# Geology/Chronology

Geochronology is the science of applying dates in the past to rocks. These rocks receive dates because they contain constituents that can be used as chronometers.

## Geologic time

On the right is a geologic clock representation. It shows some of the major units of geological time and definitive events of Earth history. The Hadean eon represents the time before fossil record of life on Earth; its upper boundary is now regarded as 4.0 Ga (billion years ago). 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 two million year Quaternary period, the time of recognizable humans, is too small to be visible at this scale.
The following four timelines show the geologic time scale. The first shows the entire time from the formation of the Earth to the present, but this compresses the most recent eon. Therefore the second scale shows the most recent eon with an expanded scale. The second scale compresses the most recent era, so the most recent era is expanded in the third scale. Since the Quaternary is a very short period with short epochs, it is further expanded in the fourth scale. The second, third, and fourth timelines are therefore each subsections of their preceding timeline as indicated by asterisks. The Holocene (the latest epoch) is too small to be shown clearly on the third timeline on the right, another reason for expanding the fourth scale. The Pleistocene (P) epoch. Q stands for the Quaternary period.
Units in geochronology and stratigraphy :
| Segments of rock (strata) in chronostratigraphy | Time spans in geochronology | Notes to geochronological units |
| Supereonothem | Supereon | 1 total, four billion years or more (e.g. Precambrian) |
| Eonothem | Eon | 4 total, half a billion years or more |
| Erathem | Era | 10 defined, several hundred million years |
| System | Period | 22 defined, tens to ~one hundred million years |
| Series | Epoch | 34 defined, tens of millions of years |
| Stage | Age | 99 defined, millions of years |
| Chronozone | Chron | subdivision of an age, not used by the ICS timescale |

## Notations

Let
* ALMA represent the Asian Land Mammal Age,
* b2k represent before AD 2000,
* BP represent before present, as the chart is for 2008, this may require an added -8 for b2k,
* ELMMZ represent the European Land Mammal Mega Zone,
* FAD represent first appearance datum,
* FO represent first occurrence,
* Ga represent Gegaannum, billion years ago, or -10$^9$ b2k,
* GICC05 represent Greenland Ice Core Chronology 2005,
* GRIP represent Greenland Ice Core Project,
* GSSP represent Global Stratotype Section and Point,
* HO represent highest occurrence,
* ICS represent the International Commission on Stratigraphy,
* IUGS represent the International Union of Geological Sciences,
* LAD represent last appearance datum,
* LO represent lowest occurrence,
* Ma represent Megaannum, million years ago, or -10$^6$ b2k,
* NALMA represent the North American Land Mammal Age,
* NGRIP represent North Greenland Ice Core Project, and
* SALMA represent South American Land Mammal Age.
"The term b2 k [b2k] refers to the ice-core zero age of AD 2000; note that this is 50 years different from the zero yr for radiocarbon, which is AD 1950 [...]."

## Phanerozoic

The Phanerozoic eon includes the Paleozoic, Mesozoic, and Cenozoic.

## Hypotheses

* Each time frame or span of time in geochronology has at least one dating technique.
* Late Jurassic is a time frame and Upper Jurassic is a stratigraphic frame so both are different time frames.
* The overall size of—or efficiency of carbon export from—the biosphere decreased at the end of the Great Oxidation Event (GOE) (ca. 2,400 to 2,050 Ma).