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The adjectives are capitalized when the subdivision is formally recognized, and lower case when not; thus "early Miocene" but "Early Jurassic."Evidence from radiometric dating indicates that Earth is about 4.54 billion years old.The geology or deep time of Earth's past has been organized into various units according to events which took place.

Corresponding to eons, eras, periods, epochs and ages, the terms "eonothem", "erathem", "system", "series", "stage" are used to refer to the layers of rock that belong to these stretches of geologic time in Earth's history.

"Our finding that these zircons are older than they should be relative to their magnetic age alters what we've thought about oceanic crust, he says.

"The ability to date zircons in ocean crust offers another and better way to determine how ocean crust is formed." Adds Cheadle, "Findings about today's ocean ridges help us to better understand how the Earth has worked in the past." Other co-authors of the paper are affiliated with the U. Geological Survey in Menlo Park, Calif., and the Woods Hole Oceanographic Institution in Woods Hole, Mass.

Other subdivisions reflect the evolution of life; the Archean and Proterozoic are both eons, the Palaeozoic, Mesozoic and Cenozoic are eras of the Phanerozoic eon.

The three million year Quaternary period, the time of recognizable humans, is too small to be visible at this scale.

Different spans of time on the GTS are usually marked by corresponding changes in the composition of strata which indicate major geological or paleontological events, such as mass extinctions.