TIME Glossary


note: The following is taken from several on-line sources, chiefly Illustrated Glossary of Geologic Terms by Steven M. Richardson, professor of geology at Iowa State University (based on the glossary in Earth: An Introduction to Geologic Change, by S. Judson and S.M. Richardson (Englewood Cliffs, NJ, Prentice Hall, 1995); definitions conform generally, and in some cases specifically, to definitions given in Robert L Bates and Julia A Jackson (editors), Glossary of Geology, 3rd ed., American Geological Institute, Alexandria, Virginia, 1987.), and Radiometric Dating A Christian Perspective Dr. Roger C. Wiens Division of Geological & Planetary Sciences California Institute of Technology reprinted from Graham Kendell archives. Also a few entries from including Wesley R. Elsberry's jargon FAQOriginal definitions by yours truly are initialed thus: "MAK"

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Absolute age The age of an object as established by some precise dating method, such as radiometric dating (compare with relative age. (Fred Bervoets - Dinodata glossary )

Absolute dating A means of estimating the age of rocks with some degree of accuracy using measurements of radioactive isotopes. (Fred Bervoets - Dinodata glossary )

Alpha decay The process of radioactive decay in which the nucleus of an atom emits an alpha particle. The new atom's atomic number is lower by two and its atomic mass number is reduced by four. (S.M. Richardson)

Alpha particle A particle consisting of two protons and two neutrons, produced during alpha decay. Identical to the nucleus of a 4He atom. (S.M. Richardson)

Archean An eon of geologic time extending from about 3.9 billion years to 2.5 billion years ago. (S.M. Richardson)

Archean The second oldest eon in Earth history. It followed the Hadean, and lasted from around 3.8 to 2.5 billion years ago. During this time prokaryote life appeared and flourished, and the Earth was characterised by a reducing (anaerobic) atmosphere. The Archean was succeeded by the Proterozoic eon. (MAK)

Atom The smallest unit that has the chemical characteristics of a particular chemical element. It is about ten billionths of an inch in diameter and is made of a dense central nucleus of protons and neutrons, surrounded by a cloud of electrons. An atom may share electrons with an adjacent atom or atoms; the result being a molecule. (MAK)

Atomic mass number The sum of the number of protons and neutrons in an atom. Approximately equal to the mass of the atom. (S.M. Richardson)

Atomic number The number of protons in an atom, a quantity that determines which element the atom represents. Example: all atoms of oxygen have 8 protons. (S.M. Richardson)


Beta decay The process of radioactive decay in which a neutron loses a beta particle (i.e. an electron). This increases the atomic number of the atom by one by turning the neutron into a proton. The atom's atomic mass number stays the same because the total number of protons and neutrons remain the same. The most common form of radioactive decay. (S.M. Richardson)

Biostratigraphy branch of stratigraphy which focuses on correlating and assigning relative ages of rock strata by using the fossil assemblages contained within them. (Wikipedia glossary)

Biostratigraphic zone unit of geologic time defined by the presence of one or more fossil species. Interval zones use the range of single species, assemblage zones use the ranges of a group of species, while abundance zones use maximum or minimum abundances instead of just presence and absence data. (University of Arizona Geosciences 308 Paleontology glossary)


14C method A method for determining the age in years of organic matter by calculating the amount of radioactive carbon still remaining, as compared to the stable isotope, 12C. (S.M. Richardson)

Carbon-14 See Radiocarbon

Cenozoic "Recent Animal Life". The third and current (most recent) geologic era of the Phanerozoic Eon, which began 65.5 million years ago; colloquially referred to the "Age of Mammals". Mammals and birds are the dominant vertebrates on land, teleost fish in the sea, and angiosperms (flowering plants) and in certain areas conifers the predominant form of plant life. (MAK)

Closed system A system (rock, planet, etc.) which has no influence or exchange with the outside world. In reality there is always some exchange or influence, but if this amount is completely insignificant for the process under consideration (e.g., for dating, if the loss or gain of atoms is insignificant) for practical purposes the system can be considered closed. (R.C. Wiens)

Cosmic rays Very high energy particles which fly through space. They are stopped by the earth's atmosphere, but in the process, they constantly produce carbon-14, beryllium-10, chlorine-36, and a few other radioactive isotopes in small quantities. Cosmogenic Produced by bombardment of cosmic rays. Carbon-14 is said to be cosmogenic because it is produced by cosmic rays. (R.C. Wiens)

