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If a radioactive isotope has a half-life of 50 million years, and a rock sample contains 1/8th of that isotope remaining overall, what is the age of the rock from which the sample was taken?

  A geologist observes an outcrop of sedimentary rocks composed of shale and sandstone with several igneous bodies cutting through the layers.  To determine age relationships, which principle best explains this example?

Bodies of rocks that are thick enough to be mappable with distinct upper and lower boundaries are officially designated as:

A guide fossil has some or all of the following desirable characteristics except:

The landward migration of a shoreline is known as a ______. 

The most accurate radiometric dates are obtained from radioactive isotopes in:

Select the item below that makes a good key bed for correlating a precise event in time:

A fossil’s “range” determines:

To determine the radiometric age of a rock, what must scientists be able to measure very accurately?

Radioactive parent Na 24 decays with a half-life of 15 hours.  Starting with 100 atoms of this isotopic parent, how many parent isotopes would remain after 45 hours?