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We must answer this question if we want to add numerical ages to the geologic column.Geologists obtain dates for sedimentary rocks by studying cross-cutting relationships between sedimentary rocks and datable igneous or metamorphic rocks.Pretty obvious that the dike came after the rocks it cuts through, right?With absolute age dating, you get a real age in actual years.We have seen that isotopic dating can be used to date the time when igneous rocks formed and when metamorphic rocks metamorphosed, but not when sedimentary rocks were deposited.
The half-life of carbon 14, for example, is 5,730 years.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.Like the other kind of dating, geologic dating isn’t always simple.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?
Look for “absolute” ages such as cornerstones, dates carved into fresh concrete, or dates stamped on manhole covers.