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Unformatted text preview: Professor Ohan Chapter 9 1- The geologic time scale provides a system of chronologic measurement relating stratigraphy to time that is used by geologists , paleontologists and other earth scientists to describe the timing and relationships between events that have occurred during the history of the Earth . 2- The table of geologic time spans presented here agrees with the dates and nomenclature proposed by the International Commission on Stratigraphy , and uses the standard color codes of the United States Geological Survey . Evidence from radiometric dating indicates that the Earth is about 4.570 billion years old. The geological or deep time of Earth's past has been organized into various units according to events which took place in each period. 3- Different spans of time on the time scale are usually delimited by major geological or paleontological events, such as mass extinctions . For example, the boundary between the Cretaceous period and the Paleogene period is defined by the CretaceousTertiary extinction event , which marked the demise of the dinosaurs and of many marine species. Older periods which predate the reliable fossil record are defined by absolute age. Each era on the scale is separated from the next by a major event or change. 4- The largest defined unit of time is the supereon, composed of eons. Eons are divided into eras, which are in turn divided into periods, epochs and ages. The terms eonothem , erathem , system, series, and stage are used to refer to the layers of rock that correspond to...
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