How geologists know how many years passed since a rock formation?
In: 6
A combination of tools are used together. In the simplest form, when you know which way up a sequence of sedimentary rocks is (they can get tilted and folded over through time in mountain building events), then we can say that the ones lying above are younger than the ones below, because sediment gradually accumulates as it’s deposited by rivers, seas, winds, etc.
To get absolute dates we can use radiometric dating techniques. When a new crystal forms it has a very specific atomic arrangement. It can only accept certain elements, and some of those elements can be radioactive. Radioactivity is the gradual (but steady) decay of one version of one element (an “isotope”) to a specific version of a different element. The first is known as the parent isotope, and it creates the daughter isotope. The daughter isotope often has no business being in the mineral crystal in which its now trapped. So we can measure the amount of the parent, the daughter (and usually another stable isotope that doesn’t decay) to work out how long ago that crystal formed. This is quite simplified, but it captures the process. There’s lots of different isotope systems that use different elements that are appropriate for different timescales, and different mineral chemistries.
Between these two lies fossil evidence. Species evolve through time, and we can use stratigraphic relationships (which lie above or below each other) together with areas where we have good radiometric ages to get really precise dates all over the world and throughout lots of geological time.
There’s several other techniques that are also used but those 3 capture the bulk of what is done.
Latest Answers