Difference between absolute and relative dating of 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.But the most accurate forms of absolute age dating are radiometric methods. Sedimentary rocks in particular are notoriously radioactive-free zones.This method works because some unstable (radioactive) isotopes of some elements decay at a known rate into daughter products. Half-life simply means the amount of time it takes for half of a remaining particular isotope to decay to a daughter product. Good discussion from the US Geological Survey: geochronolgists just measure the ratio of the remaining parent atom to the amount of daughter and voila, they know how long the molecule has been hanging out decaying. So to date those, geologists look for layers like volcanic ash that might be sandwiched between the sedimentary layers, and that tend to have radioactive elements.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?It’s based either on fossils which are recognized to represent a particular interval of time, or on radioactive decay of specific isotopes. Based on the Rule of Superposition, certain organisms clearly lived before others, during certain geologic times.After all, a dinosaur wouldn’t be caught dead next to a trilobite.
Geologists draw on it and other basic principles ( to determine the relative ages of rocks or features such as faults.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.The layers on top could only be laid down on top of the bottom layer so must be younger.However the relative ages of rocks is more commonly determined by the presumed ages of the fossils found in the sedimentary layers.