What is Geologic time scale?
[[File:Geological time spiral.png|thumb|250px|Diagram of geological time scale as a spiral.]] '''Geologic time scale''' uses the principles and techniques of [[geology]] to work out the geological history of the [[Earth]].<ref>Levin, Harold 2003. ''The Earth through time'', Hoboken, New Jersey: Wiley, p.2</ref> It looks at the processes which change the Earth's surface and [[Rock (geology)|rocks]] under the surface. Geologists use [[stratigraphy]] and [[paleontology]] to find out the sequence of the events, and show the [[plant]]s and [[animal]]s which lived at different times in the past. They worked out the sequence of rock layers. Then the discovery of [[radioactivity]] and the invention of [[radiometric dating]] techniques gave a way to get the ages of the layers ([[strata]]). We now know the timing of important events that have happened during the [[history of Earth]]. The [[age of the Earth|Earth]] is about 4.567 [[billion]] (4,567 million) years old. The geological or ''[[deep time]]'' of Earth's past has been organized into various units. Boundaries on the time scale are usually marked by major [[geology|geological]] or [[paleontology|palaeontological]] events, such as [[List of extinction events|mass extinctions]]. For example, the boundary between the [[Cretaceous]] period and the [[Palaeogene]] period is defined by the [[Cretaceous–Tertiary extinction event]]. This marked the end of the [[dinosaur]]s and of many marine [[species]]. [[Prospecting]] for [[energy]] sources and valuable [[mineral]]s depends on understanding the geological history of an area. Such knowledge can also help lessen the hazards of [[earthquake]]s and [[volcanoes]]. == Terminology == The largest defined unit of time is the supereon composed of [[Eon]]s. Eons are divided into [[Era]]s, which are in turn divided into [[Period (geology)|Periods]], [[Epoch (geology)|Epochs]] and [[Stage (geology)|Stages]]. At the same time paleontologists define a system of [[fauna]]l stages, of varying lengths, based on the kinds of animal f