thumb|right|450px|This clock representation 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 . 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
geologic time scale is a
chronologic schema (or idealized model) relating
stratigraphy to time that is used by
geologists, paleontologists and other
earth scientists to describe the timing and relationships between events that have occurred during the
history of the Earth. The table of geologic time spans 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 4.570 billion 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
paleontological events, such as
mass extinctions. For example, the boundary between the
Cretaceous period and the
Paleogene 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.
Each era on the scale is separated from the next by a major event or change.
Graphical timelines
The second and third timelines are each subsections of their preceding timeline as indicated by asterisks.
The
Holocene (the latest
epoch) is too short to be shown clearly on this timeline.
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
ages. The terms
eonothem,
erathem, system, series, and stage are used to refer to the layers of rock that correspond to these periods of geologic time.
Geologists qualify these units as Upper or Late, Middle, and Lower or Early. Examples are "Upper
Jurassic" and "Middle
Cambrian". Adjectives of depth,
Upper and
Lower, apply to the rocks themselves, as in "Upper Jurassic
sandstone," while
Late and
Early apply to time, as in "
deposition in the Early Jurassic" or "Early Jurassic climates."
Middle, applies to both. The adjectives are capitalized when the subdivision is formally recognized, and lower case when not; thus "early Miocene" but "Early Jurassic."
Geologic units from the same time but different parts of the world often look different and contain different fossils, so 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 succession of
trilobites. In
East Asia and
Siberia, the same unit is split into
Tommotian,
Atdabanian, and
Botomian stages. 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.
Galactic year
The most common large-scale time scale is millions of years (
Megaannum or
Ma). You might also see
Mya for "millions of years ago." However, for long-term measurements, this still requires rather large numbers. Yet, using the galactic year yields numbers that some people find easier to remember. The
Galactic Year (GY) is the time it takes for the solar system to revolve once around the galactic core, or about 250 Ma.
Using this scale, oceans appeared on Earth after 4 GY, life began at 5 GY, and multicellular organisms first appeared at 15 GY. Dinosaurs went extinct about 0.3 GY ago, and the true age of mammals began about 0.2 GY ago. The age of the Earth is estimated at about 20 GY.
History of the time scale
Animation showing Earth's palaeogeographic reconstruction beginning from early Cambrian period.
Diagram of geological time scale, where the past is toward the bottom of the spiral
In
classical antiquity,
Aristotle saw that
fossil seashells from rocks were similar to those found on the beach and deduced that the fossils were once part of living animals. He reasoned that the positions of land and sea had changed over long periods of time.
Leonardo da Vinci concurred with Aristotle's view that fossils were the remains of ancient life.
The 11th-century
Persian geologist,
Avicenna (Ibn Sina), examined various fossils and deduced that they originated from the
petrifaction of plants and animals. He also first proposed one of the principles underlying geologic time scales: the
law of superposition of strata. While discussing the origins of mountains in
The Book of Healing in 1027, he outlined the principle as follows:
[Stephen Toulmin and June Goodfield (1965), The Ancestry of Science: The Discovery of Time, p. 64, University of Chicago Press (cf. )]His contemporary,
Abu Rayhan Biruni (973-1048), discovered the existence of shells and fossils in regions that once were seas and later became dry land, such as the
Indian subcontinent. Based on this evidence, he realized that the Earth is constantly evolving and proposed that the Earth had an age, but that its origin was too distant to measure. Later in the 11th century, the
Chinese naturalist,
Shen Kuo (1031-1095), also recognized the concept of '
deep time'.
The principles underlying geologic (geological) time scales were later laid down by
Nicholas 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 law 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:
- Sequences of strata were often eroded, distorted, tilted, or even inverted after deposition;
- Strata laid down at the same time in different areas could have entirely different appearances;
- The strata of any given area represented only part of the Earth's long history.
A comparative geological timescale
The first serious attempts to formulate a geological time scale that could be applied anywhere on Earth were made 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 Paleocene-Pliocene) and "Quaternary" (now Pleistocene-Holocene) remained in use as names of geological periods well into the 20th century.
The
Neptunist theories popular at this time (expounded by Werner) proposed that all rocks had precipitated out of a single enormous flood. A major shift in thinking came when
James Hutton presented 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. It has been said that "as things appear from the perspective of the twentieth century, James Hutton in those reading became the founder of modern geology" Hutton proposed 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
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 Mountains. The "Cretaceous" (from Latin
creta meaning '
chalk') as a separate period was first defined by
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.
The first geologic time scale was eventually published in 1913 by the British geologist
Arthur Holmes. He greatly furthered the newly created discipline of
geochronology and published the world renowned book
The Age of the Earth in 1913 in which he estimated the
Earth's age to be at least 1.6 billion years.
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.
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. (See
lunar geologic timescale for a discussion of the geologic subdivisions of Earth's moon.) This table is arranged with the most recent geologic periods at the top, and the most ancient at the bottom. The height of each table entry does not correspond to the duration of each subdivision of time.
See also
References and footnotes
