Historical geology

Diagram of geological time scale.

The Historical geology is used by geologists and other scientists to describe the timing and relationships between events that have occurred during the history of Earth. The table of geological periods presented here agrees with the dates and nomenclature proposed by the International Commission on Stratigraphy, and uses the standard color codes of the United States Geological Survey.

Evidence from radiometric dating indicates that 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 according to events which took place in each period. Different spans of time on the time scale are usually delimited 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, which marked the demise of the dinosaurs and of many marine species. Older periods which predate the reliable fossil record are defined by absolute age.

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 changes in the observed fossil assemblages. 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 "early Miocene" but "Early Jurassic." 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 referred to as 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 that can be used around the world.

History of the time scale

Earth history mapped to 24 hours

The principles underlying geologic (geological) time scales were laid down by Nicolaus Steno in the late 17th century. Steno argued that rock layers (or strata) are laid down in succession, and that each represents a "slice" of time. He also formulated the principle of superposition, which states that any given stratum is probably older than those above it and younger than those below it. While Steno's principles were simple, applying them to real rocks proved complex. Over the course of the 18th century geologists realized that:

1. Sequences of strata were often eroded, distorted, tilted, or even inverted after deposition;
2. Strata laid down at the same time in different areas could have entirely different appearances;
3. The strata of any given area represented only part of the Earth's long history.

The first serious attempts to formulate a geological time scale that could be applied anywhere on Earth took place in the late 18th century. The most influential of those early attempts (championed by Abraham Werner, among others) divided the rocks of the Earth's crust into four types: Primary, Secondary, Tertiary, and Quaternary. Each type of rock, according to the theory, formed during a specific period in Earth history. It was thus possible to speak of a "Tertiary Period" as well as of "Tertiary Rocks." Indeed, "Tertiary" (now Palaeocene-Pliocene) and "Quaternary" (now Pleistocene-Holocene) remained in use as names of geological periods well into the 21th century.

In opposition to the then-popular Neptunist theories expounded by Werner (that all rocks had precipitated out of a single enormous flood), a major shift in thinking came with the reading by James Hutton of his Theory of the Earth; or, an Investigation of the Laws Observable in the Composition, Dissolution, and Restoration of Land Upon the Globe before the Royal Society of Edinburgh in March and April 1785, events which "as things appear from the perspective of the twentieth century, James Hutton in those reading became the founder of modern geology" What Hutton proposed was that the interior of the Earth was hot, and that this heat was the engine which drove the creation of new rock: land was eroded by air and water and deposited as layers in the sea; heat then consolidated the sediment into stone, and uplifted it into new lands. This theory was dubbed "Plutonist" in contrast to the flood-oriented theory.

The identification of strata by the fossils they contained, pioneered by William Smith, Georges Cuvier, Jean d'Omalius d'Halloy and Alexandre Brogniart in the early 19th century, enabled geologists to divide Earth history more precisely. It also enabled them to correlate strata across national (or even continental) boundaries. If two strata (however distant in space or different in composition) contained the same fossils, chances were good that they had been laid down at the same time. Detailed studies between 1820 and 1850 of the strata and fossils of Europe produced the sequence of geological periods still used today.

The process was dominated by British geologists, and the names of the periods reflect that dominance. The "Cambrian," (the Roman name for Wales) and the "Ordovician," and "Silurian", named after ancient Welsh tribes, were periods defined using stratigraphic sequences from Wales. The "Devonian" was named for the English county of Devon, and the name "Carboniferous" was simply an adaptation of "the Coal Measures," the old British geologists' term for the same set of strata. The "Permian" was named after Perm, Russia, because it was defined using strata in that region by a Scottish geologist Roderick Murchison. However, some periods were defined by geologists from other countries. The "Triassic" was named in 1834 by a German geologist Friedrich Von Alberti from the three distinct layers (Latin trias meaning triad) —red beds, capped by chalk, followed by black shales— that are found throughout Germany and Northwest Europe, called the 'Trias'. The "Jurassic" was named by a French geologist Alexandre Brogniart for the extensive marine limestone exposures of the Jura Hills. The "Cretaceous" (from Latin creta meaning 'chalk') as a separate period was first defined by a Belgian geologist Jean d'Omalius d'Halloy in 1822, using strata in the Paris basin and named for the extensive beds of chalk (calcium carbonate deposited by the shells of marine invertebrates).

British geologists were also responsible for the grouping of periods into Eras and the subdivision of the Tertiary and Quaternary periods into epochs.

