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<h2 id="dating-methods">Dating methods</h2> <a href="/facts/Geologic_time_scale/vBAm0D8Q">Units in geochronology and stratigraphy</a><a class="footnote-ref" id="fnref:1" href="#fn:1"><sup>1</sup></a><table><tbody><tr><th>Segments of rock (<a href="/facts/Stratum/w5D4VAwa">strata</a>) in <a href="/facts/Chronostratigraphy/L3XU1Uh3">chronostratigraphy</a></th><th>Time spans in geochronology</th><th>Notes togeochronological units</th></tr><tr><td><a href="/facts/Eonothem/bQhsV7sg">Eonothem</a></td><td><a href="/facts/Geologic_time_scale/vBAm0D8Q">Eon</a></td><td>4 total, half a billion years or more</td></tr><tr><td><a href="/facts/Erathem/IGVtnJ3d">Erathem</a></td><td><a href="/facts/Geologic_time_scale/vBAm0D8Q">Era</a></td><td>10 defined, several hundred million years</td></tr><tr><td><a href="/facts/System_(stratigraphy)/eEt8kSgg">System</a></td><td><a href="/facts/Geologic_time_scale/vBAm0D8Q">Period</a></td><td>22 defined, tens to ~one hundred million years</td></tr><tr><td><a href="/facts/Series_(stratigraphy)/KgmTDDy5">Series</a></td><td><a href="/facts/Geologic_time_scale/vBAm0D8Q">Epoch</a></td><td>34 defined, tens of millions of years</td></tr><tr><td><a href="/facts/Stage_(stratigraphy)/6HActMK7">Stage</a></td><td><a href="/facts/Geologic_time_scale/vBAm0D8Q">Age</a></td><td>99 defined, millions of years</td></tr><tr><td><a href="/facts/Chronozone/SCqGb2Bs">Chronozone</a></td><td><a href="/facts/Polarity_chron/k3tAxhpq">Chron</a></td><td>subdivision of an age, not used by the ICS timescale</td></tr></tbody></table> <h3>Radiometric dating</h3> <p class="note">Main article: <a href="/facts/Radiometric_dating/bDFjdPqV">Radiometric dating</a></p> <p>By measuring the amount of <a href="/facts/Radioactive_decay/Ya6FVjt4">radioactive decay</a> of a <a href="/facts/Radioactive_isotope/5WpCfbdq">radioactive isotope</a> with a known <a href="/facts/Half-life/kGVH8BAB">half-life</a>, geologists can establish the absolute age of the parent material. A number of radioactive isotopes are used for this purpose, and depending on the rate of decay, are used for dating different geological periods. More slowly decaying isotopes are useful for longer periods of time, but less accurate in absolute years. With the exception of the <a href="/facts/Radiocarbon_dating/qjNglhKP">radiocarbon method</a>, most of these techniques are actually based on measuring an increase in the abundance of a <a href="/facts/Radiogenic/KXP6MKk6">radiogenic</a> isotope, which is the decay-product of the radioactive parent isotope.<a class="footnote-ref" id="fnref:2" href="#fn:2"><sup>2</sup></a><a class="footnote-ref" id="fnref:3" href="#fn:3"><sup>3</sup></a><a class="footnote-ref" id="fnref:4" href="#fn:4"><sup>4</sup></a> Two or more radiometric methods can be used in concert to achieve more robust results.<a class="footnote-ref" id="fnref:5" href="#fn:5"><sup>5</sup></a> Most radiometric methods are suitable for geological time only, but some such as the radiocarbon method and the 40Ar/39Ar dating method can be extended into the time of early human life<a class="footnote-ref" id="fnref:6" href="#fn:6"><sup>6</sup></a> and into recorded history.<a class="footnote-ref" id="fnref:7" href="#fn:7"><sup>7</sup></a> </p><p>Some of the commonly used techniques are: </p> <ul><li><a href="/facts/Radiocarbon_dating/qjNglhKP">Radiocarbon dating</a>. This technique measures the decay of <a href="/facts/Carbon-14/Ut8SBNNa">carbon-14</a> in organic material and can be best applied to samples younger than about 60,000 years.