![]() ![]() The dating is precise, and the iridium layer has been identified in more than 100 places around the Earth. In the Pacific Ocean and the Caribbean the iridium-bearing clay forms a layer in ocean floor sediments it is found in continental shelf deposits in Europe and in North America, from Canada to New Mexico, it occurs in coal-bearing rock sequences laid down on floodplains and deltas. It doesn't seem to matter whether the boundary rocks were laid down on land or under the sea. ![]() They found that rocks laid down precisely at the K-T boundary contain extraordinary amounts of the metal iridium (Figure 18.1). The evidence for the impact was first discovered by Walter Alvarez and colleagues. The most recent work on the K-T extinction has centered on two hypotheses that suggest a violent end to the Cretaceous: a large asteroid impact and a giant volcanic eruption.Ī meteorite big enough to be called a small asteroid hit Earth precisely at the time of the K-T extinction. The K-T extinctions were a global event, so we should examine globally effective agents: geographic change, oceanographic change, climatic change, or an extraterrestrial event. But no matter how convincing or how silly they are, any of the theories that try to explain only the extinction of the dinosaurs ignore the fact that extinctions took place in land, sea, and aerial faunas, and were truly worldwide. For example, even in the 1980s a new book on dinosaur extinctions suggested that they spent too much time in the sun, got cataracts, and because they couldn't see very well, fell over cliffs to their doom. More bad science is described in this chapter than in all the rest of the book. There have been many bad theories to explain dinosaur extinctions. The K-T casualties included most of the large creatures of the time, but also some of the smallest, in particular the plankton that generate most of the primary production in the oceans. Insects, mammals, birds, and flowering plants on land, and fishes, corals, and molluscs in the ocean went on to diversify tremendously soon after the end of the Cretaceous. It was certainly sudden in geological terms and may have been catastrophic by anyone's standards.ĭespite the scale of the extinctions, however, we must not be trapped into thinking that the K-T boundary marked a disaster for all living things. There are still arguments about just how short the event was. The K-T extinctions were worldwide, affecting all the major continents and oceans. This extinction event marks a major boundary in Earth's history, the K-T or Cretaceous-Tertiary boundary, and the end of the Mesozoic Era. ![]() At the same time, most plankton and many tropical invertebrates, especially reef-dwellers, became extinct, and many land plants were severely affected. The End of the Dinosaurs: The K-T extinctionĪlmost all the large vertebrates on Earth, on land, at sea, and in the air (all dinosaurs, plesiosaurs, mosasaurs, and pterosaurs) suddenly became extinct about 65 Ma, at the end of the Cretaceous Period.
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