Geologic Time Scale: Eons, Eras, and Periods

The geologic time scale is a system used by scientists to describe Earth's history in terms of major geological or paleontological events (such as the formation of a new rock layer or the appearance or demise of certain lifeforms). Geologic time spans are divided into units and subunits, the largest of which are eons. Eons are divided into eras, which are further divided into periods, epochs, and ages. Geologic dating is extremely imprecise. For example, although the date listed for the beginning of the Ordovician period is 485 million years ago, it is actually 485.4 with an uncertainty (plus or minus) of 1.9 million years.

What Is Geologic Dating?

Geologic dating allows scientists to better understand ancient history, including the evolution of plant and animal life from single-celled organisms to dinosaurs to primates to early humans. It also helps them learn more about how human activity has transformed the planet.

_{(c) 2013 Andrew Alden, licensed to About.com, Inc. (fair use policy). Data from Geologic Time Scale of 2015.} 

The dates shown on this geologic time scale were specified by the International Commission on Stratigraphy in 2015. The colors were specified by the Committee for the Geologic Map of the World in 2009.

Of course, these geologic units are not equal in length. Eons, eras, and periods are usually separated by a significant geologic event and are unique in their climate, landscape, and biodiversity. The Cenozoic era, for example, is known as the "Age of Mammals." The Carboniferous period, on the other hand, is named for the large coal beds that were formed during this time ("carboniferous" means coal-bearing). The Cryogenian period, as its name suggests, was a time of great glaciations.

Hadean

The oldest of the geologic eons is the Hadean, which began about 4.6 billion years ago with the formation of Earth and ended about 4 billion years ago with the appearance of the first single-celled organisms. This eon is named after Hades, the Greek god of the underworld, and during this period the Earth was extremely hot. Artist renderings of the Hadean Earth depict a hellish, molten world of fire and lava. Although water was present at this time, the heat would have boiled it away into steam. Oceans as we know them today did not appear until the Earth's crust began to cool many years later.

Archean

The next geologic eon, the Archean, began about 4 billion years ago. During this period, the cooling of the Earth's crust allowed for the formation of the first oceans and continents. Scientists are not exactly sure what these continents looked like since there is so little evidence from the period. However, some believe the first landmass on Earth was a supercontinent known as Ur. Others believe it was a supercontinent known as Vaalbara.

Scientists believe that the first single-celled lifeforms developed during the Archean. These tiny microbes left their mark in layered rocks known as stromatolites, some of which are nearly 3.5 billion years old.

Unlike the Hadean, the Archean eon is divided into eras: the Eoarchean, Paleoarchean, Mesoarchean, and Neoarchean. The Neoarchean, which began about 2.8 billion years ago, was the era in which oxygenic photosynthesis began. This process, performed by algae and other microorganisms, caused oxygen molecules in water to be released into the atmosphere. Prior to oxygenic photosynthesis, Earth's atmosphere had no free oxygen, a huge impediment to the evolution of life.

Proterozoic

The Proterozoic eon began about 2.5 billion years ago and ended about 500 million years ago when the first complex lifeforms appeared. During this period, the Great Oxygenation Event transformed the Earth's atmosphere, allowing for the evolution of aerobic organisms. The Proterozoic was also the period in which the Earth's first glaciers formed. Some scientists even believe that during the Neoproterozoic era, about 650 million years ago, the surface of the Earth became frozen. Proponents of the "Snowball Earth" theory point to certain sedimentary deposits that are best explained by the presence of ice.

The first multicellular organisms developed during the Proterozoic eon, including early forms of algae. Fossils from this eon are very small. Some of the most notable from this time are the Gabon macrofossils, which were discovered in Gabon, West Africa. The fossils include flattened disks up to 17 centimeters long.

Phanerozoic

The most recent geologic eon is the Phanerozoic, which began about 540 million years ago. This eon is very distinct from the previous three—the Hadean, Archean, and Proterozoic—which are sometimes known as the Precambrian era. During the Cambrian period—the earliest part of the Phanerozoic—the first complex organisms appeared. Most of them were aquatic; the most famous examples are trilobites, small arthropods (creatures with exoskeletons) whose distinct fossils are still being discovered today. During the Ordovician period, fish, cephalopods, and corals first appeared; over time, these creatures eventually evolved into amphibians and dinosaurs.

During the Mesozoic era, which began about 250 million years ago, dinosaurs ruled the planet. These creatures were the largest to ever walk the Earth. Titanosaur, for example, grew up to 120 feet long, five times as long as an African elephant. The dinosaurs were eventually wiped out during the K-2 Extinction, an event that killed about 75 percent of the life on Earth.

Following the Mesozoic era was the Cenozoic, which began about 66 million years ago. This period is also known as the "Age of Mammals," as large mammals, following the extinction of the dinosaurs, became the dominant creatures on the planet. In the process, mammals diversified into the many species still present on the Earth today. Early humans, including Homo habilis, first appeared about 2.8 million years ago, and modern humans (Homo sapiens) first appeared about 300,000 years ago. These enormous changes to life on Earth have taken place over a period of time that, compared to geologic history, is relatively small. Human activity has transformed the planet; some scientists have proposed a new epoch, the "anthropocene," to describe this new period of life on Earth.