A period of geologic time from about 4000 to 2500 million years ago (MYA). During the Archean Eon (Fig. 1) a large percentage of the Earth's continental crust formed, plate tectonics began, very warm climates and oceans existed, and life appeared on Earth in the form of unicellular organisms.

An interval of time in Earth history (Cambrian Period) and its rock record (Cambrian System). The Cambrian Period spanned about 60 million years and began with the first appearance of marine animals with mineralized (calcium carbonate, calcium phosphate) shells. The Cambrian System includes many different kinds of marine sandstones, shales, limestones, dolomites, and volcanics. Apart from the occurrence of an alkaline playa containing deposits of trona (hydrated basic sodium carbonate) in the Officer Basin of South Australia, there is very little provable record of nonmarine Cambrian environments.

The interval of geological time, and rocks deposited during that time, from the end of the Devonian Period to the beginning of the Permian Period. The Carboniferous Period spanned from about 359 to 299 million years ago during the Paleozoic Era (Fig. 1). The rocks that formed during this time interval are known as the Carboniferous System; they include a wide variety of sedimentary, igneous, and metamorphic rocks. Sedimentary rocks in the lower portion of the Carboniferous are typically carbonates, such as limestones and dolostones, and locally some evaporites. The upper portions of the system are usually composed of cyclically repeated successions of sandstones, coals, shales, and thin limestones. See also: Sedimentary rocks

The youngest and shortest of the three Phanerozoic geological eras. It represents the geological time (and rocks deposited during that time) extending from the end of the Mesozoic Era to the present day (see illustration).

In geological time, the last period of the Mesozoic Era, preceded by the Jurassic Period and followed by the Tertiary Period. The rocks formed during Cretaceous time constitute the Cretaceous System. Omalius d'Halloys first recognized the widespread chalks of Europe as a stratigraphic unit. W. O. Conybeare and W. Phillips (1822) formally established the period, noting that whereas chalks were remarkably widespread deposits at this time, the Cretaceous System includes rocks of all sorts and its ultimate basis for recognition must lie in its fossil remains. See also: Chalk; Fossil; Jurassic; Rock age determination; Stratigraphy; Tertiary

The geologic period and system of the Paleozoic Era from the end of the Silurian Period to the beginning of the Carboniferous Period. The Devonian Peroid encompasses an interval of geologic time between 419 and 359 million years ago (MYA) based on radiometric data (Fig. 1). The Devonian System encompasses all sedimentary rocks deposited, and all igneous and metamorphic rocks formed, during the Devonian Period. It is conventional that recognition of Devonian time is determined by the definition of Devonian rocks. See also: Paleozoic

The second oldest of the five major worldwide divisions (epochs) of the Tertiary Period; also referred to as the middle epoch of the Paleogene. The Eocene (Fig. 1) extends from 56.0 to 33.9 million years ago (MYA). See also: Tertiary

An ordered, internally consistent, internationally recognized sequence of time intervals, each distinct in its own history and record of life on Earth, including the assignment of absolute time in years to each geologic interval. The geologic time scale (see figure) has two essential components: a relative scale, consisting of named intervals of geologic history arranged in chronologic sequence from oldest (bottom) to youngest (top); and a numerical (or absolute) time scale, providing estimates of absolute ages for the boundaries of these intervals.

The period extending for several hundred million years from the formation of Earth, probably around 4500 million years ago (MYA) to the first known occurrence of a preserved rock record around 4000 MYA. The length of the Hadean is comparable to that of the Phanerozoic, the current geological eon that began approximately 500 million years ago. The absence of an unequivocal rock record has not prevented—indeed, has encouraged—speculation about the nature of the Hadean Earth (see figure). See also: Earth; Earth's age; Rock

That portion of geologic time that postdates the latest episode of continental glaciation. The Holocene Epoch is synonymous with the Recent or Postglacial interval of Earth's geologic history and extends from 10,000 years ago to the present day (Fig. 1). It was preceded by the Pleistocene Epoch and is part of the Quaternary Period, a time characterized by dramatic climatic oscillations from warm (interglacial) to cold (glacial) conditions that began about 1.6 million years ago. The term Holocene is also applied to the sediments, processes, events, and environments of the epoch.