Correlation Process of establishing contemporaneity of rocks or events in one area with rocks or events in another area. (S.M. Richardson)

Cross-cutting relationships Geologic discontinuities that suggest relative ages: A geologic feature is younger than the feature it cuts. Thus, a fault cutting across a rock is younger than the rock. (S.M. Richardson)


Daughter The element or isotope which is produced by radioactive decay. (R.C. Wiens)

Decay The change from one element or isotope to another. Only certain isotopes decay. The rest are said to be stable. (R.C. Wiens)

Decay rate The rate at which a population of radioactive atoms decays into stable daughter atoms. Rate often expressed in terms of half life of the parent isotope. (S.M. Richardson)


Early In the geological timescale, the chronological equivalent to Lower. Often refers to a subdivision or epoch of a period, especially when the period is divided into three; e.g. "Early Jurassic". (MAK)

Ediacaran most recent period of the Proterozoic era, characterised by the appearance of both enigmatic Vendobionta and trace fossils that seem to pertain to more conventional organisms. The term Ediacaran was replaced for a while by Vendian, but now it seems that Ediacaran is back in fashion. (MAK)

Electron A fundamental unit of matter, negatively charged and disposed in a cloud surrounding the nucleus of an atom. (S.M. Richardson)

Electron capture Nuclear decay in which a proton in the nucleus acquires an electron from the outer cloud of the atom's electrons. This converts the proton to a neutron, reduces the number of protons in the nucleus by one and atomic number of the original element by one. Atomic mass number remains constant because the total number of protons and neutrons is unchanged. (S.M. Richardson)

Electron-capture decay The only type of radioactive decay that requires the presence of something — an electron — outside the atom's nucleus. Electron capture decay of light atoms — those having the fewest electrons — can be very slightly affected by extremely high pressures or certain chemical bonds, so as to change their half-lives by a fraction of a percent. But no change in the half-lives of elements used for radiometric dating has ever been verified. (R.C. Wiens)

Element A substance, made up of atoms, that cannot be further subdivided by chemical means. It has a certain number of protons in the nucleus and unique properties. Each element consists of a number of isotope, which have nearly all of the same properties except for their mass and their radioactive decay characteristics. (R.C. Wiens, modified)

Eon The largest division of geologic time in the geological timescale, embracing several Eras (for example, the Phanerozoic, 540 m.y. ago to present); also any span of one billion years. (

Epoch A division of the geologic time shorter than a period. Epochs are further divided into several ages. Generally, the geological timescale and the history of life is described in terms of periods, but in the case of the 65 million year long Cenozoic era, epoches are used instead (Paleocene, Eocene, etc). With most earlier periods, epoches are often equivalent to the Early, Middle, and Late part of the period. (MAK)

Era A division of the geologic time shorter than an eon, and measuring major stages in the evolution of life - e.g. Paleozoic, Mesozoic, Cenozoic. Eras are further divided into periods. (MAK)


Fission The spontaneous or induced splitting, by particle collision, of a heavy atomic nucleus into a pair of fragments plus some neutrons. Controlled induced fission can be used as a source of nuclear power. (S.M. Richardson)

Fission track dating Dating of minerals by fission tracks, damage tracks left in a mineral by spontaneous alpha emissions. (S.M. Richardson)

Fossil Evidence or trace left in rock of an ancient organism. A fossil may be a bone, shell, leaf impression, footprint, insect in amber, etc.


Geologic(al) column The arrangement of rock units in chronological order from the youngest (at the top) to the oldest (at the bottom). (S.M. Richardson). See also Geologic(al) time scale (below)

Geologic(al) time scale a system of chronologic measurement 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. (Wikipedia)


Hadean The oldest eon in the history of the Earth, extending from the origin of the Earth about 4.5 billion years ago to around 3.8 billion years ago. During the time the planet was formed, but conditions were unsuitable for life. It was succeeded by the Archean eon. (MAK)

Half-life The amount of time it takes for half the atoms of a radioactive isotope to decay. (R.C. Wiens)


Ice core Long sections of ice brought up by special drilling rigs on the ice sheets of Greenland and Antarctica (R.C. Wiens); A sample of ice taken from glaciers or other accumulations, often showing seasonal layering. [den., science] Ice cores can be dated by multiple independent means, with the oldest published date of 140,000 ± 15,000 years for the Vostok core from Antarctica. (W. R. Elsberry -

