Geologic time scale

Geologic time scale uses the principles and techniques of geology to work out the geological history of the Earth.^[1] It looks at the processes which change the Earth's surface and rocks under the surface.

Diagram of geological time scale as a spiral.

Geologists use stratigraphy and paleontology to find out the sequence of the events, and show the plants and animals which lived at different times in the past. They worked out the sequence of rock layers. Then the discovery of radioactivity and the invention of radiometric dating techniques gave a way to get the ages of the layers (strata).

We now know the timing of important events that have happened during the history of Earth. The Earth is about 4.567 billion (4,567 million) years old. The geological or deep time of Earth's past has been organized into various units. Boundaries on the time scale are usually marked by major geological or palaeontological events, such as mass extinctions. For example, the boundary between the Cretaceous period and the Palaeogene period is defined by the Cretaceous–Tertiary extinction event. This marked the end of the dinosaurs and of many marine species.

Prospecting for energy sources and valuable minerals depends on understanding the geological history of an area. Such knowledge can also help lessen the hazards of earthquakes and volcanoes.

Terminology

The largest defined unit of time is the supereon composed of Eons. Eons are divided into Eras, which are in turn divided into Periods, Epochs and Stages. At the same time paleontologists define a system of faunal stages, of varying lengths, based on the kinds of animal fossils found there. In many cases, such faunal stages have been adopted in building the geological nomenclature, though in general there are far more recognized faunal stages than defined geological time units.

Geologists tend to talk in terms of Upper/Late, Lower/Early and Middle parts of periods and other units, such as "Upper Jurassic", and "Middle Cambrian". Upper, Middle, and Lower are terms applied to the rocks themselves, as in "Upper Jurassic sandstone," while Late, Middle, and Early are applied to time, as in "Early Jurassic deposition" or "fossils of Early Jurassic age." The adjectives are capitalized when the subdivision is formally recognized, and lower case when not. Thus in 2018 when three divisions (subepochs) of the Holocene were formally recognized it became correct to write "Early Holocene" instead of "early Holocene".^[2][3]

Because geologic units occurring at the same time but from different parts of the world can often look different and contain different fossils, there are many examples where the same period was historically given different names in different locales. For example, in North America the Lower Cambrian is called the Waucoban series that is then subdivided into zones based on trilobites. The same timespan is split into Tommotian, Atdabanian and Botomian stages in East Asia and Siberia. A key aspect of the work of the International Commission on Stratigraphy is to reconcile this conflicting terminology and define universal horizons (time division) that can be used around the world.

Table of geologic time

The following table summarizes the major events and characteristics of the periods of time making up the geologic time scale. As above, this time scale is based on the International Commission on Stratigraphy. The height of each table entry does not correspond to the duration of each subdivision of time. (not shown to scale)

Geologic time
Eon	Era	Period/Age4,5		Epoch	Major Events	Start (Years Ago)3,6
Phanerozoic	Cainozoic	Quaternary		Holocene	Rise of human population; Last ice age ends	11,700
				Pleistocene	Ice ages and warmer periods; extinction of many large mammals; evolution of fully modern humans	2.588 million
		Tertiary	Neogene	Pliocene	Climate cools further; Australopithecine hominins evolve	5.333 million
				Miocene	Earth has many forests; animals flourish but later temperatures start to cool	23.03 million
			Palaeogene	Oligocene	The continents move into their current places	33.9 million
				Eocene	The Himalayas are formed as India moves into Asia	56 million
				Palaeocene	India reaches Asia; mammals evolve into new groups; birds survive extinction	66 million
	Mesozoic	Cretaceous		Upper Cretaceous	Dinosaurs become extinct in K/T extinction event.	100.5 million
				Lower Cretaceous	Dinosaurs continue to flourish; marsupial and placental mammals appear; first flowering plants	145 million
		Jurassic		Upper Jurassic	Dinosaurs dominate on land; first birds, early mammals; conifers, cycads and other seed plants. Supercontinent Pangaea begins to break up	163.5 million
				Middle Jurassic		174.1 million
				Lower Jurassic		201.3 million
		Triassic		Upper Triassic	First dinosaurs; pterosaurs; ichthyosaurs; plesiosaurs; turtles; egg-laying mammals	237 million
				Middle Triassic		247.2 million
				Lower Triassic		252.17 million
	Palaeozoic	Permian			P/Tr extinction event – 95% of species become extinct. Supercontinent Pangaea forms.	298.9 million
		Carboniferous		Pennsylvanian	Tropical climate: abundant insects, first synapsids and reptiles; coal forests	323.2 million
				Mississippian	Large primitive trees	358.9 million
		Devonian			Age of fish; first amphibia; clubmosses and horsetails appear; progymnosperms (first seed bearing plants) appear	419.2 million
		Silurian			First land plant fossils	443.4 million
		Ordovician			Invertebrates dominant	485.4 million
		Cambrian			Major diversification of life in the Cambrian adaptive radiation	541 million
Proterozoic	Neoproterozoic2	Ediacaran			First multi-celled animals	635 million
		Cryogenian			Possible Snowball Earth period	720 million
		Tonian			Supercontinent Rodinia breaks up	1 billion
	Mesoproterozoic	Stenian			The supercontinent Rodinia forms	1.2 billion
		Ectasian			First sexually reproducing organism	1.4 billion
		Calymmian			The supercontinent of Columbia breaks up	1,6 billion
	Palaeoproterozoic	Statherian			Formation of the Columbia (supercontinent) happens during this period	1.8 billion
		Orosirian			First complex single-celled life	2.05 billion
		Rhyacian			Replacement of CO2 by oxygen triggers the Huronian glaciation in this period	2.3 billion
		Siderian			The breakup of the supercontinent Kenorland occurs	2.5 billion
Archaean	Neoarchaean				The supercontinent Kenorland forms	2.8 billion
	Mesoarchaean				The supercontinet Ur is from this era	3.2 billion
	Palaeoarchaean				Bacteria build stromatolites	3.6 billion
	Eoarchaean				1st supercontinet Vaalbara existed during this era	4 billion
Hadean					Formation of Earth 4.6 billion years ago; formation of Moon 4.5 bya	4.54 billion (~4.6 bya)
In North America, the Carboniferous is subdivided into Mississippian and Pennsylvanian sub-periods or epochs. Discoveries in the past quarter century have substantially changed the view of geologic and paleontologic events just before the Cambrian. The term Neoproterozoic is used now, but older writers might have used 'Ediacaran', 'Vendian', 'Varangian', 'Precambrian', 'Protocambrian', 'Eocambrian', or might have extended the Cambrian further back in time. Dates are slightly uncertain, and differences of a few percent between sources are common. This is because deposits suitable for radiometric dating seldom occur exactly at the places in the geologic column where we would most like to have them. Dates with an * are radiometrically determined based on internationally agreed GSSPs. Paleontologists often refer to faunal stages rather than geologic periods. The faunal stage nomenclature is quite complex. See http://flatpebble.nceas.ucsb.edu/public/harland.html Archived 2002-02-19 at the Wayback Machine for an excellent time ordered list of faunal stages. In common usage the Tertiary-Quaternary and Palaeogene-Neogene-Quaternary are treated as periods. The term 'age' (e.g. 'Neogene Age') is sometimes used instead of 'period'. The time shown in the "Years Ago" column is that of the start of the Epoch in the "Epoch" column.

Related pages

• History of the Earth
• Age of the Earth
• History of the geologic time scale