| A | B |
|---|
| correlation | establishing the equivalence of rocks of similar age in different areas |
| eon | the largest time unit on the geologic time scale, next in order of magnitude above era |
| epoch | a unit of the geologic time scale that is a subdivision of a period |
| era | a major division on the geologic time scale; Eras are divided into shorter units called periods. |
| geologic time scale | the division of Earth history into blocks of time—eons, eras, periods, and epochs; The time scale was created using relative dating principles. |
| half-life | the time required for one half of the atoms of a radioactive substance to decay |
| period | a basic unit of the geologic time scale that is a subdivision of an era; Periods may be divided into smaller units called epochs. |
| radioactivity | the spontaneous decay of certain unstable atomic nuclei |
| radiocarbon (carbon-14) dating | method for determining age by comparing the amount of carbon-14 to the amount of carbon-12 in a sample |
| radiometric dating | the procedure of calculating the absolute ages of rocks and minerals that contain radioactive isotopes |
| relative dating | process by which rocks are placed in their proper sequence or order; Only the chronological order of events is determined, not the absolute age in years. |
| unconformity | a surface that represents a break in the rock record, caused by erosion or lack of deposition |
| uniformitarianism | the concept that the processes that have shaped Earth in the geologic past are essentially the same as those operating today |
| fossil | the remains or traces of an organism preserved from the geologic past |
| index fossil | a fossil that is associated with a particular span of geologic time |
