| USGS artistic
depiction of geological time scale, with caption: The Earth is very old
-- 4.5 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 3 billion years ago.
Also contained in rocks once molten are radioactive elements whose isotopes provide Earth 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 fossils (paleontology), coupled with the ages of certain rocks as measured by atomic clocks (geochronology), attest to a very old Earth! |