Passage A
The Cretaceous-Tertiary extinction event, or the K-T event, is the name given to the die-off of the dinosaurs and other species that took place some 65.5 million years ago. For many years, paleontologists believed this event was caused by climate and geological changes that interrupted the dinosaurs' food supply. However, that view changed in the 1980s.
Luis Alvarez was a Nobel Prize-winning physicist, inventor and pioneer in the field of radiation and nuclear research. He and his son, noted geologist Walter Alvarez, were conducting research in Italy when they discovered a centimeter-thick layer of iridium-enriched clay at the K-T boundary. Iridium is rare on earth, but more common in space. The Alvarezes published their findings in 1981, postulating that the thin layer of iridium was deposited following the impact of a large meteor, comet or asteroid with the earth. Furthermore, this bolide impact - the meteor, comet or asteroid colliding with the earth's surface - could have caused the extinction of the dinosaurs. At the time, the Alvarez theory was so far removed from prevailing hypotheses that it was ridiculed. Slowly, though, other scientists began finding iridium evidence at various places around the globe that corroborated the Alvarez theory. There was, however, no smoking gun in the form of an impact site.
Then in 1991, a massive meteor crater, 110 miles in diameter was discovered on the edge of the Yucatán Peninsula, extending into the Gulf of Mexico. Scientists believe the bolide that formed it was roughly 6 miles in diameter, struck the earth at 40,000 miles per hour and released 2 million times more energy than the most powerful nuclear bomb ever detonated. The heat would have broiled the earth's surface, ignited wildfires worldwide and plunged the planet into darkness as debris clouded the atmosphere. Miles-high tsunamis would have washed over the continents, drowning many forms of life. Shock waves would have triggered earthquakes and volcanic eruptions.
The resulting darkness could have lasted for months, possibly years. It would have plunged the earth's temperatures into the freezing zone, killing plants and leaving herbivores with nothing to eat. Many dinosaurs would have died within weeks. The carnivores who feasted on the herbivores would have died a month or two later. Overall, the loss of biodiversity would have been tremendous. Only small scavenging mammals that could burrow into the ground and eat whatever remained would have survived. The iridium layer plus the Chicxulub Crater were evidence enough to convince many scientists that the bolide impact theory was credible. It explained much of what previous theories could not.
Passage B
What killed the dinosaurs? Few questions in science have been more mysterious—and more contentious. Today, most textbooks and teachers tell us that nonavian dinosaurs, along with three-fourths of all species on Earth, disappeared when a massive asteroid hit the planet near the Yucatán Peninsula some 66 million years ago. But a new study published in the journal Geology shows that an episode of intense volcanism in present-day India wiped out several species before that impact occurred. The result adds to arguments that eruptions plus the asteroid caused a one-two punch. The volcanism provided the first strike, weakening the climate so much that a meteor—the more deafening blow—was able to spell disaster for Tyrannosaurs rex and its late Cretaceous kin.
The team analyzed ancient sediments at the bottom of a lake basin and found that the lake's temperature started to rise hundreds of thousands of years before the impact. That warming coincides with eruptions at the Deccan Traps in India, which likely spewed immense amounts of carbon dioxide into the atmosphere well before the impact.
Although that agrees with previous data showing pre-impact temperature swings across the world, the team also made a more alarming find. Zhang and his colleagues discovered that many fossils within the ancient sediments disappeared at the time of this warming. Two-thirds of the extinctions within the region actually occurred after the onset of volcanism and before the impact. As such, the study provides proof that the volcanoes destabilized the climate, priming the world for catastrophe.
A hotter climate certainly helped send the nonavian dinosaurs to their early grave, says Paul Renne, a geochronologist at the University of California, Berkeley, who was not involved in the study. That's because the uptick in temperature was immediately followed by a cold snap—a drastic change that likely set the stage for planet-wide disaster. In this scenario, volcanism likely sent the world into chaos, driving many extinctions alone and increasing temperatures so drastically that most of Earth's remaining species couldn't protect themselves from that second punch when the asteroid hit.
(Passage 1 has been picked from history.com and has been edited for use. Passage 2 has been picked from nationalgeographic.com and has been edited for use.)
