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.