Until the 1980s, most scientists believed that noncatastrophic geological processes caused the extinction of dinosaurs that occurred approximately 66 million years ago, at the end of the Cretaceous period. Geologists argued that a dramatic drop in sea level coincided with the extinction of the dinosaurs and could have caused the climatic changes that resulted in this extinction as well as the extinction of many ocean species. This view was seriously challenged in the 1980s by the discovery of large amounts of iridium in a layer of clay deposited at the end of the Cretaceous period. Because iridium is extremely rare in rocks on the Earth's surface but common in meteorites, researchers theorized that it was the impact of a large meteorite that dramatically changed the earth's climate and thus triggered the extinction of the dinosaurs. Currently available evidence, however, offers more support for a new theory, the volcanic-eruption theory. A vast eruption of lava in India coincided with the extinctions that occurred at the end of the Cretaceous period, and the release of carbon dioxide from this episode of volcanism could have caused the climatic change responsible for the demise of the dinosaurs. Such outpourings of lava are caused by instability in the lowest layer of the Earth's mantle, located just above the Earth's core. As the rock that constitutes this layer is heated by the Earth's core, it becomes less dense and portions of it eventually escape upward as blobs or molten rock, called "diapirs," that can, under certain circumstances, erupt violently through the Earth's crust. Moreover, the volcanic-eruption theory, like the impact theory, accounts for the presence of iridium in sedimentary deposits; it also explains matters that the meteorite-impact theory does not. Although iridium is extremely rare on the Earth's surface, the lower regions of the Earth's mantle have roughly the same composition as meteorites and contain large amounts of iridium, which in the case of a diapir eruption would probably be emitted as iridium hexafluoride, a gas that would disperse more uniformly in the atmosphere than the iridium-containing matter thrown out from a meteorite impact. In addition, the volcanic-eruption theory may explain why the end of the Cretaceous period was marked by a gradual change in sea level. Fossil records indicate that for several hundred thousand years prior to the relatively sudden disappearance of the dinosaurs, the level of the sea gradually fell, causing many marine organisms to die out. This change in sea level might well have been the result of a distortion in the Earth's surface that resulted from the movement of diapirs upward toward the Earth's crust, and the more cataclysmic extinction of the dinosaurs could have resulted from the explosive volcanism that occurred as material from the diapirs erupted onto the Earth's surface.