Question

which characteristic of potassium makes it useful for dating rocks?
it has a long half - life.
it decays at an unknown rate.
it produces lead as it breaks down.
it can be used to date all types of rock.

Explanation:

Potassium - argon dating is a radiometric dating method. The half - life of potassium - 40 is about 1.25 billion years. Since rocks (especially igneous rocks) can contain potassium - 40, and with its long half - life, it can be used to date rocks that are very old (hundreds of millions of years old).

• Option "It decays at an unknown rate" is incorrect because the decay rate of potassium - 40 is known.
• Option "It produces lead as it breaks down" is incorrect. Potassium - 40 decays to argon - 40 (about 11%) and calcium - 40 (about 89%).
• Option "It can be used to date all types of rock" is incorrect. It is mainly used for igneous rocks. Sedimentary rocks are difficult to date using potassium - argon dating as they are formed from sediments of various ages, and metamorphic rocks can reset the radiometric clock depending on the degree of metamorphism.

Answer:

It has a long half - life.