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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.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.The narrower a range of time that an animal lived, the better it is as an index of a specific time.No bones about it, fossils are important age markers.For example, which is older, the bricks in a building or the building itself?Are there repairs or cracks in the sidewalk that came after the sidewalk was built?
Geologists draw on it and other basic principles ( to determine the relative ages of rocks or features such as faults.
There are three main types of fossils: body, trace and chemical.
As well as body fossils, which are actual skeletal remains, there are impressions of soft animals, plants and footprints which have been left as mud hardened.
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.
Here is an easy-to understand analogy for your students: relative age dating is like saying that your grandfather is older than you.