| A | B |
|---|
| absolute dating | any method of measuring the age of an event or object in years |
| cast | a type of fossil that forms when sediments fill in the cavity left by a decomposed organism |
| catastrophism | a principle that states that geologic change occurs suddenly |
| eon | the largest division of geologic time |
| epoch | a subdivision of a geologic period |
| era | a unit of geologic time that includes two or more periods |
| extinction | the death of every member of a species |
| fossil | the remains or physical evidence of an organism preserved by geological processes |
| geologic column | an arrangement of rock layers in which the oldest rocks are at the bottom |
| geologic time scale | the standard method used to divide the Earth's long natural history into manageable parts |
| half-life | the time needed for half of a sample of a radioactive substance to undergo radioactive decay |
| index fossil | a fossil that is found in the rock layers of only one geologic age and that is used to establish the age of the rock layers |
| isotope | an atom that has the same number of protons (or the same atomic number) as other atoms of the same element do but has a different number of neutrons (and thus a different atomic mass) |
| mold | a mark or cavity made in a sedimentary surface by a shell or other body |
| paleontology | the scientific study of fossils |
| period | a unit of geologic time into which eras are divided |
| radioactive decay | the process in which a radioactive isotope tends to break down into a stable isotope of the same element or another element |
| radiometric dating | a method of determinining the age of an object by estimating the relative percentages of a radioactive (parent) isotope and a stable (daughter) isotope |
| relative dating | any method of determining whether an object is older or younger than other events or objects |
| superposition | a principle that states that younger rocks lie above older rocks if the layers have not been disturbed |
| trace fossil | a fossilized mark that is formed in soft sediment by the movement of an animal |
| unconformity | a break in the geologic record created when rock layers are eroded or when sediment is not deposited for a long period of time |
| uniformitarianism | a prinicple that states that geologic processes that occurred in the past can be explained by current geologic processes |
