Halo dating 101
It is based on the radioactivity of Ar, however, is an inert gas that escapes easily from rocks when they are heated but is trapped within the crystal structures of many minerals after a rock cools. This correction can be made very accurately and has no appreciable effect on the calculated age unless the atmospheric argon is a very large proportion of the total argon in the analysis.The geochronologist takes this factor into account when assigning experimental errors to the calculated ages. First, there must be no argon other than that of atmospheric composition trapped in the rock or mineral when it forms.
Unbeknownst to the scientists engaged in this controversy, however, geology was about to be profoundly affected by the same discoveries that revolutionized physics at the turn of the 20th century.Bishop James Ussher, a 17th-century Irish cleric, for example, calculated that creation occurred in 4004 B. There were many other such estimates, but they invariably resulted in an Earth only a few thousand years old.By the late 18th century, some naturalists had begun to look closely at the ancient rocks of the Earth.he question of the ages of the Earth and its rock formations and features has fascinated philosophers, theologians, and scientists for centuries, primarily because the answers put our lives in temporal perspective.Until the 18th century, this question was principally in the hands of theologians, who based their calculations on biblical chronology.For example, a method based on a parent isotope with a very long half-life, such as C method can only be used to determine the ages of certain types of young organic material and is useless on old granites.
Some methods work only on closed systems, whereas others work on open systems.
The point is that not all methods are applicable to all rocks of all ages.
One of the primary functions of the dating specialist (sometimes called a geochronologist) is to select the applicable method for the particular problem to be solved, and to design the experiment in such a way that there will be checks on the reliability of the results.
The main point is that the ages of rock formations are rarely based on a single, isolated age measurement.
On the contrary, radiometric ages are verified whenever possible and practical, and are evaluated by considering other relevant data.
Sometimes these decay schemes are used individually to determine an age (e.g., Rb-Sr) and sometimes in combinations (e.g., U-Th-Pb).