Geochronometry is a branch of

stratigraphy Stratigraphy is a branch of geology concerned with the study of rock (geology), rock layers (Stratum, strata) and layering (stratification). It is primarily used in the study of sedimentary rock, sedimentary and layered volcanic rocks. Stratigrap ...

aimed at the quantitative measurement of geologic

time Time is the continued sequence of existence and events that occurs in an apparently irreversible succession from the past, through the present, into the future. It is a component quantity of various measurements used to sequence events, to ...

. It is considered a branch of

geochronology Geochronology is the science of determining the age of rocks, fossils, and sediments using signatures inherent in the rocks themselves. Absolute geochronology can be accomplished through radioactive isotopes, whereas relative geochronology is pr ...

Brief history

The measurement of geologic

is a long-standing problem of

geology Geology () is a branch of natural science concerned with Earth and other astronomical objects, the features or rocks of which it is composed, and the processes by which they change over time. Modern geology significantly overlaps all other Ear ...

Hallam, Anthony Anthony Hallam, aka Tony Hallam, (23 December 1933 – 23 October 2017) was a British geologist, palaeontologist and writer. His research interests concentrated on the Jurassic Period, with particular reference to stratigraphy, sea level c ...

, 1983 – ''Great geological controversies''. Oxford University Press, Oxford, UK. When geology was at its beginnings, a major problem for stratigraphers was to find a reliable method for the measurement of

. In the eighteenth century, and during most of the nineteenth century, the ideas on the geologic time were indeed so controversial that the estimates for the

age of the Earth The age of Earth is estimated to be 4.54 ± 0.05 billion years This age may represent the age of Earth's accretion, or core formation, or of the material from which Earth formed. This dating is based on evidence from radiometric age-dating of ...

encompassed the whole range from ca. 6000 years to 300 million years. The longer estimate came from

Charles Darwin Charles Robert Darwin ( ; 12 February 1809 – 19 April 1882) was an English naturalist, geologist, and biologist, widely known for his contributions to evolutionary biology. His proposition that all species of life have descended fr ...

, who probably went closer to the truth because he had clear in mind that the

evolution Evolution is change in the heritable characteristics of biological populations over successive generations. These characteristics are the expressions of genes, which are passed on from parent to offspring during reproduction. Variation ...

of life must have required a lot of time to take place. The current estimate of the age of the Earth is ca. 4500 million years. The solution of the dating problem arrived only with the discovery that some natural elements undergo a continuous decay. This led to the first

radiometric dating Radiometric dating, radioactive dating or radioisotope dating is a technique which is used to date materials such as rocks or carbon, in which trace radioactive impurities were selectively incorporated when they were formed. The method compares t ...

s by Boltwood and Strutt. Today, the determination of the age of the Earth is not a primary scope of geochronometry anymore, and most efforts are rather aimed at obtaining increasingly precise radiometric datings. At the same time, other methods for the measurement of time were developed, so the quantification of geologic time can now be endeavored with a variety of approaches.

Radiometric dating

All methods based on the

radioactive decay Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration, or nuclear disintegration) is the process by which an unstable atomic nucleus loses energy by radiation. A material containing unstable nuclei is consid ...

belong to this category. The principle at the base of

is that natural unstable

isotope Isotopes are two or more types of atoms that have the same atomic number (number of protons in their nuclei) and position in the periodic table (and hence belong to the same chemical element), and that differ in nucleon numbers (mass numbers) ...

s, called 'parent isotopes', decay to some isotope which is instead stable, called the 'daughter isotope'. Under the assumptions that: (1) the initial amount of parent and daughter isotopes can be estimated, and (2) after the geologic material formed, parent and daughter isotopes did not escape the system, the age of the material can be obtained from the measurement of isotope concentrations, through the laws of radioactive decay. Methods of this kind are usually identified with the names of the parent/daughter elements. The radiometric methods under this category are: * U/Pb * U/Th * K-Ar (and Ar-Ar) * Rb/Sr * Sm/Nd * Re/Os * Lu/Hf Each of these methods perform better in different time ranges and has different limitations. However, uranium–lead dating on

zircon Zircon () is a mineral belonging to the group of nesosilicates and is a source of the metal zirconium. Its chemical name is zirconium(IV) silicate, and its corresponding chemical formula is Zr SiO4. An empirical formula showing some of the r ...

Hanchar J.M, Hoskin P.W.O. (editors), 2003, Zircon. Reviews in Mineralogy and Geochemistry, v. 53, Mineralogical Society of America. ISSN 1529-6466 and Argon-argon dating on

sanidine Sanidine is the high temperature form of potassium feldspar with a general formula K(AlSi3O8). Sanidine is found most typically in felsic volcanic rocks such as obsidian, rhyolite and trachyte. Sanidine crystallizes in the monoclinic crystal system ...

and

hornblende Hornblende is a complex inosilicate series of minerals. It is not a recognized mineral in its own right, but the name is used as a general or field term, to refer to a dark amphibole. Hornblende minerals are common in igneous and metamorphic rocks ...

are the two single methods that achieve today the best results.Erwin D.H., 2006, Dates and Rates: Temporal resolution in the deep time stratigraphic record. Annual Review of Earth and Planetary Sciences, v. 34, p. 569-590. Other methods of

are also available, that are based on slightly or largely different principles, but always rely on the phenomenon of

. These alternative radiometric methods are: * 14C, or

radiocarbon Carbon-14, C-14, or radiocarbon, is a radioactive isotope of carbon with an atomic nucleus containing 6 protons and 8 neutrons. Its presence in organic materials is the basis of the radiocarbon dating method pioneered by Willard Libby and coll ...

