Question

The age of rocks on earth or the samples of rocks and dust brought back form the moon can be found by determining the proportion of radioactive ______in the rock of dust.

• A. potassium and stable calcium
• B. uranium and stable lead
• C. carbon and stable carbon
• D. radium and stable lead

Answer 1

Answer: (B)

uranium and stable lead

Radiometric dating (often called radioactive dating) is a technique used to date materials such as rocks or carbon, usually based on a comparison between the observed abundance of a naturally occurring radioactive isotope and its decay products, using known decay rates.

The uranium-lead radiometric dating scheme has been refined to the point that the error margin in dates of rocks can be as low as less than two million years in two-and-a-half billion years.

One of its great advantages is that any sample provides two clocks, one based on $uranium-{235}^{\prime }s$ decay to $Lead-207$ with a half-life of about $700$ million years, and one based on $Uranium-{238}^{\prime }s$ decay to lead-Uranium-lead dating is often performed on the mineral zircon $206$ with a half-life of about $4.5$ billion years, providing a built-in cross-check that allows accurate determination of the age of the sample even if some of the lead has been lost.