Which Percent Of Earth's History Does Epoch Makeup
Affiliate eight Measuring Geological Time
8.1 The Geological Time Scale
William Smith worked as a surveyor in the coal-mining and canal-building industries in southwestern England in the late 1700s and early on 1800s. While doing his work, he had many opportunities to wait at the Paleozoic and Mesozoic sedimentary rocks of the region, and he did and then in a way that few had done before. Smith noticed the textural similarities and differences betwixt rocks in different locations, and more importantly, he discovered that fossils could be used to correlate rocks of the same age. Smith is credited with formulating the (the concept that specific types of organisms lived during different time intervals), and he used it to great effect in his monumental project to create a geological map of England and Wales, published in 1815. For more on William Smith, including a large-scale digital copy of the famous map, see the William Smith Wikipedia page.
Inset into Smith'due south neat geological map is a small-scale diagram showing a schematic geological cross-section extending from the Thames estuary of eastern England all the way to the west declension of Wales. Smith shows the sequence of rocks, from the Paleozoic rocks of Wales and western England, through the Mesozoic rocks of central England, to the Cenozoic rocks of the area around London (Figure 8.1.1). Although Smith did non put any dates on these—because he didn't know them—he was enlightened of the (the thought, developed much earlier by the Danish theologian and scientist Nicholas Steno, that young sedimentary rocks form on top of older ones), and so he knew that this diagram represented a stratigraphic column. And because most every menstruation of the Phanerozoic is represented along that department through Wales and England, it is a primitive geological fourth dimension scale.
Smith'south work prepare the stage for the naming and ordering of the geological periods, which was initiated around 1820, beginning by British geologists, and later by other European geologists. Many of the periods are named for places where rocks of that age are establish in Europe, such as Cambrian for Cambria (Wales), Devonian for Devon in England, Jurassic for the Jura Mountains in France and Switzerland, and Permian for the Perm region of Russian federation. Some are named for the blazon of stone that is common during that age, such as Carboniferous for the coal- and carbonate-bearing rocks of England, and Cretaceous for the chalks of England and France.
The early time scales were simply relative considering 19th century geologists did non know the ages of the rocks. That data was not available until the development of isotopic dating techniques early in the 20th century.
The geological time scale is currently maintained by the International Commission on Stratigraphy (ICS), which is part of the International Marriage of Geological Sciences. The time scale is continuously being updated every bit we learn more nigh the timing and nature of by geological events. You tin can view the ICS time calibration online. It would be a good idea to print a copy (in colour) to put on your wall while yous are studying geology.
Geological time has been divided into 4 eons: Hadean (4570 to 4850 Ma), Archean (3850 to 2500 Ma), Proterozoic (2500 to 540 Ma), and Phanerozoic (540 Ma to present). Equally shown in Figure viii.1.2, the start three of these stand for almost 90% of World's history. The last one, the Phanerozoic (meaning "visible life"), is the time that we are nearly familiar with considering Phanerozoic rocks are the most common on Globe, and they incorporate bear witness of the life forms that we are familiar with to varying degrees.
The Phanerozoic eon—the past 540 Ma of Earth's history—is divided into three eras: the Paleozoic ("early on life"), the Mesozoic ("middle life"), and the Cenozoic ("new life"), and each of these is divided into a number of periods (Effigy 8.i.3). Most of the organisms that we share Earth with evolved at various times during the Phanerozoic.
The Cenozoic era, which represents the past 65.5 Ma, is divided into iii periods: Paleogene, Neogene, and Quaternary, and seven epochs (Figure 8.ane.4). Dinosaurs became extinct at the start of the Cenozoic, after which birds and mammals radiated to fill up the bachelor habitats. World was very warm during the early Eocene and has steadily cooled e'er since. Glaciers first appeared on Antarctica in the Oligocene and then on Greenland in the Miocene, and covered much of N America and Europe by the Pleistocene. The most recent of the Pleistocene glaciations ended around 11,700 years agone. The electric current epoch is known as the Holocene. Epochs are further divided into ages (a.k.a. stages), but we won't exist going into that level of particular here.
Most of the boundaries between the periods and epochs of the geological time calibration accept been fixed on the footing of significant changes in the fossil record. For case, as already noted, the boundary between the Cretaceous and the Paleogene coincides exactly with a devastating mass extinction. That's not a coincidence. The dinosaurs and many other types of organisms went extinct at this time, and the boundary betwixt the two periods marks the division between sedimentary rocks with Cretaceous organisms (including dinosaurs) below, and Paleogene organisms in a higher place.
Paradigm Descriptions
| Era | Period | Time span |
|---|---|---|
| Paleozoic | Cambrian | 488 to 540 Ma |
| Paleozoic | Ordovician | 488 to 444 Ma |
| Paleozoic | Silurian | 444 to 416 Ma |
| Paleozoic | Devonian | 416 to 359 Ma |
| Paleozoic | Carboniferous | 359 to 299 Ma |
| Paleozoic | Permian | 299 to 251 Ma |
| Mesozoic | Triassic | 251 to 202 Ma |
| Mesozoic | Jurassic | 202 to 146 Ma |
| Mesozoic | Cretaceous | 146 to 65.5 Ma |
| Cenozoic | Paleogene | 65.5 to 23 Ma |
| Cenozoic | Neogene | 23 to 2.half dozen Ma |
| Cenozoic | Quaternary | 2.half dozen Ma to nowadays |
[Render to Figure 8.i.3]
| Period | Epoch | Time span |
|---|---|---|
| Paleogene | Paleocene | 65.5 to 55.8 Ma |
| Paleogene | Eocene | 55.8 to 33.nine Ma |
| Paleogene | Oligocene | 33.ix to 23.0 Ma |
| Neogene | Miocene | 23.0 to 5.3 Ma |
| Neogene | Pliocene | 5.3 to 2.6 Ma |
| 4th | Pleistocene | 2.6 Ma to 11,700 years ago |
| Quaternary | Holocene | 11,700 years agone to the present |
[Return to Figure 8.1.4]
Media Attributions
- Figure 8.one.1: "Sketch of the succession of strata and their relative altitudes" by William Smith. Adjusted by Steven Earle. Public domain.
- Figures viii.1.ii, 8.ane.three, eight.1.4: © Steven Earle. CC By.
Source: https://opentextbc.ca/physicalgeology2ed/chapter/8-1-the-geological-time-scale/
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