonic booms ever heard on this planet. Compression heated the air almost instantaneously until it reached a temperature 4 or 5 times that of the Sun, generating a searing flash of light during that one-second traverse of the atmosphere.
At the instant of contact with the Earth’s surface, where the Yucatán Peninsula now lies, two shock waves were triggered. One shock wave plowed forward into the bedrock, passing through a three-kilometer-thick layer of limestone near the surface, and down into the granitic crust beneath. The onrushing shock wave drove forward through the bedrock, crushing shut all cracks and pore spaces and destroying much of the orderly crystal structure of minerals.
Meanwhile, a second shock wave flashed backward into the onrushing comet. Reflecting off the back of the impactor, it tore apart the trailing edge of the comet. In the second or so it took for this to happen, the comet ceased to be recognizable as a spherical body. With its enormous momentum driving it forward, the comet penetrated deep into the Yucatán bedrock, forcing open a huge hole and molding itself into an incandescent coating on the inside of the growing hole, which was now opening out into an expanding crater. But the comet coating on the inside of the crater did not last more than a moment before it was mostly vaporized, along with much of the original target rock.
As the rapidly vaporizing comet wreckage was carried forward into the growing crater, the shock wave curved back up to the surface and spewed out ejecta—melted blobs and solid fragments of target rock—upward and outward on high, arching trajectories that flung them through the thin outer fringes of the atmosphere and beyond. Falling back to Earth within a few hundred kilometers of the rim of the crater, this debris built up a vast blanket of ejecta.
Even this did not exhaust the pyrotechnic potential of the impacting comet. The huge cloud of vaporized rock generated at ground zero was driven outward by its own heat and pressure in a colossal fireball. The explosion of a nuclear bomb—tiny by comparison—produces a hot-gas fireball which flashes outward to a diameter of a kilometer or so, until it can push no farther against the atmospheric pressure, and then floats upward to an altitude of 10 km where it spreads out into a mushroom cloud. The incomparably greater fireball of the Yucatán impact overwhelmed the atmosphere, blowing right through the entire blanket of air, expanding and accelerating out into space and launching particles of rock into trajectories which carried them far around the Earth before they fell back to the ground.
And still the fireworks continued. Even as the scorching fireball of rock vapor blew away into outer space, it was followed by a second fireball, not as hot, but almost as dramatic. For about three kilometers down from the surface, the Yucatán was covered with a thick layer of limestone. Limestone is Nature’s way of storing carbon dioxide gas as a solid, by combining it with calcium. Shocked limestone suddenly releases its stored CO2, and in an impact as large as this, enormous quantities of this gas were almost instantaneously released like popping the cork on a colossal bottle of champagne. Still more rock debris was carried aloft in this second exploding gas ball as it, too, blew through the atmosphere and into outer space.
Meanwhile, the expanding crater had reached its maximum depth of perhaps 40 km. This hemi...

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