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Earth Science Topic4
| Term | Definition |
|---|---|
| Period (of Geologic Time) | A geological time period is a standard unit of geologic time scale that spans tens to hundreds of millions of years |
| Epoch (of Geologic Time) | An epoch is a subdivision of a geological period on the Geologic Time Scale, typically lasting for several million years |
| Era (of Geologic Time) | An era is the second-longest unit of geological time, lasting tens to hundreds of millions of years and marked by major changes in Earth's life forms and geology |
| Eon (of Geologic Time) | An eon is the largest division of geologic time, spanning hundreds of millions to billions of years |
| Evolution | Evolution is the change in the inherited traits of a biological population over successive generations |
| Fossil | A fossil is the preserved remains, impression, or trace of any once-living organism from a past geological age |
| Geologic column | A geologic column is an idealized, composite diagram showing the vertical sequence of rock layers (strata) across Earth's crust, with the oldest layers at the bottom and the youngest at the top |
| Half-life | A half-life is the time required for a quantity of a substance to reduce to half of its initial value |
| Index fossil | An index fossil is the preserved remain of an organism that lived for a brief period of geologic time and is used to date and correlate rock layers |
| Varve | A varve is a pair of thin sedimentary layers (a couplet) laid down in a body of still water—usually a lake—over the course of a single year |
| Law of Cross-Cutting Relationships | The law of cross-cutting relationships states that any geologic feature that cuts across or disrupts a body of rock or strata must be younger than the rock or feature it affect |
| Law of Superposition | The law of superposition is a core geologic principle stating that in any undisturbed sequence of rock layers, the oldest layer is at the bottom and the youngest layer is at the top |
| Law of Intrusive Relationships | The law of intrusive relationships is a geologic principle stating that any igneous intrusion is younger than the pre-existing rock layers it cuts across |
| Law of Faunal Succession | The law of faunal succession states that fossil plants and animals appear in rock layers in a predictable, specific order that does not change over time |
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