When William Smith and Sir Charles Lyell first recognized that rock strata represented successive time periods, time scales could be estimated only very imprecisely since various kinds of rates of change used in estimation were highly variable. While creationists had been proposing dates of around six or seven thousand years for the age of the Earth based on the Bible, early geologists were suggesting millions of years for geologic periods with some even suggesting a virtually infinite age for the Earth. Geologists and paleontologists constructed the geologic table based on the relative positions of different strata and fossils, and estimated the time scales based on studying rates of various kinds of weathering, erosion, sedimentation, and lithification. Until the discovery of radioactivity in 1896 and the development of its geological applications through radiometric dating during the first half of the 20th century (pioneered by such geologists as Arthur Holmes) which allowed for more precise absolute dating of rocks, the ages of various rock strata and the age of the Earth were the subject of considerable debate.

In 1977, the Global Commission on Stratigraphy (now the International Commission on Stratigraphy) started an effort to define global references (Global Boundary Stratotype Sections and Points) for geologic periods and faunal stages. The commission's most recent work is described in the 2004 geologic time scale of Gradstein et al.. A UML model for how the timescale is structured, relating it to the GSSP, is also available.

Geologic history shown to scale

Below is the geologic history shown to scale.

From the formation of the Earth to today.

The Phanerozoic eon.

The Cainozoic era.

Cryo = Cryogenian

Ed. = Ediacaran

Meso = Mesozoic

C = Cainozoic

Neo = Neogene

Q = Quaternary

P = Pleistocene

H = Holocene

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. To see the table click "show" the the far right on the gray bar with the black line. The height of each table entry does not correspond to the duration of each subdivision of time. (not shown to scale)

Geologic time
Supereon	Eon	Era	Period/Age4,5		Epoch	Major Events	Years Ago3,6
	Phanerozoic	Cainozoic	Quaternary		Holocene	Rise of Human population. Last Ice age ends.	11,700
					Pleistocene	Extinction of many large mammals. Evolution of fully modern humans	2.588 million
			Tertiary	Neogene	Pliocene	The continents drift into their current place. Temperatures cool into Ice age late in the Neogene	5.333 million
					Miocene		23.03 million
				Palaeogene	Oligocene		33.9 million
					Eocene		56 million
					Palaeocene		66 million
		Mesozoic	Cretaceous		Upper Cretaceous	Dinosaurs reach peak, become extinct in K/T extinction event; marsupial and placental mammals appear; first flowering plants	100.5 million
					Lower Cretaceous		145 million
			Jurassic		Upper Jurassic	first birds, early mammals; conifers, cycads and other seed plants. Supercontinent Pangaea breaks up.	163.5 million
					Middle Jurassic		174.1 million
					Lower Jurassic		201.3 million
			Triassic		Upper Triassic	First dinosaurs; pterosaurs; ichthyosaurs; plesiosaurs; 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	Abundant insects, first reptiles, coal forests	323.2 million
					Mississippian	Large primitive trees	358.9 million
			Devonian			First amphibians, 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
Precambrian	Proterozoic	Neoproterozoic2	Ediacaran			First multi-celled animals	635 million
			Cryogenian			Possible Snowball Earth Period.	850 million
			Tonian			Supercontinent Rodinia breaks up.	1,000 million
		Mesoproterozoic				First sexually reproducing organism. The supercontinent Rodinia forms.	1,600 million
		Palaeoproterozoic				First complex single-celled life	2,500 million
	Archaean					Bacteria build stromatolites. 1st supercontinet Vaalbara forms and breaks up.	4,000 million
	Hadean					Formation of Earth 4.6 Gya; formation of Moon 4.5 Gya	4,567 million
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 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.

Proposed Precambrian Timeline

The Geologic Time Scale 2012 book from which the ICS approved the new time scale also included a proposal to revise the Precambrian Timeline.

• Hadean Eon - 4567-4030 MYA
• Archaean Eon - 4030-2420 MYA
• Proterozoic Eon - 2420-541 MYA
  • Palaeoproterozoic Era - 2420-1780 MYA
  • Mesoproterozoic Era - 1780-850 MYA
  • Neoproterozoic Era - 850-541 MYA
    • Cryogenian Period - 850-635 MYA
    • Ediacaran Period - 635-541 MYA

Related pages

• History of the Earth
• Age of the Earth

Other websites

• GSA: Geologic Time Scale
• CHRONOS
• National Museum of Natural History - Geologic Time
• Geological Time Systems - Information model for the geologic time scale
• Exploring Time from Planck Time to the lifespan of the universe

Geologic History

Source

International Chronostratigraphic Chart 2013. International Commission on Stratigraphy, retrieved 8 April 2013. Divisions of geologic time – major chronostratigraphic and geochronologic units USGS, retrieved 8 April 2013.