When the dinosaurs disappeared, so did numbers of plant and animal species. That was not the only major extinction however. There have been five in the past 600 million years. The asteroid theory is one explanation of what caused them. It is a controversial hypothesis that many now disbelieve, but it is still useful in thinking about the distant past.
The asteroid theory is also called the Alvarez hypothesis, because it was first made public by a father and son team, Luis and Walter Alvarez, in 1980. Luis Alvarez, a physics Nobelist, worked together with his son Walter, a geologist, to discover and explain the presence of anomalous amounts of iridium in one thin layer of earth’s crust. Their work led them to formulate a brilliant explanation for the great extinctions. It involves asteroids striking the earth, and the damage such impacts might do.
Chicxulub Crater is 112 miles in diameter, and a mile deep. Identified at the edge of the Yucatan Peninsula in 1990, it is a filled-in impact scar from an enormous bolide, a meteor or asteroid, which hit the earth about 65 million years ago. At about the same time, according to advocates of the asteroid theory, 65% of the species on earth died, including the dinosaurs.
The Shiva Crater, in the Arabian Sea off the coast of Mumbai, is even larger. It could measure 370 by 280 miles. It is seven and one-half miles deep. Sankar Chatterjee named it for Shiva, the Hindu goddess of destruction and renewal. The bolide that made it would have measured 25 miles in diameter.
Impacts like the ones that made these craters would have had a huge effect of the planet. Dust and debris would fill the air. Volcanoes would erupt. Tsunamis would batter coasts and spread destruction inland. Enormous fires would rage, and incalculably large storms. Acid rain would fall.
An asteroid winter might have lasted for years. As dust and smoke filled the air, temperatures would fall, and plants would receive no light. As they died, oxygen levels would fall. Herbivores would die of hunger, and then large carnivores would start eating one another. Eventually, smaller animals might survive by feeding on the bodies of larger animals. When the air cleared, a new era would dawn, dominated by new species.
Some geologic evidence supports this theory. There are layers of deposits in the earth associated with possible asteroid catastrophes. They are quite different from the deposits above and below them and their structure and characteristics are strikingly alien. One marks the K-T boundary, also known as the Cretaceous Tertiary boundary, the mark of the great extinction.
Siderophiles are common in this layer. They are a group of elements that are rare on the surface of the earth but common in certain meteors. These elements bind with iron, but not so easily with oxygen or silicon, so they are found mostly in earth’s core, and are generally rare in the crust. The siderophile element iridium, especially, is extremely rare in the crust in general, yet very common at the K-T boundary.
Tektites contain quartz grains that vaporized and then cooled without a crystal structure. They are found in meteors and in layers associated with the great extinction. Shocked quartz is a form of mineral found at nuclear bomb sites, in known meteor impact craters, and in layers associated with the end of the dinosaurs.
Yet some scientists doubt the Alvarez theory. Gerta Keller of Princeton and Thierry Adatta of Lausanne University believe that the impact that caused the Chicxulub Crater happened 300,000 years earlier than the disappearance of the dinosaurs. They say they have found evidence of species change at a very different time than the Alvarez theory calls for.
Those who disagree with these new findings say that that earthquakes or tsunamis may have distorted the geologic record by depositing material quickly or displacing it. Keller and Adatta counter that the layers that indicate the passage of time must have been deposited slowly because they show evidence of many generations of burrowing creatures in the rock.
Gerta Keller agrees that something catastrophic did happen to end the Age of Reptiles. She thinks that event might have been huge volcanic explosions in the area that is now the Deccan Traps in India. This igneous feature covers 500,000 square kilometers and is perhaps 2,000 meters thick.
The eruptions that produced the volumes of lava here could quite possibly have altered the climate sufficiently to end an age. Some scientists think that global warming caused by events at the Deccan Traps contributed to a long decline of the dinosaurs and other species, and that Chicxulub only finished them off.
Some hypothesize that the Shiva Crater formation and volcanism at the Deccan Traps are somehow linked. Possibly, the bolides that caused Shiva crater and Chicxulub were fragments of a body that broke up above earth. Astronomers watched that happen to a comet nearing Jupiter, and saw the event’s multiple scars.
The Siberian Traps, an igneous region in Russia, is associated with the Permian-Triassic event, another mass extinction. There is also debate about this transition, which some scientists think was associated with a bolide impact.
It is not certain yet what caused the death of the dinosaurs. The Asteroid theory is one explanation, and a good one. The research of other scientists will either refine this theory or replace it with something which fits the facts even better. In any case, Walter and Luis Alvarez laid a useful foundation for the study of great extinctions.
