Rocks and relative age
Very early in the science of geology it was recognized that in many places one can tell the comparative ages of rocks by their relations to one another. For example, most sedimentary rocks are consolidated accumulations of large or small rock fragments and were deposited as nearly horizontal layers of gravel, sand, or mud. In an undisturbed sequence of sedimentary rocks, the layer on the bottom was deposited first and the layer on top was deposited last. All of these must, of course, be younger than any previously formed rock fragments incorporated in them.
Igneous rocks are those formed by solidification of molten material, either as lava flows on the earth’s surface (extrusive igneous rocks) or at depth within the earth (intrusive igneous rocks). The relative ages of extrusive igneous rocks can often be determined in much the same way as those of sedimentary strata. A lava flow is younger than the rocks on which it rests, but older than those that rest on top of it.
An intrusive igneous rock must be younger than the rocks that enclosed it at the time it solidified. It may contain pieces of the enclosing rocks that broke off the walls and fell into the liquid. Pebbles of the igneous rock that are incorporated in nearby sedimentary layers indicate that the sediments must be somewhat younger.
All of these criteria tell us only that one rock is older or younger than another. They tell us little about the absolute age of the rocks or about how much older one is than the other.