Eon

Eon, main division of geologic time. Eons are the largest units that geologists use to measure the age of the Earth. Eons are divided into eras. Each era has a number of periods, each of which is divided into epochs. The eons of the Earth are the Archean (about 3.8 billion to 2.5 billion years before present), the Proterozoic (about 2.5 billion to 542 million years before present), and the Phanerozoic (about 542 million years before present to present). The Archean and Proterozoic eons and pre-Archean time (from Earth’s formation about 4.6 billion years before present to 3.8 billion years before present) are often called Precambrian time.

The oldest dated rocks on Earth are just over 4 billion years old, but rocks gathered on the Moon and models of the formation of the Earth help geologists estimate that Earth is really about half a billion years older than the oldest rocks so far recorded on it. The first life on Earth appeared during the Archean (Greek for “ancient”) Eon. Earth scientists have found microscopic fossil remains left behind by simple, single-celled organisms that are 3.8 billion years old. Geologists define the end of the Archean Eon as 2.5 billion years before present. The continents, oceans, and atmosphere formed during this eon. Archean rocks form the ancient cores of all of the present-day continents.

The Proterozoic (Greek for “earlier life”) Eon is the second eon of Earth’s history, starting about 2.5 billion years before present and lasting to 542 million years before present. Proterozoic rocks contain abundant life. Early in the Proterozoic, complex cells called eukaryotes and photosynthesizing blue-green algae called stromatolites appeared. By the end of the eon, diverse and advanced organisms began appearing. The oxygen content of the atmosphere increased greatly during the Proterozoic, gradually changing the world’s surface and paving the way for the variety of animals that emerged in the Phanerozoic.

The Phanerozoic (Greek for “appearance of life”) Eon is the youngest and shortest eon in Earth’s history. The eon began about 542 million years before present and continues to the present. Phanerozoic rocks occur on all continents. The fossils in Phanerozoic rocks illustrate a progression of life from early marine organisms to terrestrial plants and animals, ending with the wide variety of species on Earth today.

The fossil record also shows several major extinctions, in which a large percentage of the number of species on the earth died out and were replaced by new species. Asteroids crashing into Earth probably caused some of the extinctions, while others may have resulted from unknown terrestrial or astronomical forces. Rocks of the Phanerozoic reveal that our present-day continents are continuously moving, a process called plate tectonics. Phanerozoic rocks are responsible for providing all of our fossil fuels, most sources of groundwater, many building supplies, and many commercial mineral products.