Glossary radiometric dating
The impact also created shocked quartz crystals that were blasted into the air and subsequently fell to the west into the inland sea that occupied much of central North America at that time.Today this shocked quartz is found in South Dakota, Colorado, and Nebraska in a thin layer (the Crow Creek Member) within a thick rock formation known as the Pierre Shale.Impact melts and glasses (or minerals that have recrystallized from the melt; e.g., Krogh et al., 1993; Izett et al., 1994) have another important use, as they often are the most suitable material for the dating of an impact structure.The methods most commonly used for dating of impact melt rocks or glasses include the K-Ar, Ar, fission track, Rb-Sr, Sm-Nd, or U- Th-Pb isotope methods.The largest difference between these mineral pairs, in the ash from the Gregory Member, is less than 1%.Third, the radiometric ages agree, within analytical error, with the relative positions of the dated ash beds as determined by the geologic mapping and the fossil assemblages; that is, the ages get older from top to bottom as they should.
Relative dating to determine the age of rocks and fossils: Geologists have established a set of principles that can be applied to sedimentary and volcanic rocks that are exposed at the Earth’s surface to determine the relative ages of geological events preserved in the rock record.
First, each age is based on numerous measurements; laboratory errors, had there been any, would be readily apparent.
Second, ages were measured on two very different minerals, sanidine and biotite, from several of the ash beds.
A process for determining the age of an object by measuring the amount of a given radioactive material it contains.
If one knows how much of this radioactive material was present initially in the object (by determining how much of the material has decayed), and one knows the half-life of the material, one can deduce the age of the object.