Cretaceous Catastrophe
While mass extinctions of numerous animal species have occurred throughout geologic history, it is the unexplainable disappearance of the great dinosaurs at the end of the Cretaceous Period (approximately 66 million years ago) that has generated a tremendous amount of debate. Some scientists speculate that severe changes in the Earth's environment were the cause, while others surmise receding seas and widespread disease. Still others contend that the dinosaurs' demise might have been the result of a widespread disaster. However, consensus has not yet been achieved.
In 1980, a disaster began to look more likely. A team of scientists headed by Luis Alvarez (1911-1988), who received the 1968 Nobel Prize in Physics, proposed the Cretaceous impact theory, which asserts that an enormous asteroid impact marked the end of the Cretaceous Period, causing the extinction of the dinosaurs. Alvarez, his son, Walter Alvarez (1940-) and scientists Frank Asaro and Helen Michel published their findings in the June 6, 1980 issue of Science magazine. In their study, they argued that the Earth had been hit by an asteroid roughly the size of Manhattan. After discovering high levels of iridium at the Cretaceous-Tertiary ("K-T") boundary in Denmark, Italy, and New Zealand, they theorized that a massive extraterrestrial object or objects had crashed into the Earth. Such an impact would have sent a huge volume of dust into the atmosphere, blocking sunlight and gradually disrupting ecosystems everywhere.
Their theory, backed by geologic evidence, drew initial support in the scientific community, but lacked the final key to solving the mystery: If a massive extraterrestrial object had made such a powerful impact, why were its remains still undiscovered? Part of the problem lies in the geologic processes of the Earth. Since the time of the crash, approximately 20 percent of the Earth's crust has been recycled by processes associated with plate tectonics. This means that geologists might have to travel far and wide, and dig deep into the Earth to find any remaining evidence.
The purported crater remained undiscovered until the early 1990s, when several scientific teams began to uncover evidence in the Gulf of Mexico region. Then, in 1992, a team led by Alan Hildebrand of the Geological Survey of Canada in Ottawa announced the discovery of a large crater in the Yucatán region of Mexico, which left ample evidence of a massive crash. Rocks found in this area contained quartz grains that showed evidence of shock waves from an impact. This provided the much needed evidence to support earlier discoveries. Actually, the Mexican national petroleum company (PEMEX) had began uncovering clues as early as the 1950s. While drilling exploratory wells in the Yucatán region, they penetrated layers of rocks, unusual for that region. Upon careful study, geologists discovered that these rocks had solidified from a molten state. In the 1970s PEMEX conducted a magnetic survey of the area, which revealed a large circular structure buried beneath the surface. Centered near the town of Chicxulub on the north coast, the feature extended into the Gulf of Mexico, measuring 112 mi. (180 km) in diameter. Despite the magnitude of these discoveries, PEMEX had stored away the information.
In the nearly two decades since the impact theory was first proposed, if anything, debate about the demise of the dinosaurs has become more heated. Although the majority of geologists support a catastrophic event to explain the Cretaceous extinction, not everyone agrees on its source. Some geologists contend that volcanoes were the source of the iridium layer as well as the ash that triggered the extinctions. Some paleontologists don't even support a catastrophic event. They claim that the fossil record does not show a sudden, almost instantaneous extinction, but rather a slow decline over several million years.
Whatever the cause, hopefully, further research will someday put to rest the controversy over the source of the extinction at the Cretaceous-Tertiary boundary. Even if it turns out that a collision with an extraterrestrial body was not at fault, the impact theory has served to generate considerable discussion both about the extinction event and the possibility of future extraterrestrial impacts.
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