Igneous rocks and stratigraphyEdit
When they are first formed, extrusive igneous rocks (lava flows and volcanic ash) will be younger than the sedimentary rocks below them, and will be older than the sedimentary rocks which subsequently form above them.
Note that as usual the original position of the rocks can be altered by tectonic events, and if this has happened the original order must be recovered by studying way-up structures and observation of the faults and folds that have been formed. Using our < notation, we may write that if rock A < rock B < rock C, where rocks A and C are sedimentary and rock B is extrusive, then rock A was deposited before rock B, and rock C was deposited after it.
Now consider intrusive igneous rocks. If we have an intrusive sheet-like structure (a sill) then this is necessarily younger than the sedimentary rocks below it. But it is also younger than the sedimentary rocks that were above it when it formed. (Note that something must have been above it when it formed, otherwise it would not be intrusive.)
But it is only older than the sedimentary rocks above it when it formed. Suppose that igneous rock B intrudes between sedimentary rocks A and C. Then further sediment is deposited on top of C, forming sedimentary rock D. Then B is younger than C but older than D.
When considering intrusive igneous structures such as dikes and plutons, we may appeal to the principle of cross-cutting relationships to say that they are younger than the sedimentary rocks through which they penetrate.
The reader should bear in mind that we can tell the difference between intrusive and extrusive igneous rocks just by looking at them: intrusive igneous rocks are coarse-grained, and extrusive igneous rocks are fine-grained. So even though a sill and a lava flow are the same shape and may have just the same chemical composition, we can still tell the difference between them by their texture.
Why is this important?Edit
So far, we have only been able to consider the relative ages of features in the geological record; we can say that this stratum is was deposited before that, and that the deposition of this fossil preceded the deposition of the other. But this only provides us with relative ages: we can say that A is older than B, but how much older? Older by a minute? A day? A million years? We have relative ages, but so far we have no actual dates.
A geologist, if handed (for example) a fossil trilobite, cannot perform a physical or chemical analysis of the fossil and tell you how old it is. But if that same geologist finds the trilobite in the field lying in sedimentary rocks between two lava flows, then s/he can find the actual dates of the lava flows, and can then tell you that the trilobite is younger than the oldest lava flow and older than the youngest lava flow, and by assigning dates to the lava flows can tell you that the date of the fossil lies between them.
This brings us on to the topic of absolute dating, how it is done, and why it works.