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    By Steven Jaret | September 14th 2009 08:37 AM | Print | E-mail
        For centuries astronomers and star-gazers alike have looked into the night sky and seen the Moon with its surface dotted by impact craters.  Yet, only within the last half-century have geologists and Earth scientists accepted meteorite impacts as a process that affects the Earth. Prior to geologist Eugene Shoemaker’s 1963 landmark study of Meteor Crater in Arizona, meteorite impacts were thought to be solely an extraterrestrial occurrence. Today we know that is not the case. In just over 40 years the idea of a meteorite impact on Earth has gone from virtually nonexistent to the heart of popular culture with Blockbuster hits such as Deep Impact or Armageddon.
        After the initial discover of Meteor Crater the next major turning point of impact crater research came in 1991, when Alan Hilderbrand, of the University of Arizona, published an article confirming the existence of a 180 kilometers wide impact crater off the coast of the Yucatan Peninsula. In that article, Hilderbrand presents evidence that the impact, named the “Chixulub impact” (or “Chixulub” for short), occurred 65 million years ago, at the boundary between the Cretaceous and Tertiary periods. This time boundary (known to geologists as the “K/T boundary”1) marks the second largest extinction event in Earth’s history, and is most notably known for the loss of large dinosaurs. Thus, the discovery of Chixulub ignited a flurry of research and inevitable scientific debate as to the exact role the Chixulub event had in the global mass extinction at the K/T boundary.   This discovery bolstered the emerging field of impact cratering research – especially studies of dynamic and mechanical processes associated with impacts – in order understand what happened during the Chixulub event.
        An impact event is a violent process which causes sudden and instantaneous geologic change.  Within hours or even minutes of impact, the landscape is transformed by an intense explosion; melting and vaporization of both the meteorite and the target ground rock; tremendous faulting and seismic activity; and the formation of a topographic basin out of which ash and debris is ejected from the crater. The ejected material, appropriately named “ejecta,” is thrown outward and upward into the atmosphere and is then redeposited as a sedimentary rock layer which geologists can later identify in the field. For impacts like Chixulub which occur in water, there will also be a large tsunami and intense wave action.  Simulations and modeling for an impact similar to Chixulub suggest that the energy released during the impact event was close to 1.5 x 1024 joules (roughly the equivalent of 360 million megatons of TNT!).
        At Chixulub, geologic evidence of large-scale effects of the impact are strong.  Thick tsunami and ejecta deposits are found throughout the Caribbean and Gulf of Mexico as far away as Haiti and the southern coastal plain of Alabama. Smaller deposits are also found throughout the world. Within this ejecta, geologists have found microscopic features in individual mineral grains called Planar Deformation Features (PDF’s) which are only found in meteorite impact craters and confirm that Chixulub is an impact crater.
        The direct link between the K/T mass extinction and the Chixulub impact, however, is not as clear-cut. The impact event was not the only active process during the end of the Cretaceous.  An increase in volcanic activity, increased global CO2 levels, and a global marine regression also occurred prior to and at the K/T boundary.  The Chixulub impact was probably only one of multiple factors that eventually led to the demise of over 50% of species on Earth, including the large dinosaurs.
        Two problems exist for geologists trying to understand the how impacts effect Earth’s surface. First, the K/T extinction is not a single unique event. Since life arose on our planet some 3.5 billion years ago, there have been 5 major global extinctions and numerous localized extinctions. Second, according to the Earth Impact Database run by the University of New Brunswick, there are currently 179 confirmed impact craters across the globe. Using Chixulub as a global model is difficult because the impact-extinction link is not statistically significant.
        In recent years geologists have been investigating the other known extinctions with new insights about impact cratering, in hopes of clarifying the connection between impacts and extinctions.   In a 2004 Science article, Dr. Luann Becker, a geologist at the University of California, Santa Barbra, proposed an impact crater in Australia and tied it to largest extinction, the Permian-Triassic extinction. This article, however, was met with much skepticism and is not widely accepted within the impact cratering community. A critical response to this article, also published in Science, by Andrew Glikson of the Research School of Earth Science in Canberra, Australia, questioned the impact interpretation. Glikson claimed that Becker did not provide enough quantitative measurements of PDF’s nor did she convincingly show that the rocks in her study are not of volcanic origin (this is important because volcanic events and impacts are capable of deforming rocks at high pressures, and can both generate melted rocks and glass).         Likewise, a 2009 Science article suggest the Younger Dryas Boundary (about 12900 years ago) and the extinction of the Clovis people in North America are tied to an impact event. This study, however, is regarded as speculative because it lacks the concrete evidence seen at Chixulub. The main supporting evidence in this article is the presence of nanodiamonds, which can form during the high pressure associated with impacts. However, of the major types of evidence for meteorite impact craters, nanodiamonds alone are the least diagnostic feature. Furthermore, since no physical crater at this site has been discovered  many scientists hesitant to accept this hypothesis.  
        Today researchers are try to understand what, if anything is special about the Chixulub event and the K/T boundary. Were there impacts at the time of other extinctions?  What is the effect of the size or composition of rocks at Chixulub? Did the impact just hasten an extinction that was already in progress? All these questions remain to be answered.  While we currently do not know the exact role impact craters have on Earth’s environment, the evidence at Chixulub and possibility of other impact-related extinctions remains a puzzling mystery. What is clear, however, is that as we work to understand current changes in Earth’s environment it is necessary to examine the past – a past which is dotted by impact craters. In order to fully understand the Earth we must understand all the processes acting on the planet – including meteorite impacts.

    1  “K/T” as the shorthand for Cretaceous/Tertiary may seem odd as Cretaceous starts with a C not a K but geologists do indeed use this abbreviation.  Each geologic period has its own symbol and the older, longer, Cambrian Period is given the symbol C so we abbreviate Cretaceous with a K.