| A | B |
|---|
| Uniformitarianism | The concept that processes that have shaped Earth in the past are essentially the same as those operating today. |
| 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. |
| Law of superposition | In any undeformed sequence of sedimentary rocks, each bed is older than the layers above and younger than the layers below. |
| Principle of original horizontality | A principle of relative dating; Layers of sediments are generally deposited in a horizontal or nearly horizontal position. |
| Principle of cross-cutting relationships | A principle of relative dating; A rock or fault is younger than any rock or fault through which it cuts. |
| Unconformity | A surface that represents a break in the rock record, caused by erosion or lack of deposition. |
| Correlation | Establishing the equivalence of rocks of similar age in different areas. |
| Extinct | No longer burning, no longer active, no longer living. |
| Fossil | The remains or traces of an organism preserved from the geologic past. |
| Principle of fossil succession | Fossilized organisms succeed one another in a definite and determinable order. Therefore, any time period can be recognized by its fossil content. |
| Index fossil | A fossil that is associated with a particular span of geologic time. |
| Radioactivity | The spontaneous decay of certain unstable atomic nuclei. |
| Half-life | The time required for one half of the atoms of a radioactive substance to decay. |
| Radiometric dating | The procedure of calculating the absolute ages of rocks and minerals that contain radioactive isotopes. |
| Radiocarbon dating | Method for determining age by comparing the amount of carbon-14 to the amount of carbon-12 in a sample. |
| Era | A major division on the geologic time scale sometimes 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. |
| Eon | The largest time unit on the geologic time scale, next in order of magnitude above era. |
| Period | A basic unit of the geologic time scale that is a subdivision of an era. It can also be divided into smaller units called epochs. |
| Epoch | A unit of the geologic time scale that is a subdivision of a period. |
