Geologic Time - Key terms


The absolute age of a geologic phenomenon is its age in Earthyears. Compare with relative age.


In general, an atmosphere is a blanket of gases surrounding a planet. Unless otherwise identified, however, the term refers to the atmosphere of Earth, which consists of nitrogen (78%), oxygen (21%), argon (0.93%), and other substances that include water vapor, carbon dioxide, ozone, and noble gases such as neon, which together comprise 0.07%.


The smallest particle of an element, consisting of protons, neutrons, and electrons. An atom can exist either alone or in combination with other atoms in a molecule.


A combination of all living things on Earth—plants, animals, birds, marine life, insects, viruses, single-cell organisms, and so on—as well as all formerly living things that have not yet decomposed.


A subdiscipline of stratigraphy devoted to studying the ages of rocks and what they reveal about geologic time.


Any effort directed toward finding the age of a particular item or phenomenon. Methods of geologic dating are either relative (i.e., comparative and usually based on rock strata) or absolute. The latter, based on such methods as the study of radioactive isotopes, usually is given in terms of actual years or millions of years.


A negatively charged particle in an atom, which spins around the nucleus.


A substance made up of only one kind of atom. Unlike compounds, elements cannot be chemically broken into other substances.


The longest phase of geologic time. Earth's history has consisted of four eons, the Hadean or Priscoan, Archaean, Proterozoic, and Phanerozoic. The next-smallest subdivision of geologic time is the era.


The fourth-longest phase of geologic time, shorter than an era and longer than an age and a chron. The current epoch is the Holocene, which began about 0.01 Ma (10,000 years) ago.


The second-longest phase of geologic time, after an eon. The current eon, the Phanerozoic, has had three eras, the Paleozoic, Mesozoic, and Cenozoic, which is the current era. The next-smallest subdivision of geologic time is the period.


An abbreviation meaning "giga-years," or "billion years." The age of Earth is about 4.6 Ga.


The study of Earth's age and the dating of specific formations in terms of geologic time.


The vast stretch of time over which Earth's geologic development has occurred. This span (about 4.6 billion years) dwarfs the history of human existence, which is only about two million years. Much smaller still is the span of human civilization, only about 5,500 years.


The upper part of Earth's continental crust, or that portion of the solid earth on which human beings live and which provides them with most of their food and natural resources.


The study of Earth's physical history. Historical geology is one of two principal branches of geology, the other being physical geology.


The entirety of Earth's water, excluding water vapor in the atmosphere but including all oceans, lakes, streams, groundwater, snow, and ice.


Atoms that have an equal number of protons and hence are of the same element but differ in their number of neutrons. This results in a difference ofmass. An isotope may be either stable or radioactive.


An abbreviation used by earth scientists, meaning "million years" or "megayears." When an event is dated to, for instance, 160 Ma, it usually means that it took place 160 million years ago.


A subatomic particle that has no electric charge. Neutrons are found at the nucleus of an atom, alongside protons.


The center of an atom, a region where protons and neutrons are located and around which electrons spin.


The third-longest phase of geologic time, after an era. The current eon, the Phanerozoic, has had 11 periods, and the current era, the Cenozoic, has consisted of three periods, of which the most recent is the Quaternary. The next-smallest subdivision of geologic time is the epoch.


A term that refers to the first three of four eons in Earth's history, which lasted from about4, Ma to about 545 Ma ago.


A positively charged particle in an atom.


A term describing a phenomenon whereby certain materials are subject to a form of decay brought about by the emission of high-energy particles or radiation. Forms of particles or energy include alpha particles (positively charged helium nuclei), beta particles (either electrons or subatomic particles called positrons , or gamma rays, which occupy the highest energy level in the electromagnetic spectrum.


A method of absolute dating using ratios between "parent" isotopes and "daughter" isotopes, which are formed by the radioactive decay of parent isotopes.


The relative age of a geologic phenomenon is its age in comparison with other geologic phenomena, particularly the stratigraphic record of rock layers. Compare with absolute age.


Material deposited at or near Earth's surface from a number of sources, most notably preexisting rock.


Rock formed by compression and deposition (i.e., formation of deposits) on the part of other rock and mineral particles.


The study and interpretation of sediments, including sedimentary processes and formations.


The study of rock layers, or strata, beneath Earth's surface.

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