The Dating Process
Usually when radioactivity is
mentioned in relation to deposit dating, either uranium-lead or carbon14
comes to mind. However, many elements are radioactive, among
these an isotope of potassium called potassium40. With
time, potassium40 breaks down to an isotope or argon, argon
40. Since the rate of the change from one isotope to
another is known, measuring the ratio of these elements in a rock tells
us the number of years that have passed since the rock was formed.
potassium-argon method of dating has limitations. When you learned
that the date of the Irvington paleoecosystem was based on deposits in
Idaho and not on the Pleistocene deposits in California, did you wonder
why? It is because the K40 / Ar40 dating method can be used only
on fresh or unaltered igneous rocks and on certain sedimentary minerals
such as glauconite. The Irvingtonian sediments contain no lava
flows and no glauconite. They, therefore, cannot be directly dated
by this method.
A mammoth from the lava flows of
Bennett Mountain, Idaho, is strikingly similar to the mammoth found at
Irvington. Since both animals represent the same stage of
evolution, they are considered time-equivalent. The K40 / Ar40 age
of the Idaho lava flows (1.36 million years) is, therefore, considered
the age of the Irvingtonian deposits.
More and more scientists are
accepting potassium-argon dates as the most accurate yet available.
This means that wide differences in dates found in scientific writings
will be reduced. And the differences have been wide indeed.
For example, a scientist selected ten books (mostly texts) printed
between 1941 and 1965 and tabulated the dates each gave for the
beginning of each epoch in the Cenozoic era. Here are the results.
||Began 75 to 63 million years ago
||Began 60 to 45 million years ago
||Began 41 to 35 million years ago
||Began 28 to 18 million years ago
||Began 16 to 10 million years ago
||Began 1 million to 600 thousand years ago
||Began 50 to 11 thousand years ago