3 types of relative dating
Relative age dating also means paying attention to crosscutting relationships.Say for example that a volcanic dike, or a fault, cuts across several sedimentary layers, or maybe through another volcanic rock type.In a way this field, called geochronology, is some of the purest detective work earth scientists do.There are two basic approaches: relative age dating, and absolute age dating.What’s more, if the whole rock is badly weathered, it will be hard to find an intact mineral grain containing radioactive isotopes.You might have noticed that many of the oldest age dates come from a mineral called zircon.I also like this simple exercise, a spin-off from an activity described on the USGS site above.Take students on a neighborhood walk and see what you can observe about age dates around you.
Activity: Further discussion: Good overview as relates to the Grand Canyon: age dating: Use with this cross section of the Grand Canyon from the USGS’s teaching page: Canyon Have students reconstruct a simple geologic history — which are the oldest rocks shown? Are there any that you can’t tell using the Rule of Superposition?
Chart of a few different isotope half lifes: In reality, geologists tend to mix and match relative and absolute age dates to piece together a geologic history.
If a rock has been partially melted, or otherwise metamorphosed, that causes complications for radiometric (absolute) age dating as well.
Here is an easy-to understand analogy for your students: relative age dating is like saying that your grandfather is older than you.
Absolute age dating is like saying you are 15 years old and your grandfather is 77 years old.
Pretty obvious that the dike came after the rocks it cuts through, right?