Relative and absolute dating of geologic events
-geologists had a relative geologic time scale and knew Earth was very old, but didn't know how old until radioactivity provided a way to directly date rocks. measure parent/daughter ratio (% parent remaining) with a mass spectrometer. Geochronology is the science of dating and determining the time sequence of events in the history of the Earth.
If a geologist claims to be younger than his or her co-worker, that is a relative age.
-Guide fossil (index fossil)- fossil with a short range (lived for a short period of time). and are ~3.9 BY old -meteorites and some moon rocks, however, dated at ~4.6 BY; believed to have formed at same time as Earth; age of Earth, therefore, believed to be ~4.6 BY.
-Assemblage range zones- use more than 1 fossil in rock layer; area of age overlap is age (figs. Relative geologic time scale- early geologists constructed based on correlation of rocks from all over world (mostly by fossils); only relative geologic time periods, no actual dates (fig. Absolute Dating Specific age given in number of years before present.
Relative time was determined long before absolute time.
Index fossils are often used to determine a specific era.