Geology and Geophysics


Geologyand Geophysics

Questionsand Answers

29.Geologists know that potassium-40 decays to argon-40 with a half-lifeof 1.3 billion years. Analysis of a hypothetical sample of granitereveals that 75 percent of potassium-40 atoms decay to form argon-40.What is the age of sample of granite? The granite sampleis __0.975 million years old. Calculations:

100%= 1.3 x 109

75%= X years old

X= 75 x 1.3 x 109/100

X= 0.975 x 109yearsold

30.Determine the numerical ages of rock samples that contain a parentisotope with a half-life of 100 million years and have the followingpercentages of original parent isotope: 50% Age:__50million years old__


50x 100 / 100

=50million years old

25%Age:__ 25 million years old__


25x 100 / 100

25million years old

6%Age:___ 6 million years old

6x 100 / 100

=6million years old31. A. What assumptions have to bemade in order to correctly date a sample using radiometric data?


Thereare three major assumptions made with this type of dating:

  1. The quantity of daughter and parent isotopes available during the formation of the rock is known.

  2. There has been no gain or loss of the daughter or parent isotopes from the original rock.

  3. The speed of radioactive decay is known and has been stable since the rock formed.

B.How would having daughter isotopes present in the sample alterradiometric dating results?


Daughterisotopes are essential in determining the half-lives of decayingsubstances. If both daughter and parent isotopes are there, therelative amount of parent isotopes to daughter ones can give a hintto the era of the sample. If there is more parent to daughter isotoperatio, it means that the sample is young. If there exists fewerparent than daughter ratio, the sample is said to be old.