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Geologic Timescale

The geologic timescale is the framework for deciphering the history of planet Earth. It is used by geologists and other scientists to describe the timing and relationships between events that have occurred during the history of the earth.

Nomenclature

The history of the earth is broken up into a hierarchical set of divisions for describing geologic time. In increasingly smaller units of time, the generally accepted divisions are eon, era, period, epoch, and age. The Phanerozoic eon represents the time during which the majority of macroscopic organisms, algal, fungal, plant, and animal, lived. When first proposed as a division of geologic time, the beginning of the Phanerozoic, approximately 542 million years ago (mya), was thought to coincide with the beginning of life. In reality, this eon coincides with the appearance of animals that evolved external skeletons, like shells, and the somewhat later animals that formed internal skeletons, such as the bony elements of vertebrates. The time before the Phanerozoic is usually referred to as the Precambrian. The Phanerozoic consists of three major divisions: the Cenozoic, the Mesozoic, and the Paleozoic eras. The zoic part of the word comes from the root zoo, which means animal. Cen means recent, tneso means middle, and paleo means ancient. These divisions reflect major changes in the composition of ancient faunas, each era being recognized by its domination by a particular group of animals. The Cenozoic has sometimes been called the age of mammals, the Mesozoic the age of dinosaurs, and the Paleozoic the age of fishes. This is an overly simplified view; it has some value for the newcomer but can be a bit misleading. For instance, other groups of animals lived during the Mesozoic. In addition to the dinosaurs, animals such as mammals, turtles, crocodiles, frogs, and countless varieties of insects also lived on land. In addition, there were many kinds of plants living in the past that no longer live today. Ancient floras went through great changes too, and not always at the same times that the animal groups changed.

Few discussions in geology can occur without reference to geologic time, which is often discussed in two forms: (1) Relative time (chronos-tratic), subdivisions of the earth's geology in a specific order based upon relative age relationships; these subdivisions are given names, most of which can be recognized globally, usually on the basis of fossils. (2) Absolute time (chronometric), numerical ages in millions of years or some other measurement. These are most commonly obtained via radiometric dating methods performed on appropriate rock types.

History

The first people who needed to understand the geological relationships of different rock units were miners. Mining had been of commercial interest since at least the days of the Romans, but it wasn't until the 1500s and 1600s that these efforts produced an interest in local rock relationships. By noting the relationships of different rock units, Nicolaus Steno in 1669 described two basic geologic principles. The first stated that sedimentary rocks are laid down in a horizontal manner, and the second stated that younger rock units were deposited on top of older rock units. To envision this latter principle, think of the layers of paint on a wall. The oldest layer was put on first and is at the bottom, while the newest layer is at the top. An additional concept was introduced by James Hutton in 1795, and later emphasized by Charles Lyell in the early 1800s. This was the idea that natural geologic processes were uniform in frequency and magnitude throughout time, an idea known as the principle of uniformitarianism. Steno's principles allowed workers in the 1600s and early 1700s to begin to recognize rock successions. However, because rocks were locally described by the color, texture, or even smell, comparisons between rock sequences of different areas were often not possible. Fossils provided the opportunity for workers to correlate geographically distinct areas. This contribution was possible because fossils are found over wide regions of the earth's crust.

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