<a class="footnote-ref" id="fnref:8" href="#fn:8"><sup>8</sup></a><a class="footnote-ref" id="fnref:9" href="#fn:9"><sup>9</sup></a></li> <li><a href="/facts/Uranium%E2%80%93lead_dating/LMCJBhrQ">Uranium–lead dating</a>. This technique measures the ratio of two lead <a href="/facts/Isotope/KD9B1y6o">isotopes</a> (lead-206 and lead-207) to the amount of uranium in a mineral or rock. Often applied to the trace mineral <a href="/facts/Zircon/84APfbLt">zircon</a> in <a href="/facts/Igneous_rock/xeMJhUah">igneous rocks</a>, this method is one of the two most commonly used (along with <a href="/facts/Argon%E2%80%93argon_dating/kALXQu7H">argon–argon dating</a>) for geologic dating. <a href="/facts/Monazite_geochronology/GS7uzB6j">Monazite geochronology</a> is another example of U–Pb dating, employed for dating metamorphism in particular. Uranium–lead dating is applied to samples older than about 1 million years.</li> <li><a href="/facts/Uranium%E2%80%93thorium_dating/r0UN290Y">Uranium–thorium dating</a>. This technique is used to date <a href="/facts/Speleothem/mwwTnySG">speleothems</a>, <a href="/facts/Coral/j7fBLrpw">corals</a>, <a href="/facts/Carbonate/FES1QHkN">carbonates</a>, and fossil <a href="/facts/Bone/kwJaR6IQ">bones</a>. Its range is from a few years to about 700,000 years.</li> <li><a href="/facts/Potassium%E2%80%93argon_dating/JQdWfdIB">Potassium–argon dating</a> and <a href="/facts/Argon%E2%80%93argon_dating/kALXQu7H">argon–argon dating</a>. These techniques date <a href="/facts/Metamorphic_rock/JRp6dsCm">metamorphic</a>, <a href="/facts/Igneous_rock/xeMJhUah">igneous</a> and <a href="/facts/Volcanic_rock/ID3oPwtM">volcanic</a> rocks. They are also used to date <a href="/facts/Volcanic_ash/c3cqgBfV">volcanic ash</a> layers within or overlying <a href="/facts/Paleoanthropology/Umjg0sUn">paleoanthropologic</a> sites. The younger limit of the argon–argon method is a few thousand years.</li> <li><a href="/facts/Electron_spin_resonance/TDHV8c9e">Electron spin resonance</a> (ESR) dating</li></ul> <h3>Fission-track dating</h3> <p class="note">Main article: <a href="/facts/Fission_track_dating/IlqwpvuV">Fission track dating</a></p> <h3>Cosmogenic nuclide geochronology</h3> <p class="note">Main article: <a href="/facts/Cosmogenic_radionuclide_dating/HKFzxr4x">Cosmogenic radionuclide dating</a></p> <p>A series of related techniques for determining the age at which a geomorphic surface was created (<a href="/facts/Cosmogenic_dating/HKFzxr4x">exposure dating</a>), or at which formerly <a href="/facts/Surficial_deposit/lvPgpbWL">surficial materials</a> were buried (burial dating).<a class="footnote-ref" id="fnref:10" href="#fn:10"><sup>10</sup></a> Exposure dating uses the concentration of exotic nuclides (e.g. 10Be, 26Al, 36Cl) produced by cosmic rays interacting with Earth materials as a proxy for the age at which a surface, such as an alluvial fan, was created. Burial dating uses the differential radioactive decay of 2 cosmogenic elements as a proxy for the age at which a sediment was screened by burial from further cosmic rays exposure. </p> <h3>Luminescence dating</h3> <p>Luminescence dating techniques observe 'light' emitted from materials such as quartz, diamond, feldspar, and calcite. Many types of luminescence techniques are utilized in geology, including <a href="/facts/Optically_stimulated_luminescence/EPr2uVn4">optically stimulated luminescence</a> (OSL), <a href="/facts/Cathodoluminescence/qqCdFxtQ">cathodoluminescence</a> (CL), and <a href="/facts/Thermoluminescence/v8qxKI1n">thermoluminescence</a> (TL).<a class="footnote-ref" id="fnref:11" href="#fn:11"><sup>11</sup></a> Thermoluminescence and optically stimulated luminescence are used in archaeology to date 'fired' objects such as pottery or cooking stones and can be used to observe sand migration. </p> <h3>Incremental dating</h3> <p class="note">Main article: <a href="/facts/Incremental_dating/rzKf8jBU">Incremental dating</a></p> <p><a href="/facts/Incremental_dating/rzKf8jBU">Incremental dating</a> techniques allow the construction of year-by-year annual chronologies, which can be fixed (<i>i.e.