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The Structure of Geologic Time

The Geologic Eons of Time

Eons > Eras > Periods > Epochs

Scientists have devised the Geologic Time Chart so that Time can be understood from the largest and most general intervals, or units, to the smallest, most specific intervals.

The Eons are the largest interval units into which Geologic Time is divided and represented on the chart. There are four Geologic Eons. The first three, the Hadean, Archean, and Proterozoic Eons are frequently lumped together and referred to as the Precambrian. See illustration at right.

The Hadean Eon

The first Eon of Time is the Hadean Eon. The Hadean Eon is the oldest interval of Time and is dated from 4,600 Million Years Ago to 3,900 Million Years Ago. No rock record dating from the Hadean Eon is known on Earth with the exception of the 3.96 Billion Year old rocks found in Canada's Northwest Territories. Although, rocks on other Planets in the Solar System were possibly deposited during the Hadean Eon. Earth's earliest known life forms are found in rocks dating from the Archean Eon.

The Archean Eon

The oldest rocks known on Earth were deposited during the
Archean Eon which is dated from 3,900 Million Years Ago to 2,500 Million Years Ago. The Archean Eon is also referred to as the Archaeozoic Eon. Sedimentary rock deposited in the oceans during the Archean Eon contain microscopic life-forms that have bacterial characteristics. These organisms were
among the only living things on Earth for over one billion years.

Stromatolites, formed by colonies of photosynthesizing bacteria known as cyanobacteria (blue-green algae) lived in the shallow waters. In the Archean Eon, Earth's atmosphere was composed of methane, ammonia, hydrogen, and carbon monoxide, which would be unbreatheable by living organisms today. The extensive colonies of photosynthesizing cyanobacteria released oxygen into the atmosphere, beginning a process that eventually resulted in a breatheable atmosphere and impacted living things in future Eons.

The climate and weather in the Archean Eon was very hot, and wet. The oceans were also very warm. Extensive worldwide volcanic eruptions ejected volcanic ash and dust into the atmosphere causing violent lightening storms and continual rain.

The Proterozoic Eon

Rocks deposited during the Proterozoic Eon include evidence of multicelled organisms. The Proterozoic Eon was from 2,500 Million Years Ago to 540 Million Years Ago. The stromatolites remained common in the shallow waters of Proterozoic Eon oceans, however, many other living things were present such as archaea that lived in extreme habitats where the water was very hot and extremely salty.

By the end of the Proterozoic Eon eukaryotes, cells that have a nucleus, appeared and were the ancestors of all modern living things. One of the first extinctions occurred in the Proterozoic Eon, when oxygen accumulated in the atmosphere and caused those bacteria that required the atmospheric conditions of the Archean Eon to die. The Proterozoic Eon also experienced two ice ages, when polar ice became at least a kilometer deep and covered a area extending to the equator.

The Phanerozoic Eon

The Phanerozoic Eon began 540 Million Years Ago and continues into the Present. The Rocks deposited during the Phanerozoic Eon contain evidence of fossilized hard body parts of modern living things.