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Geologic Time
| Term | Definition |
|---|---|
| relative age | the age of an object in relation to the ages of other objects |
| law of superposition | the principle that a sedimentary rock layer is older than the layers above it and younger than the layers below it if the layers are not disturbed |
| law of crosscutting relationships | the principle that a fault or body of rock is younger than any other body of rock that it cuts through |
| absolute age | the numeric age of an object or event |
| radiometric dating | a method of determining the absolute age of an object by comparing the relative percentages of radioactive (parent) isotope and stable (daughter) isotope |
| half-life | the time required for half of a sample of a radioactive isotope to break down by radioactive decay to form a daughter isotope |
| fossil | the trace remains of an organism that lived long ago, most commonly preserved in sedimentary rock |
| era | a unit of geologic time that includes two or more periods |
| period | a unit of geologic time that is longer than an epoch but shorter than an era |
| epoch | a subdivision of geologic time that is shorter than a period |
| Precambrian time | the interval of time in the geologic time scale from Earth’s formation to the beginning of the Paleozoic Era, from 4.6 billion to 542 million years ago |
| Paleozoic Era | the geologic era that followed Precambrian time and that lasted from 542 million to 251 million years ago |
| mass extinction | an episode during which large numbers of species become extinct |
| Mesozoic Era | the geologic era that lasted from 251 million to 65.5 million years ago; also called the Age of Reptiles |
| Cenozoic Era | the current geologic era, which began 65.5 million years ago; also called the Age of Mammals |
| gas giant | a planet that has a deep, massive atmosphere, such as Jupiter, Saturn, Uranus, or Neptune |
| terrestrial planet | one of the highly dense planets nearest to the sun; Mercury, Venus, Mars, and Earth |
| planetesimal | a small body from which a planet originated in the early stages of development of the solar system |
| solar nebula | a rotating cloud of gas and dust from which the sun and planets formed; also any nebula from which stars and exoplanets may form |
| planet | a celestial body that orbits the sun, is round because of its own gravity, and has cleared the neighborhood around its orbital path |
| solar system | the sun and all of the planets and other bodies that travel around it |
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smvree
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