Geology
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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.
Geologic Time: Graphical Representation of Geologic Time
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 to as the world or the Blue Planet. [ 22 ] Earth formed approximately 4.54 billion years ago, and life appeared on its surface within one billion years. [ 23 ] Earth's biosphere then significantly altered the atmospheric and other basic physical conditions, which enabled the proliferation of organisms as well as the formation of the ozone layer , which together with Earth's magnetic field blocked harmful solar radiation , and permitted formerly ocean-confined life to move safely to land. [ 24 ] The physical properties of the Earth , as well as its geological history and orbit, have allowed life to persist.
Earth