Igneous rock Rock formed from molten lava. The other two types of rock are sedimentary, formed by the cementing together of soil or sand, and metamorphic rocks reformed by heat over long periods of time. (R.C. Wiens)

Index fossil A fossil that identifies and dates the strata in which it is typically found. To be most useful, an index fossil must have broad, even worldwide distribution and must be restricted to a narrow stratigraphic range. (S.M. Richardson)

International Commission on Stratigraphy (ICS) a subordinate body of the International Union of Geological Sciences (an international non-governmental organization devoted to international cooperation in the field of geology) that concerns itself with stratigraphy, geological, and geochronological matters on a global scale. ( modified from Wikipedia) website

Isochron dating  A self-checking method of dating used with several radioisotopes. This mechanism compares ratios of radioactive isotopes to their decay products. (W. R. Elsberry via W.J. Hudson)

Isotope Atoms of a given element that have the same atomic mass. Most elements have more than one isotope. Most radioactive elements used for dating have one radioactive isotope and at least one stable isotope. For example carbon-14 (which weighs 14 atomic mass units) is radioactive, while the more common isotopes, carbon-12 and carbon-13 are not. (R.C. Wiens). Isotopes share the same atomic number. For example, oxygen (atomic number 8) may have an atomic mass number of 16, 17, or 18, depending on whether it has 8, 9, or 10 neutrons. It therefore has three isotopes. (S.M. Richardson)



40K/40Ar method A method used for the dating of potassium-bearing rocks by using the ratio of radioactive 40K to its daughter, 40Ar. (S.M. Richardson)


Late In the geological timescale, the chronological equivalent to Upper. Often refers to a subdivision or epoch of a period, especially when the period is divided into three; e.g. "Late Jurassic". (MAK)

Lead-lead dating A variation on the uranium-lead technique in which only the isotopes of lead need to be measured. (R.C. Wiens)


Magma Hot molten material from which rocks are formed. When magma erupts on the surface of the earth, it is called lava. (R.C. Wiens)

Magnetic chron Time during which magnetic polarity is dominantly normal or dominantly reversed. (S.M. Richardson)

Magnetic polarity The direction, north (normal) or south (reversed), that a magnetic compass needle points. (S.M. Richardson)

Magnetic polarity time scale A chronology based on the shifting polarity of the Earth's magnetic field. (S.M. Richardson)

Magnetic stratigraphy A stratigraphic sequence based on the magnetic polarity of the rocks. (S.M. Richardson)

Magnetic subchron A period during a magnetic chron when the magnetic polarity is the opposite from that of the magnetic chron. (S.M. Richardson)

Mesozoic "Middle Animal Life". The second era of the Phanerozoic eon, lasting from 251 to 65.5 million ago, and characterised by the dominance of archosaurian reptiles on land, and marine reptiles in the seas. Plants are mostly gymnosperms. (MAK)

Metamorphism The heating of rocks over long time periods at temperatures which are hot enough to change the crystal structure but not hot enough to completely melt the rock. Metamorphism tends to alter or reset the radiometric time clocks, though some radiometric techniques are more resistant to resetting than others. (R.C. Wiens)

Middle In the geological timescale, the middle of the three subdivisions or epochs of a period; e.g. "Middle Jurassic". (MAK)


Neutron A nuclear particle lacking electrical charge. It is electrically neutral, hence the name. The number of neutrons determines the isotope of an element. (MAK)

Nucleons Name given to protons and neutrons, which make up the nucleus of an atom. They are more massive than electrons. The number of nucleons determines the atomic weight. (MAK)

Nucleus (atomic) The small, dense, center of an atom, which is made up of protons and neutrons (except for hydrogen which in stable form possesses only a single proton in the nucleus). (MAK)



Paleomagnetism Study of the Earth's past magnetism as it is recorded in the rocks. (S.M. Richardson)

Paleontology The scientific study of ancient life, through examination of fossil remains.

