Numerical age dating
We have seen that isotopic dating can be used to date the time when igneous rocks formed and when metamorphic rocks metamorphosed, but not when sedimentary rocks were deposited.
So how do we determine the numerical age of a sedimentary rock?
Unlike people, you can’t really guess the age of a rock from looking at it.
This method works because some unstable (radioactive) isotopes of some elements decay at a known rate into daughter products. Half-life simply means the amount of time it takes for half of a remaining particular isotope to decay to a daughter product. Good discussion from the US Geological Survey: geochronolgists just measure the ratio of the remaining parent atom to the amount of daughter and voila, they know how long the molecule has been hanging out decaying. So to date those, geologists look for layers like volcanic ash that might be sandwiched between the sedimentary layers, and that tend to have radioactive elements.What’s more, if the whole rock is badly weathered, it will be hard to find an intact mineral grain containing radioactive isotopes.You might have noticed that many of the oldest age dates come from a mineral called zircon.For example, if we ﬁnd a sequence of sedimentary strata deposited unconformably on a datable granite, the strata must be younger than the granite (figure above).If a datable basalt dike cuts the strata, the strata must be older than the dike.