Iridium

Iridium, Quartz, and Glass; The Dinosaurs Last Clues
Christian H. d’Orgeix
Walter Alvarez describes the journey to the impact hypothesis in his book T. rex and the crater of doom. The impact hypothesis plays a key role in our understanding of catastrophes effect on life’s history and revolutionized geology’s view of gradualism. The detection of high levels of iridium in Gubbio samples, the discovery of shocked quartz in bedrock in continental sites of western North America, and the finding of unaltered impact glass in spherules at the KT boundary were respectively fundamental to the development, testing, and acceptance of the impact hypothesis.
The discovery of a spike in iridium at the clay boundary between the Cretaceous and Tertiary periods in the geological record played a major role in the development of the impact hypothesis. It was previously assumed that the clay layer at the KT boundary represented millions of years of deposition (Alvarez, 1997). Dr. Alvarez wanted to determine the exact length of time which passed during which the clay was deposited (Alvarez, 1997). He assumed that as meteors and comets broke apart in the atmosphere they would spread extremely small amounts of iridium across the surface of the planet and these amounts could be measured to determine the amount of time which had passed as the clay was deposited (Alvarez, 1997). Dr. Alvarez predicted that either the clay was deposited slowly which would result in roughly 0.1 ppb of iridium in the clay bed or the clay was deposited quickly resulting in nearly zero measurable iridium (Alvarez, 1997). Dr. Alvarez gathered samples of clay marking the boundary from Gubbio (a series of rock outcrops outside a town of Italy named Gubbio) and gave them to a chemist named Frank Asaro who performed neutron activation analysis to determine the amount of iridium in the samples (Alvarez, 1997). The data showed that the amount of iridium in the clay layer marking the KT boundary was 9 ppb which was a far higher value than Dr. Alvarez expected even if the clay had been deposited slowly (Alvarez, 1997).
The discovery of high levels of iridium in the Gubbio samples was a key piece of evidence for the development of the impact hypothesis. The data was important because iridium is a very rare element on earth which meant the discovery narrowed the possible causes of the KT extinction. It also proved that the KT extinction was not caused by a gradual phenomenon but instead must have been caused by some sort of catastrophic event which released large amounts of iridium (Alvarez, 1997). This last point was very important in the discovery of the impact hypothesis because previously geologists followed a strictly gradualist approach and disregarded evidence of catastrophic events as rare anomalies which played a very small role in the history of the earth (Alvarez, 1997). This specific piece of data would lead Dr. Alvarez to further testing of the cause of the KT extinction (Alvarez, 1997).
The discovery of shocked quartz at KT sites in western North America was important to the testing of the impact hypothesis. Shocked quartz is created when shock waves pass through grains of quartz creating sets of planar deformation bands (Alvarez, 1997). The shocked quartz at the KT boundary was believed to be created by a continental-crust impact of a meteor or comet (Alvarez, 1997). The shocked quartz was initially discovered by Bruce Bohor in New Mexico (Alvarez, 1997). After his discovery Bohor concluded that the damaged quartz grains were evidence for the impact hypothesis since an impact of a meteor or comet is the only thing capable of producing the shock waves which would leave the distinct look of shocked quartz (Alvarez, 1997).
The finding of shocked quartz at KT sites in western North America was very important to the testing of the impact hypothesis because it opened up geologists to the previously discarded possibility of a continental impact (Alvarez, 1997). Before the discovery of shocked quartz geologists had proved that the cause of the KT extinction had been a catastrophic phenomenon and that it had released large amounts of iridium across the globe, however, there was no evidence of a crater and many geologists believed that the impact had occurred in the ocean and the crater had been subducted (Alvarez, 1997). Scientists had been previously mislead because evidence was found at KT boundaries that elements found within oceanic crust had been released during the impact (Alvarez, 1997). The discovery of shocked quartz reinvigorated the search for an impact crater because a continental impact crater would have been buried instead of subducted resulting in the possibility for it to be found.
After the crater had been found the discovery of unaltered impact glass in KT spherules was very important in the acceptance of the impact hypothesis. KT spherules were small spherical pieces of glass created by the heat of the impact and subsequently distributed across the planet by the explosion caused by the impact (Alvarez, 1997). These spherules were found at KT boundaries across the globe (Alvarez, 1997). Before 1990 all of the KT spherules that had been found had, had their original chemical composition altered over millions of years (Alvarez, 1997). However, in 1990 several groups of researchers discovered unaltered glass at a site in Haiti (Alvarez, 1997). The groups then performed chemical analyses that showed that the spherules chemical composition was consistent with the chemical composition of the Yucatan peninsula at the site of the Chicxulub crater (Alvarez, 1997). The spherules were made mostly of a black glass created by rocks from the continental-crust and small amounts of yellow glass created by calcium-rich rocks that had been deposited on top of the typical continental-crust rocks.
The discovery of unaltered impact glass was important to the acceptance of the impact hypothesis because it confirmed that the Chicxulub crater was the impact crater of the meteor or comet which caused the KT extinction and it provided more evidence against a volcanic hypothesis (Alvarez, 1997). Previously scientists had been confused by the simultaneous presence of shocked quartz and elements typically found in oceanic crust at the KT boundary (Alvarez, 1997). The chemical analyses of the unaltered spherules provided evidence that the crust of the Yucatan peninsula contained deposits of calcium-rich rocks which would explain why scientists were initially thrown off by the presence of elements typical to oceanic crust (Alvarez, 1997). This discovery finally put many of the counter arguments to the impact hypothesis to rest by explaining the discrepancy between the expected elements of a continental versus oceanic impact and by providing evidence that the Chicxulub crater was the impact crater of the impact which caused the KT extinction (Alvarez, 1997). The discovery of the unaltered glass also played an important role in acceptance of the impact hypothesis by providing counter evidence to the competing volcanic hypothesis. Because the glass was streaky geologists determined that it must have been created by an impact event instead of a volcanic one because melts from impacts freeze much more quickly than those from volcanic melts thereby resulting in streaky glass (Alvarez, 1997). Although the impact hypothesis still faced fierce criticism by a small portion of geologists after this discovery there was enough evidence that many accepted and supported the impact hypothesis (Alvarez, 1997).
Walter Alvarez played an important role in the creation of the impact hypothesis however the evidence supporting the hypothesis was only gained by the combined work of many scientists across various fields creating something far larger than anything one individual could do in his or her lifetime. The journey to the acceptance of the impact hypothesis was not only important in understanding a piece of geologic history but also had revolutionary effects on the geologic view of gradualism and our understanding of catastrophes in earth’s history. The three pieces of evidence discussed within this essay were pivotal to the development, testing, and acceptance of the impact hypothesis, one of the most important geological discoveries in the last century.