“T. rex and the Crater of Doom” by Walter Alvarez
Hardcover, 208 pages
Published in 2015 by Princeton University Press
ISBN-10 : 9780691169668 | ISBN-13 : 978-0691169668
Date Finished: Aug 5, 2017
How strongly I recommend it: 7/10
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The story of the asteroid that wiped out the dinosaurs 65 million years ago is one that I’ve always wanted to learn more about. This book, by the scientist who put the puzzle together, is a fascinating look at this extinction event that allowed humans to become the dominant species on earth. “We are the beneficiaries of Armageddon,” writes Alvarez.
My Notes:
… a site called Chicxulub, along the coast of the Yucatan Peninsula.
Today, the impact at the end of the Cretaceous period is one of the most strongly supported events in the geological record.
It was not long ago that this idea, known as panspermia, was dismissed as brusquely as the notion of an impact at the end of the Cretaceous. But today it looks plausible, albeit unconfirmed. Life might have originated on one planet and then spread to others, or impacts may have cross-contaminated worlds.
Today scientists generally recognize five major periods of mass extinctions. The mass extinctions at the end of the Cretaceous period, as dramatic as they may seem, were far from the biggest. Some 250 million years ago, as the Permian period gave way to the Triassic, about 70 percent of species on land and about 95 percent of species in the ocean became extinct. Along with the “big five,” scientists have identified a number of smaller bursts of extinctions.
Geologists use the term Mesozoic, or Middle Life, for the Age of Dinosaurs. The their period of the Mesozoic was the Cretaceous, following the Triassic and the Jurassic periods.
Mesozoic world… ended abruptly, and with no warming, 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.
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.
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.
How can we imagine a comet 10km 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 9km above sea level.
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.
The Moment of Impact
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 he air almost instantaneously until lit 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.
The moment of contact with the Earth’s surface, where the Yucatan 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 pores spaces and destroying much of the orderly crystal structure of minerals.
The result was a gigantic tsunami—a massive wave perhaps a kilometer high, which spreads outwards across the Gulf of Mexico at terrific speed.
Within hours of the impact, most of Mexico and the United States must have been reduced to a desolate wasteland of mostly appealing, agonizing destruction.
Vast quantities of fine dust had burst through the atmosphere in the fireball and the dust was now settling through the upper atmosphere around the world, blocking the sunlight. The land became so dark that you could not have seen you hand in front of your face, and this darkness and the accompanying cold probably lasted for a few months, until finally most of the dust had settled to the ground.
Carbon dioxide can only be removed slowly from the air, and now it trapped the heat from the Sun, raising temperatures to sweltering levels. It was probably thousands of years before the carbon dioxide was back to normal levels.
A world first dark and frozen, then deadly hot, a world poisoned by acid and soot. This was the global aftermath of the Yucatan impact.
By one estimate, half of the genera living at the time of the impact perished.
The loss of the dinosaurs is probably related to their position in the food chain, with herbivorous dinosaurs eating vegetation and carnivorous dinosaurs eating herbivores and perhaps small mammals.
Many smaller land animals survived, including mammals, as well as reptiles such as crocodiles and turtles.
For 150 million years dinosaurs had been the large land animals of the planet while mammals were confined to the role of small animals. With the disappearance of the dinosaurs, there were new opportunities for mammals, and evolution rapidly produced large ones.
Our horror at the destruction caused by the impact that ended the Cretaceous is eased by he understanding that only because of this catastrophe did evolution embark on a course which, 65 million years later, has led to us. We are the beneficiaries of Armageddon.
Rocks are the key to Earth history, because solids remember but liquids and gases forget. Retrieving these long-lost memories is the business of geologists and paleontologists, of people who have chosen to be the historians of the Earth.
As recently as 1975, the story of the impact on the Yucatan was completely unknown.
The key discovery was that history is written in rocks. Ex libra lapidum historia mundi—from the book of rocks comes the history of the Earth.
Erosion destroys the record of Earth history. On the deep sea floor, however, there is little erosion because the waves cannot reach the bottom and currents are slow and gentle.
In terms of eras—the broadest division of Earth history—the Yucatan impact separates the Mesozoic ("middle life," or the Age of Dinosaurs) from the Cenozoic (“recent life,” or the Age of Mammals).
In thinking about Earth history, we need to change our units of time from years to “millions-years,” because thinking of Earth history in terms of years is as hopeless as measuring a trip from Mexico to Italy in centimeters, or measuring a lifetime in seconds.
Earth history is, by chance, about one million times as long as the written record of human history. Writing was invented about 5,000 years ago, and the Earth was formed about 5,000 million-years ago.
