Impacts Craters

Abstract

In 1980, father and son geologists Luis and Walter Alvarez discovered unusually high traces of iridium in rock layers near Gubbio (Italy) that define the Cretaceous-Paleogene boundary (K-Pg; formerly known as the K-T boundary) when about 75 % of all species on Earth, including the dinosaurs, perished forever in a mass extinction event. The Alvarez’s put forward an extraordinary claim – that the accumulation of this much iridium-bearing dust would require an asteroid of about 10 km wide to hit the Earth. The search was on. But it proved to be difficult for two reasons: First, the event took place 65 million years ago and the crater could have been eroded or filled with sediments. And secondly, most of the Earth’s surface is covered with water making it difficult to detect craters on the ocean floor. A decade later, geologists and geophysicists searching for oil finally found a huge crater that fit the search criteria. It was almost 200 km in diameter and 1.5 km deep, but buried under sediments near a town called Chicxulub on Mexico’s Yucatán Peninsula. Chicxulub means “tail of the devil” in the local Mayan language. Today, almost all scientists agree that this gigantic asteroid impact produced a blast 6 million times more powerful than the 1980 eruption of Mt. St. Helens and caused a cataclysmic collapse of ecosystems that lead ultimately to the K-Pg mass extinction. The last piece of the puzzle was put together in 2013, when researchers provided new evidence that the age of the Chicxulub asteroid impact and the K-Pg boundary coincide precisely. This short chapter presents a few visual examples of impact craters that are visible on the face of the Earth and explains why they are so scarce and rarely preserved on Earth.