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. 

Unfortunately, the 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.