Question

Carbon-14 if often used for radioactive dating, but it has its limitations. Uranium-238 or lead-206 are most often used to date rocks.
Why would U-238 or Pb-206 be more useful than C-14 when dating rocks?

A) Rocks do not contain any carbon for dating purposes.

B) Carbon-14's half-life is much too long to use for dating rocks.

C) The half life of carbon-14 is about 5700 years and is too short date rocks.

D) Carbon-14 is only found in living things; it can only be used to date remains.

Answer 1

Answer:

C) The half life of carbon-14 is about 5700 years and is too short to date rocks.  

Step-by-step explanation:

The practical limit for carbon dating is about nine half-lives (50 000 yr).

At that point, the amount of carbon remaining has dropped to about 0.2 % of the original amount.

The experimental uncertainties involved become so large that carbon dating is no longer reliable.

Most rocks hundreds of millions or even billions of years old.

Any radioactive carbon would have long since decayed to unmeasurable levels.

A) is wrong. Many rocks contain carbon, but they are so old that the amount of radioactive carbon has dropped below measurables.

B) is wrong. The half-life of carbon is too short to use for dating rocks.

D) is wrong. About 20 % of the carbon in rocks comes from living things that were embedded in layers of mud.

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Uranium-238 and lead-206 have half-lives of 4.5 × 10⁹ yr, which is about the age of the Earth.

Even the oldest rocks would still have half of their radioactivity, and younger rocks would have more.

Thus, U-238 and Pb-206 are ideal isotopes for measuring the ages of rocks.