Historical geology

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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 data presented here agrees with the dates and nomenclature of the International Commission on Stratigraphy.

Evidence from radiometric dating shows 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. Different 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.

Terminology[change | change source]

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[change | change source]

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"[1] 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.[2] 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[3] 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.[4] A UML model for how the timescale is structured, relating it to the GSSP, is also available.[5]

Table of geologic time[change | change source]

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 5.333 million
Miocene Temperatures cool into Ice age late in the Neogene 23.03 million
Palaeogene Oligocene The continents move into their current place. 33.9 million
Eocene The Himalayas are formed during India's collison into Asia. 56 million
Palaeocene India collides into Asia. 66 million
Mesozoic Cretaceous Upper Cretaceous Dinosaurs become extinct in K/T extinction event. 100.5 million
Lower Cretaceous Dinosaurs reach peak. Marsupial and placental mammals appear; first flowering plants 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 Stenian The supercontinent Rodinia forms. 1,200 million
Ectasian First sexually reproducing organism. 1,400 million
Calymmian The supercontinent of Columbia breaks up. 1,600 million
Palaeoproterozoic Statherian Formation of the Columbia (supercontinent) happens during this period. 1,800 million
Orosirian First complex single-celled life 2,050 million
Rhyacian Any oxygen catastrophe triggers the Huronian glaciation in this period. 2,300 million
Siderian The breakup of the supercontinent Kenorland occurs. 2,500 million
Archaean Neoarchaean The supercontinent Kenorland forms. 2,800 million
Mesoarchaean The supercontinet Ur is from this era. 3,200 million
Palaeoarchaean Bacteria build stromatolites. 3,600 million
Eoarchaean 1st supercontinet Vaalbara existed during this era. 4,000 million
Hadean Formation of Earth 4.6 Gya; formation of Moon 4.5 Gya 4,567 million
  1. In North America, the Carboniferous is subdivided into Mississippian and Pennsylvanian sub-periods or epochs.
  2. 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.
  3. 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.
  4. 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.
  5. 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'.
  6. The time shown in the "Years Ago" column is that of the start of the Epoch in the "Epoch" column.

Related pages[change | change source]

References and footnotes[change | change source]

  1. John McPhee, Basin and Range, New York:Farrar, Straus and Giroux, 1981, pp.95-100.
  2. John McPhee, Basin and Range, pp.113-114.
  3. (in Russian) Great Soviet Encyclopedia (3rd ed. ed.). Moscow: Sovetskaya Enciklopediya. 1974. pp. vol. 16, p. 50.
  4. Felix M. Gradstein, James G. Ogg, Alan G. Smith (eds) 2005. A Geologic Time Scale 2004, Cambridge University Press. ISBN 0-521-78673-8
  5. Cox & Richard, A formal model for the geologic time scale and global stratotype section and point, compatible with geospatial information transfer standards, Geosphere, volume 1, pp 119-137, Geological Society of America, 2005

Other websites[change | change source]