Radioisotope dating

The Pierre Shale also contains volcanic ash that was erupted from volcanoes and then fell into the sea, where it was preserved as thin beds.

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We often test them under controlled conditions to learn when and why they fail so we will not use them incorrectly. For example, after extensive testing over many years, it was concluded that uranium-helium dating is highly unreliable because the small helium atom diffuses easily out of minerals over geologic time.As a result, this method is not used except in rare and highly specialized applications.The heat of the impact melted some of the feldspar crystals in the granitic rocks of the impact zone, thereby resetting their internal radiometric clocks.These melted crystals, and therefore the impact, have been dated by the 40Ar/39Ar method at 74.1 Ma (million years; Izett and others 1998), but that is not the whole story by a long shot.I could have selected many more examples but then this would have turned into a book rather than the intended short paper.

The Manson Meteorite Impact and the Pierre Shale In the Cretaceous Period, a large meteorite struck the earth at a location near the present town of Manson, Iowa.Scientists who use radiometric dating typically use every means at their disposal to check, recheck, and verify their results, and the more important the results the more they are apt to be checked and rechecked by others.As a result, it is nearly impossible to be completely fooled by a good set of radiometric age data collected as part of a well-designed experiment.The Pierre Shale, which is divided into identifiable sedimentary beds called members, also contains abundant fossils of numerous species of ammonites, ancestors of the chambered nautilus.The fossils, when combined with geologic mapping, allow the various exposed sections of the Pierre Shale to be pieced together in their proper relative positions to form a complete composite section (Figure 1).Such failures may be due to laboratory errors (mistakes happen), unrecognized geologic factors (nature sometimes fools us), or misapplication of the techniques (no one is perfect).