How Relative and Absolute Dating Help Scientists Organize Geologic Time

A set of vocabulary flashcards defining key terms, principles, radioisotopes, and subdivisions associated with relative and absolute dating methods used in Earth Science to organize geologic time.

Relative Dating

A method that determines the sequence or relative age of rocks and fossils (older or younger) by their position and relationships, without providing an exact numerical age.

Absolute Dating

A method that determines the actual numerical age of rocks and fossils, usually in millions or billions of years, using the radioactive decay of elements.

Law of Superposition

A principle of relative dating stating that in an undisturbed sequence of sedimentary rock layers, the oldest layer is at the bottom and the youngest layer is at the top.

Cross-Cutting Relationships

A relative dating principle stating that any geologic feature (such as a fault, dike, or canyon) that cuts across rock layers is younger than the layers it cuts.

Fossil Succession

A principle stating that certain fossils appear in rock layers in a specific, predictable vertical order through time.

Unconformity

A erosion surface where older rocks were eroded before younger rocks were deposited, representing a gap in time in the geologic record.

Rock Inclusions

Fragments of older rock found contained inside a younger rock, which are older than the rock containing them.

Baked Contacts

A feature formed when hot magma intrudes into older rock, changing or 'baking' the surrounding rock to show that the baked rock is older than the intrusion.

Original Horizontality

A relative dating principle stating that sediment layers are originally deposited roughly horizontally.

Lateral Continuity

A relative dating principle stating that rock layers extend laterally in all directions until they thin out or terminate.

Index Fossils

Fossils of organisms that were widespread, abundant, easily recognizable, and lived during a relatively short geologic time range, used to date and correlate rock layers.

Half-Life

The time required for half of a radioactive parent isotope to decay into its stable daughter isotope.

Parent Isotope

An unstable, radioactive isotope that decays over time at a known rate.

Daughter Isotope

The stable isotope product formed by the radioactive decay of a parent isotope.

Geologic Time Scale

A system that organizes Earth's $4.6\text{ billion-year}$ history into eons, eras, periods, and epochs based on evidence from rocks, fossils, and dating methods.

Eons

The largest subdivisions of time on the geologic time scale (e.g., Phanerozoic, Proterozoic, Archean, Hadean).

Eras

Subdivisions within an eon on the geologic time scale (e.g., Cenozoic, Mesozoic, Paleozoic).

Periods

Subdivisions within an era on the geologic time scale (e.g., Quaternary, Cretaceous, Cambrian).

Epochs

Smaller subdivisions within periods on the geologic time scale.

Biostratigraphic Correlation

The process of matching rock layers across different locations using shared index fossils.

Carbon-14

A radioisotope with a half-life of $5{,}730\text{ years}$, used in absolute dating of organic remains up to approximately $50{,}000\text{ years}$ old.

Potassium-40

A radioisotope with a half-life of $1.25\text{ billion years}$, used in absolute dating of volcanic rocks and certain minerals.

Uranium-238

A radioisotope with a half-life of $4.47\text{ billion years}$, used in absolute dating of very old rocks that decays into Lead-206.

Rubidium-87

A radioisotope with a half-life of $48.8\text{ billion years}$, used in absolute dating of ancient rocks and continental crust.