</i> linked to the present day and thus <a href="/facts/Calendar/mgY3sm8P">calendar</a> or <a href="/facts/Sidereal_time/2JAVf8dy">sidereal time</a>) or floating. </p> <ul><li><a href="/facts/Dendrochronology/bXK8Km2M">Dendrochronology</a></li> <li><a href="/facts/Ice_core/QnzraAoN">Ice cores</a></li> <li><a href="/facts/Lichenometry/tAoGXsJs">Lichenometry</a></li> <li><a href="/facts/Varve/qxN9be5Q">Varves</a></li></ul> <h3>Paleomagnetic dating</h3> <p>A sequence of <a href="/facts/Paleomagnetic_pole/k3lv5XaP">paleomagnetic poles</a> (usually called virtual geomagnetic poles), which are already well defined in age, constitutes an <a href="/facts/Apparent_polar_wander/2tzchSzD">apparent polar wander</a> path (APWP). Such a path is constructed for a large continental block. APWPs for different continents can be used as a reference for newly obtained poles for the rocks with unknown age. For paleomagnetic dating, it is suggested to use the APWP in order to date a pole obtained from rocks or sediments of unknown age by linking the paleopole to the nearest point on the APWP. Two methods of paleomagnetic dating have been suggested: (1) the angular method and (2) the rotation method.<a class="footnote-ref" id="fnref:12" href="#fn:12"><sup>12</sup></a> The first method is used for paleomagnetic dating of rocks inside of the same continental block. The second method is used for the folded areas where tectonic rotations are possible. </p> <h3>Magnetostratigraphy</h3> <p class="note">Main article: <a href="/facts/Magnetostratigraphy/k3tAxhpq">Magnetostratigraphy</a></p> <p><a href="/facts/Magnetostratigraphy/k3tAxhpq">Magnetostratigraphy</a> determines age from the pattern of magnetic polarity zones in a series of bedded sedimentary and/or volcanic rocks by comparison to the magnetic polarity timescale. The polarity timescale has been previously determined by dating of seafloor magnetic anomalies, radiometrically dating volcanic rocks within magnetostratigraphic sections, and astronomically dating magnetostratigraphic sections. </p> <h3>Chemostratigraphy</h3> <p>Global trends in isotope compositions, particularly carbon-13 and strontium isotopes, can be used to correlate strata.<a class="footnote-ref" id="fnref:13" href="#fn:13"><sup>13</sup></a> </p> <h3>Correlation of marker horizons</h3> <p><a href="/facts/Marker_horizon/w3IvG631">Marker horizons</a> are stratigraphic units of the same age and of such distinctive composition and appearance that, despite their presence in different geographic sites, there is certainty about their age-equivalence. Fossil faunal and floral <a href="/facts/Fossil/by7NzIA1">assemblages</a>, both marine and terrestrial, make for distinctive marker horizons.<a class="footnote-ref" id="fnref:14" href="#fn:14"><sup>14</sup></a> <a href="/facts/Tephrochronology/bWwMKSMu">Tephrochronology</a> is a method for geochemical correlation of unknown volcanic ash (tephra) to geochemically fingerprinted, dated <a href="/facts/Tephra/fYHmdLu1">tephra</a>. Tephra is also often used as a dating tool in archaeology, since the dates of some eruptions are well-established. </p> <h2 id="geological-hierarchy-of-chronological-periodization">Geological hierarchy of chronological periodization</h2> <p>Geochronology, from largest to smallest: </p> <ol><li><a href="/facts/Eon_(geology)/vBAm0D8Q">Eon</a></li> <li><a href="/facts/Era_(geology)/0gqadCDc">Era</a></li> <li><a href="/facts/Period_(geology)/6LBR11yQ">Period</a></li> <li><a href="/facts/Epoch_(geology)/RhfBWrP2">Epoch</a></li> <li><a href="/facts/Age_(geology)/qFToW7mN">Age</a></li> <li><a href="/facts/Polarity_chron/k3tAxhpq">Chron</a></li></ol> <h2 id="differences-from-chronostratigraphy">Differences from chronostratigraphy</h2> <p>It is important not to confuse geochronologic and chronostratigraphic units.<a class="footnote-ref" id="fnref:15" href="#fn:15"><sup>15</sup></a> Geochronological units are periods of time, thus it is correct to say that <i><a href="/facts/Tyrannosaurus/HQw1cywX">Tyrannosaurus</a> rex</i> lived during the Late <a href="/facts/Cretaceous/nwKtFtKx">Cretaceous</a> Epoch.<a class="footnote-ref" id="fnref:16" href="#fn:16"><sup>16</sup></a> Chronostratigraphic units are geological material, so it is also correct to say that fossils of the genus <i>Tyrannosaurus</i> have been found in the Upper Cretaceous Series.