We are told that these methods are accurate to a few percent, and that there are many different methods.
We are told that of all the radiometric dates that are measured, only a few percent are anomalous.
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.
Description: Radiometric Dating, Geologic Time, and the Age of the Radiometric Dating, Geologic Time, and the Age of the Radiometric Dating, Geologic Time, and the Age of the Radiomet...
With absolute age dating, you get a real age in actual years.
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?
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.
Look for “absolute” ages such as cornerstones, dates carved into fresh concrete, or dates stamped on manhole covers.