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time scale
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The division of geologic history into era, periods, and epoch accomplished through stratig-raphy and paleontology.
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eon
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"The largest division of geologic time, embracing several Eras, for example, the Phanerozoic, 600 m.y. ago to present); also any span of one billion years. "
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aeon
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see
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era
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A time period including several periods, but smaller than an eon. Commonly recognized eras are Precambrian, Paleozoic, Mesozoic, and Cenozoic.
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epoch
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One subdivision of a geologic period, often chosen to correspond to a stratigraphic series. Also used fo a division of time corresponding to a paleomagnetic interval.
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age
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geologic. The division of geologic time smaller than an epoch.
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formation
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The basic unit for the naming of rocks in stratigraphy
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formation
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A rock formation is a body of rock of considerable extent with distinctive characteristics that allow geologists to map, describe, and name it.
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Precambrian
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The 'unofficial' time period that encompasses all time from the Earth’s formation, 4.55 billion years ago to 570 million years ago, the beginning of the Paleozoic Era.
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Proterozoic
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The 'Precambrian' epoch from 2.5 billion to 570 million years ago
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Archean
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The time interval between 3800-2500 million years ago. The Archean is one of the Precambrian time intervals.
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basement
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The oldest rocks recognized in a given area, a complex of metamorphic and igneous rocks that underlies all the sedimentary formation. Usually Precambrian or Paleozoic in age.
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basement complex
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A series of rocks generally with complex structure beneath the dominantly sedimentary rocks. In many places, these are igneous and metamorphic rocks of either Early or Late Precambrian, but in some places these may be much younger, as Paleozoic, Mesozoic, or even Cenozoic
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basement rocks
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Rocks beneath a large geologic feature (such as the fold-and-thrust belt of a mountain system) which are genetically unrelated to the overlying feature.
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Phanerozoic
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The eon beginning about 570 million years ago and continuing to the present. The portion of Earth history with rocks containing abundant fossil.
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Paleozoic
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An era in geologic time that spans from 570 to 245 million years before the present.
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Cambrian
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A period in the geologic time scale that spans from 570 to 505 million years ago.
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Ordovician
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A period in the geologic time scale that spans from 505 to 438 million years ago.
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Silurian
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A period in the geologic time scale that spans from 438 to 408 million years ago.
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Devonian
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A period in the geologic time scale that spans from 408 to 360 million years ago.
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Mississippian
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A period in the geologic time scale that spans from 360 to 320 million years ago.
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Pennsylvanian
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A period in the geologic time scale that spans from 320 to 286 million years ago.
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Permian
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A period in the geologic time scale that spans from 286 to 245 million years ago.
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Mesozoic
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An era in geologic time that spans from 245 to 65 million years before the present and was the age of the dinosaurs.
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Triassic
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A period in the geologic time scale that spans from 245 to 208 million years ago.
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Jurassic
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A period in the geologic time scale that spans from 208 to 144 million years ago.
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Cretaceous
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A period in the geologic time scale that spans from 144 to 65 million years ago.
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Cenozoic
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An era in geologic time that spans from 65 million years ago to the present.
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Tertiary
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A period in the geologic time scale that spans from 65 to 1.6 million years ago.
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Pliocene
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The latest Epoch of the Tertiary Period, beginning about 5.3 million years ago and ending 1.6 million years ago.
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Paleogene
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The Paleogene period is the interval of geological time from 65 to 24 million years ago. Paleogene is derived from the Greek word for ancient.
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Neogene
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The Neogene period is the interval of geological time from 24 to 1.8 million years ago. Neogene is derived from the Greek word for recent.
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Quaternary
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The latest period of geologic time, covering the most-recent 1.6 million years of the Earth's history. It is divided into two epoch: the Pleistocene -- 1.6 million years ago to approximately 10,000 years ago -- and the Holocene -- the period from approximately 10,000 years ago to the present. The Quaternary period is the artificial division of time separating prehuman and human periods. It contains five ice age and four interglaqcial ages, and temperature indicators seem to show sharp abrupt changes by several degrees.
