A relative age dating activity


04-Jan-2017 00:35

To determine the relative age of different rocks, geologists start with the assumption that unless something has happened, in a sequence of sedimentary rock layers, the newer rock layers will be on top of older ones. This rule is common sense, but it serves as a powerful reference point.

Geologists draw on it and other basic principles ( to determine the relative ages of rocks or features such as faults.

Look for “absolute” ages such as cornerstones, dates carved into fresh concrete, or dates stamped on manhole covers.

Absolute age dating: Have students work alone or in pairs to find an article or paper that uses radiometric age dating.

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.

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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.

Pretty obvious that the dike came after the rocks it cuts through, right?



May 18, 2011. Relative dating is used to arrange geological events, and the rocks they leave behind, in a sequence. The method of reading the order is called stratigraphy layers of rock are called strata. Relative dating does not provide actual numerical dates for the rocks.… continue reading »


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