Dating geologic events
Development of the geologic time scale and dating of formations and rocks relies upon two fundamentally different ways of telling time: relative and absolute.
Relative dating places events or rocks in their chronologic sequence or order of occurrence.
-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.
-Key bed- distinctive or unusual bed present in different areas (coal bed, volcanic ash layer).
-Correlation by fossils- same fossil or fossil assemblage in widely separated rocks are same age. age of rock = number of half lives gone by X half life of isotope pair used How old is the Earth?
The shorter the range of the fossil, the more accurate the correlation. -oldest rocks dated are from Greenland, Minnesota, etc.
Absolute dating places events or rocks at a specific time.
If a geologist claims to be younger than his or her co-worker, that is a relative age.
This 9-12-grade activity introduces students to age dating with exercises using relative and absolute dating. Links to various activities and lesson plans concerning relative and absolute dating. Content information about absolute and relative dating methods used by the U.