Dating fossils through strata and radiometric samples
It was only in the early part of the 20th century, when isotopic dating methods were first applied, that it became possible to discover the absolute ages of the rocks containing fossils.
In most cases, we cannot use isotopic techniques to directly date fossils or the sedimentary rocks they are found in, but we can constrain their ages by dating igneous rocks that cut across sedimentary rocks, or volcanic ash layers that lie within sedimentary layers.
When radiometric techniques are applied to metamorphic rocks, the results normally tell us the date of metamorphism, not the date when the parent rock formed.
C) can be applied to many geological materials, including sediments and sedimentary rocks, but the materials in question must be younger than 60 ka.
The correlational studies described so far allow scientists to estimate the relative ages of strata.
If stratum B lies above stratum A, B is the younger of the two.
However determining the actual, or absolute, age of strata (for example, 3.5 million years old) is often difficult since the age of a fossil cannot be determined directly.
The most useful tool in dating strata is radiometric dating of materials.
That rate is known as its half-life, the time it takes for one-half of a sample of the isotope to decay.Feldspar does not have any argon in it when it forms.Over time, the Ar remain embedded within the crystal, unless the rock is subjected to high temperatures after it forms.[SE] K-Ar is just one of many isotope-pairs that are useful for dating geological materials.
Some of the other important pairs are listed in Table 8.2, along with the age ranges that they apply to and some comments on their applications.The sample must be analyzed using a very sensitive mass-spectrometer, which can detect the differences between the masses of atoms, and can therefore distinguish between Ar in the case of the K-Ar method).