2.2: Geology and Geologic Time

Geology describes the structures of the Earth at and beneath its surface, and the processes that have shaped that structure. It also provides tools to determine the relative and absolute ages of rocks found in each location, and to describe the histories of those rocks. By combining these tools, geologists can chronicle the geological history of the Earth as a whole, and to demonstrate the age of Earth. Geology provides the primary evidence for plate tectonics, the evolutionary history of life, and the Earth's past climates.

Geology often applies information from physics and chemistry to the natural world, like understanding the physical forces in a landslide or the chemical interaction between water and rocks. The term comes from the Greek word geo, meaning Earth, and logos, meaning to think or reckon with.

Geologic Time Scale

William “Strata” 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), and he used it to great effect in his monumental project to create a geological map of England and Wales, published in 1815.

Geologic Time has been subdivided into a series of divisions by geologists. Eon is the largest division of time, followed by era, period, epoch, and age. The partitions of the geologic time scale are the same everywhere on Earth; however, rocks may or may not be present at a given location depending on the geologic activity going on during a particular period. Thus, we have the concept of time vs. rock, in which time is an unbroken continuum but rocks may be missing and/or unavailable for study. The figure of the geologic time scale represents time flowing continuously from the beginning of the Earth, with the time units presented in an unbroken sequence. But that does not mean there are rocks available for study for all these time units, since erosion has removed much of the depositional sequences.

Geological Time has been divided into four eons: Hadean, Archean, Proterozoic, and Phanerozoic, and as shown below, the first three of these represent almost 90% of Earth’s history. 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 life forms that we are all somewhat familiar with.

Figure 2.2: The Eons of Earth's History. Image is used under a CC-BY 4.0 license.

The Phanerozoic, 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 several periods. Most of the organisms that we share Earth with, evolved at various times during the Phanerozoic.

Figure 2.3: The Eras and Periods of the Phanerozoic. Image is used under a CC-BY 4.0 license.

The Cenozoic, which represents the past 65.5 Ma, is divided into three periods: Paleogene, Neogene, and Quaternary, and seven epochs. 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 in Antarctica in the Oligocene and then in 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 or stages.

Figure 2.4: The Periods and Epochs of the Cenozoic. Image is used under a CC-BY 4.0 license.

Most of the boundaries between the periods and epochs of the geological time scale have been fixed based on significant changes in the fossil record, due to significant geologic events that significantly changed the environments. For example, as already noted, the boundary between the Cretaceous and the Paleogene coincides exactly with the extinction of the dinosaurs. That’s not a coincidence. 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 below and Paleogene organisms above.