Chapter 4 Measuring Geological Time
If you read any book or article about geology, you are confronted with strange names of the geologic past such as Phanerozoic, Jurassic, Holocene, Hadean, and so on. These words come from the various Eon, Eras, Epochs, or Periods of the geologic times scale. The geologic timescale is a representation of time based on the rock record of Earth (e.g., the timing and relationships of events in geologic history). It is the history of our Earth, in chronological order. It was first developed in the 18th century, built by using the principles of relative age dating you read about in the previous section of this chapter. The geologic time scale was completed long before scientists were ever able to assign numerical ages to events in the geologic past, and remains largely unchanged. The main difference is that we can now know the time at which the events occurred.
Development of the geologic timescale begins with the work of William Smith. Smith worked as a surveyor in the coal-mining and canal-building industries in southwestern England in the late 1700s and early 1800s. While doing his work, he had many opportunities to look at the Paleozoic and Mesozoic sedimentary rocks of the region, and he did so in a way that few had done before. Smith noticed the textural similarities and differences between 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 principle of faunal succession (the concept that specific types of organisms lived during different time intervals, the types of organisms on Earth have changed and evolved through geologic time, and once an organism goes extinct it is no longer observed in younger layers in the rock record), 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. The principle of faunal succession is also the geologic foundation for the Theory of Evolution.
Inset into Smith’s great geological map is a small diagram showing a schematic geological cross-section extending from the Thames estuary of eastern England all the way to the west coast 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 4.3.1). Although Smith did not put any dates on these—because he didn’t know them—he was aware of the principle of superposition (young sedimentary rocks form on top of older ones), and so he knew that this diagram represented a stratigraphic column. And because almost every period of the Phanerozoic is represented along that section through Wales and England, it is a primitive geological time scale that records the geologic history of Great Britain.
Smith’s work set the stage for the naming and ordering of the geological periods, which was initiated around 1820, first by British geologists, and later by other European geologists. Many of the periods are named for places where rocks of that age are found 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 Russia. Some are named for the type of rock 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 only relative because 19th century geologists did not know the ages of the rocks. That information was not available until the development of isotopic dating techniques early in the 20th century.
Today, the geological time scale is currently maintained by the International Commission on Stratigraphy (ICS), which is part of the International Union of Geological Sciences. The time scale is continuously being updated as we learn more about the timing and nature of past geological events. You can view the ICS time scale online. It would be a good idea to print a copy (in color) to put on your wall while you are studying geology.
Geological time has been divided into four eons: Hadean (4570 to 3850 Ma), Archean (3850 to 2500 Ma), Proterozoic (2500 to 540 Ma), and Phanerozoic (540 Ma to present). As shown in Figure 4.3.2, the first three of these represent almost 90% of Earth’s history. The middle part of the Proterozoic (1.8-0.8 Ga) is even sometimes referred to as the “boring billion”, which is frankly rude to simple single celled organisms, or organism that appreciate breathing oxygen. The last one, the Phanerozoic (meaning “visible life”), is the time that we are most familiar with because Phanerozoic rocks are the most common on Earth, and they contain evidence 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 life”), the Mesozoic (“middle life”), and the Cenozoic (“new life”), and each of these is divided into a number of periods (Figure 4.3.3). 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 three periods: Paleogene, Neogene, and Quaternary, and seven epochs (Figure 4.3.4). Dinosaurs became extinct at the start of the Cenozoic, after which birds and mammals radiated to fill the available habitats. Earth was very warm during the early Eocene and has steadily cooled ever since. Glaciers first appeared on Antarctica in the Oligocene and then on Greenland in the Miocene, and covered much of North America and Europe by the Pleistocene. The most recent of the Pleistocene glaciations ended around 11,700 years ago. The current epoch is known as the Holocene. Epochs are further divided into ages (a.k.a. stages), but we won’t be going into that level of detail here.
Most of the boundaries between the periods and epochs of the geological time scale have been fixed on the basis of significant changes in the fossil record. For example, 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 between the two periods marks the division between sedimentary rocks with Cretaceous organisms (including dinosaurs) below, and Paleogene organisms above.
Image Descriptions
Media Attributions
- Physical Geology-2nd Edition, by Steven Earle is licensed under CC BY 4.0, Adapted from the original by Ryan B. Anderson.
- Figure 4.3.1: “Sketch of the succession of strata and their relative altitudes” by William Smith. Adapted by Steven Earle. Public domain.
- Figures 4.3.2, 4.3.3, 4.3.4: © Steven Earle. CC BY.
the principle that life on Earth has evolved in an orderly way, and that we can expect to always find fossils of a specific type in rocks of a specific age
the principle that in a sequence of layered rocks that is not overturned or interrupted by faulting, the oldest will be at the bottom and the youngest at the top