Paleozoic "Ancient Animal Life". The earliest era of the Phanerozoic eon, but also the longest, lasting from 542 to 251 million years ago. It includes what are colloquially referred to as the ages of invertebrates, fish, amphibians, and the start of the age of reptiles. Plants include Pteridophytes, later supplemented by early gymnosperms. (MAK)

Parent A radioactive element whose decay produces stable daughter elements. (S.M. Richardson)

Phanerozoic literally "visible animal life", the most recent of the four eons of geologic time. Characterised by complex multicellular life. Includes the Palaeozoic, Mesozoic, and Cenozoic. Usually contrasted with the Precambrian, in which life is little known and generally microscopic. (MAK)

Polonium haloes Radiation induced pleochroic haloes in mica. (W. R. Elsberry -

Precambrian An informal term, found mostly in earlier books, to refer to the sum of geologic time from the formation of the Earth to the start of the Cambrian period. (MAK)

Principle of faunal and floral succession Groups of animals and plants have succeeded one another in a definite and discernible order. (S.M. Richardson)

Proterozoic The name means the era of "first (animal) life". This was the third eon in Earth history, during which eukaryote life and an oxygen atmosphere appeared. It followed the Archean, and lasted from around 2.5 to half a billion years, making it the longest eon, in terms of years. The end of the Proterozoic saw the rise of complex life. The Proterozoic was followed by the Paleozoic era. (MAK)

Proton A nuclear particle possessing a positive electrical charge. The number of protons in the nucleus constitute the atomic number of an element. (MAK)



Radioactivity The spontaneous decay of the nucleus of an element. It involves the change in the number of protons in the nucleus and therefore creates an atom of a new element. During the change, or decay, energy is released either as light or energetic particles. (S.M. Richardson, R.C. Wiens)

Radiocarbon Carbon-14, a radioactive isotope of Carbon which contains 14 nucleons (6 protons and 8 neutrons) rather than the usual 12 (6 protons and 6 neutrons). It is used to date dead plant and animal matter. Radiocarbon is not used for dating rocks. (MAK, R.C. Wiens)

Radiometric dating Determination of a time interval (e.g., the time since formation of a rock) based on nuclear decay of naturally occurring radioactive isotopes. Radiometric dating is one subset of the many dating methods used in geology. (R.C. Wiens)

Relative age, Relative time A statement of the approximate age of an object or feature in comparison with some other object or feature, rather than in terms of its age in years. (Fred Bervoets - Dinodata glossary ) Dating of rocks and geologic events by their positions in chronological order without reference to number of years before the present. (S.M. Richardson). compare with Absolute age.

Remanent magnetism Magnetism acquired by a rock as some time in the past. (S.M. Richardson)

Reversed polarity Time when a magnetic needle points to the south pole. (S.M. Richardson)


Sediment Any of a number of materials deposited at Earth's surface by physical agents (such as wind, water, and ice), chemical agents (precipitation from oceans, lakes, and rivers), or biological agents (organisms, living and dead). (Hugh Rance)

Stratification The accumulation of material in layers or beds. (S.M. Richardson)

Stratigraphy The succession and age relation of layered rocks. (S.M. Richardson)

Superposition A statement of relative age in layered rocks: In a series of sedimentary rocks that has not been overturned, the topmost layer is always the youngest and the bottommost layer is always the oldest. (S.M. Richardson)


Three-isotope plot In dating, this is a plot in which one axis represents the parent isotope and the other axis represents the daughter isotope. Both parent and daughter isotopes are ratioed to a daughter element isotope that is not produced by radioactive decay This type of plot gives the age independent of the original amounts of the isotopes. (R.C. Wiens)

Tree ring A ring visible in the stump or sawed section of a tree which indicates how much it grew in a year. The age of a tree can be determined by counting the growth rings. (R.C. Wiens)

Two-component mixing The mixing of two different source materials to produce a rock. On rare occasions this can result in an incorrect age for certain techniques that use three-isotope plots. Two component mixing can be recognized if more than one dating technique is used, or if surrounding rocks are dated. (R.C. Wiens)



Vendian The latest period of the Proterozoic era, spanning the time between 650 and 544 million years ago. Also referred to as the Ediacaran period, the Vendian is distinguished by fossils representing a characteristic collection of complex soft-bodied organisms found at several localities around the world. (USGS Paleontology glossary) During the '90s Vendian replaced Ediacaran it seems the pendulum has now swung the other way, with Ediacaran is now the preferred term for this period. (MAK)



Xenolith Literally, a foreign chunk of rock within a rock. Some rocks contain pieces of older rocks within them. These pieces were ripped off the magma chamber in which the main rock formed and were incorporated into the rock without melting. Xenoliths do not occur in most rocks, and they are usually recognizable by eye where they do occur. If unrecognized, they can result in an incorrect date for a rock (the date may be of the older xenolith). (R.C. Wiens)



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