<a class="footnote-ref" id="fnref:17" href="#fn:17"><sup>17</sup></a> In the same way, it is entirely possible to go and visit an Upper Cretaceous Series deposit – such as the <a href="/facts/Hell_Creek/eMYgR66e">Hell Creek</a> deposit where the <i>Tyrannosaurus</i> fossils were found – but it is naturally impossible to visit the Late Cretaceous Epoch as that is a period of time. </p> <h2 id="see-also">See also</h2> <ul><li><a href="/facts/Astronomical_chronology/vQhPjman">Astronomical chronology</a> <ul><li><a href="/facts/Age_of_Earth/MxNh2Q11">Age of Earth</a></li> <li><a href="/facts/Age_of_the_universe/BaByputq">Age of the universe</a></li></ul></li> <li><a href="/facts/Chronological_dating/xYYqj6qK">Chronological dating</a>, archaeological chronology <ul><li><a href="/facts/Absolute_dating/qYHIGBEu">Absolute dating</a></li> <li><a href="/facts/Relative_dating/PyvmNRU6">Relative dating</a></li> <li><a href="/facts/Phase_(archaeology)/2wKHluC8">Phase (archaeology)</a></li> <li><a href="/facts/Archaeological_association/5axlcVQ4">Archaeological association</a></li></ul></li> <li>Geochronology <ul><li><a href="/facts/Closure_temperature/9kFe21m6">Closure temperature</a></li> <li><a href="/facts/Geologic_time_scale/vBAm0D8Q">Geologic time scale</a></li> <li><a href="/facts/Geological_history_of_Earth/Jx7ClmF4">Geological history of Earth</a></li> <li><a href="/facts/Thermochronology/Pf6c83VM">Thermochronology</a></li> <li><a href="/facts/List_of_geochronologic_names/x69zhphS">List of geochronologic names</a></li></ul></li> <li>General <ul><li><a href="/facts/Consilience/W2oKy2XX">Consilience</a>, evidence from independent, unrelated sources can "converge" on strong conclusions</li></ul></li></ul> <h2 id="further-reading">Further reading</h2> <ul><li>Smart, P.L., and Frances, P.D. (1991), <i>Quaternary dating methods - a user's guide</i>. Quaternary Research Association Technical Guide No.4 <a href="/facts/ISBN_(identifier)/15AdSPa9">ISBN</a> 0-907780-08-3</li> <li>Lowe, J.J., and Walker, M.J.C. (1997), <i>Reconstructing Quaternary Environments</i> (2nd edition). Longman publishing <a href="/facts/ISBN_(identifier)/15AdSPa9">ISBN</a> 0-582-10166-2</li> <li>Mattinson, J. M. (2013), <i>Revolution and evolution: 100 years of U-Pb geochronology</i>. Elements 9, 53–57.</li> <li><i>Geochronology bibliography</i> <a href="http://www.talkorigins.org/origins/biblio/geochronology.html">Talk:Origins Archive</a></li></ul> <h2 id="external-links">External links</h2> <ul><li><a href="https://portal.ga.gov.au/persona/geochronology">Geochronology and Isotopes Data Portal </a></li> <li><a href="http://stratigraphy.org/">International Commission on Stratigraphy </a></li> <li><a href="http://data.bgs.ac.uk/home.html">BGS Open Data </a> <a href="http://trimc-nlp.blogspot.com/2013/09/ontology-models-for-geochronology-and.html">Geochronological Ontologies</a></li></ul> <h2 id="references">References</h2> <ol> <li id="fn:1"><p>Cohen, K.M.; Finney, S.; Gibbard, P.L. (2015), International Chronostratigraphic Chart (PDF), International Commission on Stratigraphy. <a href="http://www.stratigraphy.org/ICSchart/ChronostratChart2015-01.pdf" target="_blank">http://www.stratigraphy.org/ICSchart/ChronostratChart2015-01.pdf</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></p></li> <li id="fn:2"><p>Dickin, Alan P. (2000). Radiogenic isotope geology (1 ed.). Cambridge: Cambridge Univ. Press. ISBN 978-0-521-59891-0. <a href="978-0-521-59891-0" target="_blank">978-0-521-59891-0</a> <a href="#fnref:2" class="footnote-back-ref">↩</a></p></li> <li id="fn:3"><p>Faure, Gunter (1986). Principles of isotope geology (2 ed.). New York: Wiley. ISBN 978-0-471-86412-7. <a href="978-0-471-86412-7" target="_blank">978-0-471-86412-7</a> <a href="#fnref:3" class="footnote-back-ref">↩</a></p></li> <li id="fn:4"><p>Faure, Gunter; Mensing, Teresa M.; Faure, Gunter (2005). Isotopes: principles and applications (3 ed.). Hoboken, N.J: Wiley. 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