Question

List and describe two ways that scientists can date fossils found in the earth.

Answer 1

1. Radiometric dating: This is based on dating the rocks found in the same layers of soil. Because we know how many kinds of rocks form volcanically, and we also know that they contain ratios of elements that change at a known rate (because some elements undergo radioactive decay at a known rate), we can compute the approximate age of the rocks, and thus the fossils.

2. Index fossils: There are certain species of organisms that we find unique to certain layers of soil of a certain age (as determined by #1). For example, trilobites are found only in Cambrian layers (about 540 to 490 million years ago), and ammonites are found only in late Silurian and early Devonian layers (about 400 million years ago). Once we have documented this pattern, those fossils are "index fossils" that allow us to date other fossils in the same layer.

-----

As you research this, I should warn you that there is a lot of *misinformation* about fossil dating that you will find on the Internet. This is because Creationists (people who deny evolution and/or an old age of the earth), have the misconception that the only evidence of evolution is *fossils* (which is untrue), and thus by attacking the science behind the dating of fossils, they can disprove evolution.

With this in mind, let me clear up three misconceptions that Creationists repeatedly bring up ... demonstrating that they have no real understanding of fossil dating at all ... and have no desire whatsoever to actually understand the concepts they reject:

First, radiometric dating is different from "carbon dating". Carbon dating is a *type* of radiometric dating ... but it is not used on fossils, but rather on much younger objects, like bones, hair, leather, wood and paper, or cloth ... all *once-living* tissue (fossils are rock). Secondly, carbon dating has a known range of about 60,000 years ... which is not even a fraction of the range needed to date fossils that are in the millions, much less rocks that can go into the billions of years old. So for both these reasons, if you ever see a Creationist web site or poster attacking carbon dating's accuracy for dating fossils or the age of the earth, you can be quite sure they have no earthly clue what they're talking about.

Second, radiometric dating is actually *many* different techniques based on many different elements with different rates of decay. This allows us to confirm the results. Creationists often say that radiometric dating is based on arbitrary "assumptions" about how rocks form or how fast an element decays ... but they miss the point that these assumptions are not "arbitrary" but confirmable, and that each of these dozens of techniques use *different* assumptions that all produce the *same* results (the same dates). Creationists even challenge the fact that particle decay is *constant* (in other words, in order to attack the concept of evolution as accepted by biologists, they have to enlarge their attack on the *physicists* and the fundamentals of radioactive decay) ... and completely miss the point that all these elements would have to have have *hugely* variable rates of decay, with this variability different for every element, in order to converge on the same results! In other words, the Creationists want to alter the laws of physics by many orders of magnitude, and in completely arbitrary amounts.

Third, Creationists will often call fossil dating "circular reasoning" ... and they do this by pointing only to index fossils (#2) and say "those stupid geologists date the rocks by the fossils, and then date the fossils by the rocks." They are just being dishonest. The age of index fossils are established by radiometry (#1) BEFORE they are useful as index fossils (#2). Only the most dishonest (or stupid) interpretation of these techniques would call this "circular reasoning."


Sorry if this was more than you asked for ... but this environment (the Internet) is full of noise. It is not possible to give a short answer to questions like this, without also anticipating all the BAD information that anti-science people throw out there *deliberately* to confuse people.