There is a widely held belief that 65 million years ago a massive object crashed to earth from outer space, an event that resulted in the extinction of the dinosaurs, and most of the other life forms then existing at that ancient time.
Substantial geological evidence has been uncovered to show that such an impact occurred. A crater site has been identified in the Yucatan Peninsula in eastern Mexico, known as the Chicxulub Crater, and geologists who are in agreement with the impact theory are satisfied the event that created Chicxulub is the culprit leading to the mass extinctions.
The impact theory was developed in the late1970’s by geologist Walter Alvarez and his father, Luis Alvarez, a Nobel prize-winning scientist. It came after geological field-work done by Walter Alvarez in Gubbio, Italy, where he was investigating sedimentary deposits at the boundary layer between rocks dated to two geological periods, one of which was the period in which it was known that dinosaurs became extinct. There he discovered a thin band of a mineral identified as iridium, an element normally rare at the surface of the earth but known to be much more common in meteorites, solar system debris that has fallen to earth from outer space. After investigation, a similar iridium layer was found at many other locations around the world where rock types of the same geological periods existed.
From this it was hypothesized that the iridium was of extraterrestrial origin and had arrived on earth when a six mile sized asteroid crashed into the earth from space with earth shattering force and with the release of stupendous energy, ejecting matter up into the atmosphere to be borne around the globe by air currents and subsequently deposited at many far away locations.
From this assumption, scenarios were developed to describe what could have happened following the moment of impact that would then lead to the eventual extinctions of the dinosaurs and also of approximately 70% of all other life forms on land and in the seas. Thus the impact theory gained recognition.
This conclusion was immediately contested by scientists who believed the extinctions were caused instead by massive volcanic eruptions in India, known as the Deccan Traps, that took place during the age of the extinctions.
Dewey McLean of Virginia Tech was an early opponent who argued, in the face of much unpleasant resistance, that the dinosaur extinctions occurred during a Deccan Traps volcanism-induced greenhouse climate change. In the words of Walter Alvarez, from his book “T.Rex and the Crater of Doom” published in 1997, referring to discussions with Maclean: “heated exchanges enlivened a few scientific meetings”. The debate became ugly. In referring to Maclean, Luis Alvarez wrote in the New York Times and elsewhere, critical and insulting comments. In 1988 Dewey Maclean responded in an open letter to Alvarez with: “Operating in a science you do not comprehend, you publicly insult paleontologists. In the New York Times you abased paleontologists as “not very good scientists…more like stamp collectors,” and attacked opponents by name as “weak sister,” “incompetent,” and “publishing scientific nonsense.”
But many other leading scientists continued to support the volcanism view. Research in 1986 by Vincent Courtillot, an important scientist from France had confirmed regions of the Deccan Traps to be of the right age.
And so it continued, back and forth until the discovery, actually a re-discovery and identification, by the Alvarez team, of the Chicxulub crater in Yucatan in 1991, a discovery that did much to turn the tide of popular opinion to the Impact camp. Other supporting discoveries had been made meanwhile, including deposits that geologists describe as: shocked quartz, tektites small round glassy objects, and sand deposits typical of those deposited by action of a tsunami, all of these are thought to be characteristic of what would result from a massive impact on the shores of the Yucatan, so the case was closed in the view of Walter Alvarez and others.
Nevertheless, other geologists held to their view, offering a counter hypothesis pointing to other evidence including climate change, massive volcanic eruptions or even a combination of those events that all took place 65 million years ago. Extensive field investigations were carried out and reported by Coutillot, Chenet, and Fluteau of France leading to convincing evidence to support the volcanism theory to the satisfaction of its supporters.
Mass extinctions have occurred several times before in the Earth’s history. Chenet has claimed that most mass extinctions of the past coincided with massive volcanic eruptions and only in the case of the death of the dinosaurs does it appear that both volcanism and an impact were involved, but in any case the extinction would have taken place even if the impact had not occurred, according to Chenet.
As long ago as 2003, Gerta Keller of Princeton University and her colleagues had reported on geological research carried out in India and Mexico, uncovering geologic evidence that the meteorite impact occurred 300,000 years before the mass extinctions. Most of the current scientific work is being done by the supporters of volcanism. Additional field-work by Keller and her colleagues supports the conclusion that the extinctions happened at the same time as the end of the main phase of India’s Deccan eruptions, which points to it being volcanism that killed the dinosaurs. At the December 2008 meeting of the American Geophysical Union she emphatically repeated her earlier claim: “The Chicxulub impact hit the Yucatan about 300,000 years before the mass extinction that included the dinosaurs and, therefore, could not have caused it.” Keller also said that the Chicxsulub crater has been vastly overestimated as evidence. More information on these findings will be released later in 2009.
Contacted after those assertions were made, Walter Alvarez continues to reject the idea of volcanism as the cause of the mass extinctions, saying that other scientists have also, after detailed examination, found it unacceptable and that few experts would agree that Chicxulub is older than the extinctions. As a reminder, he said that it was the discovery of the thin layers of iridium, rare at the earth’s crust but more abundant in meteorites, that triggered his impact theory. And subsequently, around the world, similar debris from a giant impact was identified in sediments deposited at the same time as the mass extinction happened. Alvarez said that volcanism may have played a contributing role but that the Chicxsulub event was the primary trigger.
So where does that leave us? It is well known that many times in the past the Earth has suffered major consequences from the impacts of asteroids and comets that have reached Earth from far out in the Solar System. Huge craters caused by such impacts, some many millions of years old, can still be identified in some places even though in most cases time and erosion has eradicated or disguised their presence. Or perhaps they are hidden beneath the waters of the ocean. Well known In the United States is the 50,000 year old Meteor Crater in Arizona and in Canada, the Sudbury Basin in Ontario is a 1.85 billion year old meteorite impact site.
About the object that impacted at Chicxulub, senior scientist David Brez Carlisle, in his book “Dinosaurs, Diamonds and Things from Outer Space” published in 1995 by Stanford University Press, states that the impactor was not an asteroid but a comet of about 20 miles in diameter. This interpretation being based, among other things, on the composition of the non-terrestrial rock materials at the crater site. The home of most asteroids is the Asteroid Belt, between the planets Mars and Jupiter. The comets originate in the Oort Cloud at the far reaches of the Solar System, some of them, after being dislodged from orbit there, spend time in the Kuiper Belt located just beyond planet Neptune and some orbit within the asteroid belt. Comets, at their core, contain the oldest material in the solar System.
I have not heard it expressed elsewhere, but could Walter Alvarez have located the wrong crater, a crater 300,000 years too old? And is it possible that the real culprit, if it was a meteorite, is still waiting to be found? The combination of the evidence of the Deccan Traps volcanism together with a new discovery of a younger dated catastrophic impact would seem to be the best of both worlds and might even satisfy both camps.