Such meteorites are believed to reach the upper atmosphere at a constant rate, so that the metallic rain falls steadily to earth, where it joins with terrestrial material-dust eroded from the continents and the skeletons of microscopic marine animals. The iridium found in sedimentary rocks (and often it is too scarce to be detected) appears to have settled from space in a steady rain of microscopic fragments-a kind of cosmic dust-worn from tiny meteorites that form the shooting stars that flame out high above the earth. Their abundance in meteorites and in average material of the solar system is many times higher than in the earth's crust. When the earth formed, iridium, like other elements of the platinum group (which includes osmium, palladium, rhodium, and ruthenium), accompanied iron into the molten core, leaving these elements so rare in the earth's crust that we call some of them precious. To do so, he looked not down to the earth but up to the heavens, postulating that the amount of a rare metallic element called iridium might provide the clock. Therefore, as he had done so many times in his career, Luis Alvarez invented a new technique. None of the standard geologic clocks-the ones based on radioactive parent-daughter pairs of atoms that are used to calculate exact ages-had enough sensitivity or would work on the chemical elements in the clay layer. Because no one knew how much time the clay layer might represent, the clock might have to measure small differences. What was needed, he reasoned, was a geologic clock that had been operating at the time the clay layer formed but that could be read today. The attempt to determine the time interval using the magnetic chronology thus failed, but in another way the effort succeeded, for the mind of Luis Alvarez was now locked in. ![]() This appeared to be an improvement over Walter's rough estimate, but since the clay is quite different from the limestone, there really was no basis for assuming the same sedimentation rate for both. ![]() Since the boundary clay is about 1 cm thick, at that rate it would have taken a little more than 1,000 years to form. Six meters in 750,000 years is equivalent to 0.8 cm of sediment deposited every 1,000 years. Alas, during this period of geologic history the reversals had not happened often enough: All that could be told is that the clay layer fell within a 6-m section of limestone deposited during a single period of magnetism, called 29 R (for reversed), that was known to have lasted for about 750,000 years. The magnetic reversal time scale offered one possibility for determining how much time the clay layer represented: The particular pattern of reversals above and below the clay might bracket its age of formation and allow an upper limit to be placed on how long it could have taken to deposit the layer.
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