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Pleistocene
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The earlier of the two periods of the Quaternary period, starting 1.6 million years before the present and ending about 10,000 years ago. It was a time of glacial activity. A vast amount of evidence has been collected that indicates dramatic oscillations in climatic conditions of global proportion throughout the Pleistocene. It has been well documented that global ice mass has undergone huge fluctuations on the order of 10E19 kg (equivalent to more than 100 meters of sea level) during the last million years. In addition, it has been demonstrated that the concentrations of atmospheric CO2 vary by at least 30% through a glacial cycle, with glacial periods characterized by low pCO2 (~190 ppm) and interglacials by high pCO2 (~280 ppm). Ocean temperature and circulation have also undergone glacial-interglacial change.
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Miocene
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An Epoch that includes the time interval of about 23.7 to 5.3 million years ago.
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Holocene
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The most recent epoch of the Quaternary period covering approximately the last 10,000 years.
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Anthropocene
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New proposed epoch covering human geological impact on the Earth
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integrated time scale (Eocene)
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"The Eocene rock record initially was and still is recognized by its characteristic fossil; fossils are cross-checked against the geomagnetic reversal scale. But rates of change and other quantifications demand numerical calibration -- time in millions of years -- which is founded ultimately in isotopic decay in the potassium-argon and other decay series. The geomagnetic scale consists of chrons, of which Chron C24 and Chron C19 are important here. Some fossil succession have their own names; thus, e.g., the Wasatchian North American land mammal age."
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geochronology
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The science of absolute dating and relative dating of geologic formation and events, primarily through the measurement of daughter element produced by radioactive decay in mineral.
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radiometric age
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The approximate age of a geologic event, feature, fossil, or rock in years. Radiometric ages, sometimes termed 'absolute' ages, are determined by using natural radioactive 'clocks'. See radiocarbon dating.
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radiocarbon dating
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"The age of organic material determined by the amounts of carbon isotope 12, 13 and 14. The ratio of 12 to 14 is about the same in all living things but when a plant or animal dies, no more carbon is taken on. Carbon 12 and 13 are stable isotopes and the amounts remain the same even in dead material. Carbon 14 is an radioactive isotope that decays radioactively until none is left; . Thus, the ratio records the time elapsed since death. Since carbon 14 decays relatively rapidly, the method is only reliable for the last 40,000 years. See radiometric age."
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radiometric dating
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A dating method that uses measurements of certain radioactive isotope to calculate the age in years (absolute age) of rocks and mineral.
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isotope geology
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The study of the relative abundance of isotope in rocks to determine their age (see geochronology) or conditions of formation.
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isotopic age
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Age of a rock or mineral that is determined by isotopic methods in which the initial isotopic ratio of the sample assumed.
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absolute date
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An estimate of the true age of a mineral or rock based on the rate of decay of radioactive minerals.
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absolute time
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Geological time measured in a specific number of years.
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varve
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"A layer of sediment deposited in lakes during one year. Each layer consists of two parts, grading upward from coarse to fine and light to dark, which are deposited at different seasons and which differ in color and texture; thus the layers can be counted and measured. In a complete series the number of layers gives the date on which the ground was vacated by retreating ice."
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dendrogeomorphology
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Application of tree-ring studies (dendrochronology) to the interpretation of geomorphologic processes. Often supplemented by related botanical observations.
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lichenometry
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A dating method that uses the growth rate of certain lichen species as an indicator of the age of the surface the lichen is growing on.
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relative dating
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The process of placing rocks and geologic structure in the correct chronological order. This process does not yield age in number of years. See radiometric dating.
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quantitative stratigraphy
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Use of computer-based mathematical methods to calibrate stratigraphic models that with a minimum of information provide maximum predictive potency and include formulation of confidence limits.
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paleoclimatology
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The study of how global climate has changed through time.
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paleoclimate
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The average state or typical conditions of climate during some past geologic period.
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