The geologic time scale is a system of chronological measurement that relates stratigraphy to time, and is used by geologists, paleontologists, and other earth scientists to describe the timing and relationships between events that have occurred throughout Earth's history. The table of geologic time spans presented here agrees with the dates and nomenclature set forth 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 4.54 billion years old. The geology or deep time of Earth's past has been organized into various units according to events which took place in each period. Different spans of time on the GTS are usually delimited by changes in the composition of strata which correspond to them, indicating 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 Cretaceous–Paleogene extinction event, which marked the demise of the dinosaurs and many other groups of life. Older time spans which predate the reliable fossil record (before the Proterozoic Eon) are defined by absolute age. (source: Wikipedia). To understand the enormous span of time we often use an analogy. A common analogy is to compare the 4.6 billion years with 1 hour or one 24 hour period (see above video) Another common example is to create a timeline using a fixed distance like a football field or stretch of straight road. Today we will use wildcat walk to illustrate geologic time. in our analogy 1 million years equals 3.5 cm (yes an arbitrary number but it fits the distance from the classroom building to the main building). We will build our geologic time using a collaborative Presentation on Google Docs. Each person will select and research one period of time. You will need to insert a picture that represents the time period and a brief description of event that make the time period significant. |
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