The determination of numerical ages is based on the decay of radioactive atoms. A few of the elements that occur in minerals have unstable nuclei which change into other elements. This offers us a clock, because as time passes and the atoms of the parent element in a mineral grain gradually change into daughter element, the ratio of daughter to parent increases. Radioactive decay takes place at an unvarying rate, no matter what changes in pressure or temperature, or what chemical reactions go on, so it provides a very reliable clock.
Some rocks contain magnetic minerals grains which record the direction of the Earth’s magnetic field at the time they were deposited as sediments, or cooled as Laval flows.
It had been a real surprise to geophysicists, just a decade earlier, to find that at many times in the past, the Earth’s magnetic field had reversed. The Earth acts as if it had a huge bar magnet in its interior, aligned roughly north-south, producing the global magnetic field which aligns the compasses on ships and the fossil compasses in rocks.
The rotation and the orientation of the Earth have not changed. Only the direction of its magnetic field has reversed.
The Earth has two tape recorders recording the polarity of the magnetic field, and they give the same reversal history, even though the sea-floor recorder runs 6,000 times faster than the deep-sea sediment recorder.
BIBLICAL CHRONOLOGY AND CATASTROPHISM
However, the early geologists automatically assumed a brief history, because the Bible actually lists the generations of our forefathers back to the creation of the Earth, and the Bible was accepted as an accurate account of history.
With so little available for its formation, a mountain range like the Alps could only be seen as the wreckage from a catastrophe, and perhaps this view resonated with the gloom that travelers evidently felt while crossing mountain barriers.
Geology could not become a real science until the stranglehold of Biblical chronology was broken.
John Muir poetically but correctly attributed the vertical walls of Yosemite to the slow grinding of glaciers, rather than to violence and catastrophe: “Nature chose for a tool not the earthquake or lining to rend and split asunder, not the stormy torrent or eroding rain, but the tender snow-flowers noiselessly falling through unnumbered centuries, the offspring of the sun and the sea.
Up through the 1950s, most geologists believe that the continents had always been where they are now.
… the German meteorologist Alfred Wegener argued in the 1920s that the matching coasts of South America and Africa do provide a marker for horizontal motions.
… the central concept of plate tectonics is gradual change. Oceans so wide that they take hours to cross in jet aircraft have grown by sea-floor spreading over tens of millions of years, at a rate of a few centimeters per year—about the rate at which your fingernails grow.
Normal stars shine because atomic nuclei of hydrogen at their centers are fusing together to form helium and heavier elements, with the release of prodigious amounts of energy. The energy escapes in the form of photons which ricochet around inside the star, sustaining the pressure that keeps the gravity of the star from shrinking it to a much smaller size.
The rocky Earth and our carbon-based bodies are largely composed of the debris of stars that blew up. Supernovas made life possible, but if one occurred in a star in the Sun’s neighborhood, it would be a major disaster, with the Earth’s surface bathed in dangerous or lethal radiation and the climate seriously disrupted.
From a review of the paleontological literature, Dale estimated that almost half of the genera of animals, plants, and single-celled organisms had died out at the end of the Cretaceous.
The exact mechanism is not yet clear, but immediately after the comet hit the Yucatan, the tsunami sped away from the impact site. It left evidence of its passage in the form of a torn-up sea bottom covered by sedimentary debris—the evidence we were seeking.
It was the tsunami that gave the mystery away. After years of frustration the tsunami deposits had finally led the detectives to the scene of the crime, and at last everything was coming together.
The Cretaceous-Tertiary boundary marks a profound discontinuity in Earth history. The early geologists were right to choose it as the dividing line to separate fundamental eras in the history of life—the Mesozoic and the Cenozoic—the era of Middle Life and the era of Recent Life.
The one terrible day undid the benefits which 150 million years of natural selection had conferred upon the dinosaurs, making them ever fitter to be the large land animals of Earth. Evolution had not equipped them to survive the environmental disasters infected by a huge impact, and when the holocaust was over, they were gone.
But one of the most remarkable features of mammal evolution after the KT extinction was the rapidity with which large land mammals evolved. In addition, the number of mammal species quickly went up, as mammals evidently found all kinds of new niches—new ways to exploit the world around them.
Perhaps the fragments come not from the main crater, but from secondary craters, where really large ejecta blocks fell, making their own small craters, which only penetrated the shallow dolomite.
The ejecta curtain .. . roughly 45 degree trajectories, …
There was almost no fossils, and thus no evidence of biological extinction, in Precambrian strata, but the detailed fossil record of the 570 million years since the end of the Precambrian gives evidence of five great mass extinctions and about five smaller ones.
The greatest of all the mass extinctions was at the Permian-Triassic boundary, 250 million years ago. There is no evidence either for or against impact at that time, because there is virtually no preserved stratigraphic record across the Permian-Traissic boundary anywhere in the world.
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