Abosolute dating

29 Oct

In the articles on stratigraphy we looked at what is called relative dating, where we could say that one geological feature was older or younger than another but without actually putting dates on them.By contrast, absolute dating allows us to assign dates to geological features.To avoid confusion later on, let us say at once that the "absolute" in "absolute dating" is not short for "absolutely correct".In this context, the word "absolute" is not the opposite of "approximate" but just of the word "relative".All rocks and minerals contain tiny amounts of these radioactive elements.Radioactive elements are unstable; they breakdown spontaneously into more stable atoms over time, a process known as radioactive decay.If they don't, then it's not just a question of geologists being wrong about geology, but of physicists being wrong about physics and chemists being wrong about chemistry; if the geologists are wrong, entire laws of nature will have to be rewritten.

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If we wish to use a geological process as the geological equivalent of an hourglass, we would want to have similar conditions: we would like to find some quantity which we can measure reliably (corresponding to the condition that we can measure the amount of sand in the lower bulb of the hourglass); which increases or decreases from a known quantity (corresponding to the lower bulb of the hourglass being empty when it starts running) at a known rate (corresponding to knowing the rate of flow of sand); and so forth.

The time that it takes for half of a sample to decay is known as the half life of the isotope.

Some isotopes have half lives longer than the present age of the universe, but they are still subject to the same laws of quantum physics and will eventually decay, even if doing so at a time when all remaining atoms in the universe are separated by astronomical distances.

It's this resetting process that gives us the ability to date rocks that formed at different times in earth history.

A commonly used radiometric dating technique relies on the breakdown of potassium (Ar in an igneous rock can tell us the amount of time that has passed since the rock crystallized.