The Earth is very old—four and a half billion years or more according to
recent estimates. Most of the evidence for an ancient Earth is contained
in the rocks that form the Earth’s crust. The rock layers
themselves—like pages in a long and complicated History—record the
surface-shaping events of the past, and buried within them are traces of
life—the plants and animals that evolved from organic structures that
existed perhaps three billion years ago.
Also contained in rocks once molten are radioactive elements whose
isotopes provide earth scientists with an atomic clock. Within these
rocks, “parent” isotopes decay at a predictable rate to form “daughter”
isotopes. By determining the relative amounts of parent and daughter
isotopes, the age of these rocks can be calculated.
Thus the results of studies of rock layers (stratigraphy), and of the
progressive development of life (paleontology), coupled with the ages of
certain rocks as measured by atomic clocks (geochronology), attest to a
very old Earth!