Extinction: Evolution and the End of Man. Michael Boulter
them to test the Umbria sample for metal elements. Iridium is rare on this planet, only occurring in very small quantities, so it came as a great surprise when the clay layer showed relatively massive quantities of the metal, nine parts per billion. Suddenly the dating experiment didn’t seem to matter. The important thing now was to explain the high concentration of iridium. What made that observation really exciting was the knowledge that most iridium comes from space, carried on Earth in micrometeorite showers. So this was obviously some shower. Even greater excitement lay ahead when they had the fragment dated by the new method. Luis Alvarez’s rock sample from Umbria was 65 million years old, from the K–T boundary itself.
From calculations based on the amount of iridium in the sample, the Alvarezes’ colleagues in the University of California at Berkeley proposed a theory that a 20km-diameter extraterrestrial object hit the Earth, spreading the iridium it contained in its spray. Throughout the 1980s and 1990s what we think is the same ‘iridium layer’ has turned up in North America, Essex, Denmark, Asia and elsewhere. A speculative idea came from a small amount of evidence and some highly imaginative but testable ideas. What began as a risky expression of grey fantasy by the Alvarezes has turned into a largely black or white fact well supported by other evidence from a variety of greyer disciplines.
Not least impressive is evidence that buried remains of the meteorite have been detected in Mexico. Rediscovered is a better word, for the structure was first mapped in 1962. Then, the ash and lava flows were thought to be part of a normal volcanic structure. There was no reason to get excited and the maps were filed away for normal everyday use. Then, in 1981, five years after the Alvarezes’ suggestion of an impact, two petroleum geologists found the Chicxulub crater off the Yucatán peninsula. Impact craters don’t lead to oil, so they were told to go away and get on with something else, not quite in disgrace, but not knowing how close to fame they were.
Other evidence of the impact includes glass beads that have also been found in the thin iridium layer sediments throughout the world. It’s been known for centuries that when you shoot a hot cannon ball into sand the energy melts the sand and the glass forms into small beads that splash out all around. With big chunks of rock like the Chicxulub meteorite it happens on a different scale, producing a wide variety of different minerals. Some of them form crystals less than a millimetre in diameter and are made of very particular materials such as magnetites containing nickel, as well as quartz, only found in remnants of meteoritic material. A range of clues like these can be identified very easily by looking at the shape of crystals in a scanning electron microscope. Very unusual crystals are well known in the scientific literature and are found frequently around meteorite impact craters of very different ages. They’ve also been found on the moon, where of course there’s no shortage of such craters. Other larger beads are found in the iridium layer at several localities as well as at the site of the impact, while others are pure glass.
Конец ознакомительного фрагмента.
Текст предоставлен ООО «ЛитРес».
Прочитайте эту книгу целиком, купив полную легальную версию на ЛитРес.
Безопасно оплатить книгу можно банковской картой Visa, MasterCard, Maestro, со счета мобильного телефона, с платежного терминала, в салоне МТС или Связной, через PayPal, WebMoney, Яндекс.Деньги, QIWI Кошелек, бонусными картами или другим удобным Вам способом.