Using Radioactive Decay as a Geological Clock

Students in Earth Science 1, Earth Science 10, Geology 1 and Geology 10 should all be familiar with the three isotope systems which have been most widely applied to dating the earth.  These dating methods are carbon 14 dating, postassium-argon dating, and uranium-lead dating.  You should know the half-life for each radioactive isotope, and the types of materials which can be dated by each one. 

Additional systems are considered in Geology 3 (Earth History).  A more detailed tutorial will be prepared for that course  in the future.

Review your lecture notes and textbook information, and then see if you can answer the following questions:

1.  For each of the following materials, list the dating method or methods which would work best for determining its age in years before the present:

Dinosaur bone:

Mesozoic granite:

Skeleton of an ancient Egyptian prince:

Miocene basaltic lava flow:

Log found in glacial till from the most recent ice advance:

2.  The micas from a sample of andesite have been extracted, and the ratio between ⁴⁰K and ⁴⁰Ar has been measured.  Based on this ratio, 25% of the original ⁴⁰K is still in the rock.  According to the K-Ar data, when did the micas in the andesite cool and crystallize?

3.  A log from a glacial deposit is 11,150 years old.  What percentage of the original ¹⁴C remains in the log?

4.  You've been asked to determine the age of metamorphism of a sample of gneiss.  A sample of zircon crystals from the gneiss yields a uranium-lead age of 2.3 billion years.  Potassium-argon dating of the mica crystals yields an age of 140 million years.  What is the most likely explanation for these results?

A) Radioactive dating methods are always inaccurate
B) Metamorphism of the gneiss occurred 2.3 billion years ago
C) Metamorphism of the gneiss began 2.3 billion years ago, and ended 140 million years ago
D) The protolith (or parent rock) formed 2.3 billion years ago, metamorphism began much later and ended 140 million years ago
E) Metamorphic rocks of this type must be dated using carbon 14, not potassium-argon dating

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