Get flash to fully experience Pearltrees 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.
Earth is the third planet from the Sun , and the densest and
fifth-largest of the eight planets in the Solar System . It is
also the largest of the Solar System's four terrestrial planets
. It is sometimes referred