Fission track dating Fission track dating is a radiometric dating technique based on analyses of the damage trails, or tracks, left by fission fragments in certain uranium-bearing minerals and glasses. Fission-track dating is a relatively simple method of radiomet ...

* Optical luminescence dating and

Thermoluminescence dating Thermoluminescence dating (TL) is the determination, by means of measuring the accumulated radiation dose, of the time elapsed since material containing crystalline minerals was either heated (lava, ceramics) or exposed to sunlight (sediments ...

* Cosmic ray exposure dating These methods, especially

, are particularly reliable for recent samples, but are much less accurate for deep geologic time. More specifically, radiocarbon becomes unreliable already for samples >50000 years old.

Incremental dating

These methods are based on the building of incremental chronologies from a point of known age, which is usually the present. When a chronology is not tied to such a known age point, it is called a ''floating chronology''. Incremental dating methods include: *

Dendrochronology Dendrochronology (or tree-ring dating) is the scientific method of dating tree rings (also called growth rings) to the exact year they were formed. As well as dating them, this can give data for dendroclimatology, the study of climate and atmos ...

Lichenometry In archaeology, palaeontology, and geomorphology, lichenometry is a geomorphic method of geochronologic dating that uses lichen growth to determine the age of exposed rock, based on a presumed specific rate of increase in radial size over time.L ...

Sclerochronology Sclerochronology is the study of periodic physical and chemical features in the hard tissues of animals that grow by accretion, including invertebrates and coralline red algae, and the temporal context in which they formed. It is particularly usefu ...

mollusc Mollusca is the second-largest phylum of invertebrate animals after the Arthropoda, the members of which are known as molluscs or mollusks (). Around 85,000 extant species of molluscs are recognized. The number of fossil species is esti ...

shells or

coral Corals are marine invertebrates within the class Anthozoa of the phylum Cnidaria. They typically form compact colonies of many identical individual polyps. Coral species include the important reef builders that inhabit tropical oceans and sec ...

skeletons *

Varve A varve is an annual layer of sediment or sedimentary rock. The word 'varve' derives from the Swedish word ''varv'' whose meanings and connotations include 'revolution', 'in layers', and 'circle'. The term first appeared as ''Hvarfig lera'' (var ...

chronology (e.g., in

lake A lake is an area filled with water, localized in a basin, surrounded by land, and distinct from any river or other outlet that serves to feed or drain the lake. Lakes lie on land and are not part of the ocean, although, like the much large ...

sediments or

ice cores An ice core is a core sample that is typically removed from an ice sheet or a high mountain glacier. Since the ice forms from the incremental buildup of annual layers of snow, lower layers are older than upper ones, and an ice core contains ic ...

) * Lamina counting in

speleothem A speleothem (; ) is a geological formation by mineral deposits that accumulate over time in natural caves. Speleothems most commonly form in calcareous caves due to carbonate dissolution reactions. They can take a variety of forms, depending on ...

s *

Cyclostratigraphy Cyclostratigraphy is a subdiscipline of stratigraphy that studies astronomically forced climate cycles within sedimentary successions. Orbital changes Astronomical cycles (also known as Milankovitch cycles) are variations of the Earth's or ...

based on Milankovitch cycles

Geologic time scale

A major achievement of geochronometry is the documentation of geologic

, as represented in geologic time scales. A

geologic time scale The geologic time scale, or geological time scale, (GTS) is a representation of time based on the rock record of Earth. It is a system of chronological dating that uses chronostratigraphy (the process of relating strata to time) and geochrono ...

is a scheme that integrates the

geochronologic Geochronology is the science of determining the age of rocks, fossils, and sediments using signatures inherent in the rocks themselves. Absolute geochronology can be accomplished through radioactive isotopes, whereas relative geochronology is pr ...

subdivisions of geologic time and their absolute ages and durations. The latest version of the geologic time scale was published in 2004,Gradstein F.M., Ogg J.G. and Smith A.G., 2004, A Geologic Time Scale 2004, Cambridge University Press, Cambridge, UK. and includes a comparison of present and past time scales. The greater efforts of geochronometry today are aimed at retrieving accurate ages of major events in the Earth's history and of

stage Stage or stages may refer to: Acting * Stage (theatre), a space for the performance of theatrical productions * Theatre, a branch of the performing arts, often referred to as "the stage" * ''The Stage'', a weekly British theatre newspaper * Sta ...

age Age or AGE may refer to: Time and its effects * Age, the amount of time someone or something has been alive or has existed ** East Asian age reckoning, an Asian system of marking age starting at 1 * Ageing or aging, the process of becoming older ...

boundaries.

References

External links