Question

8. Analysis of a basalt rock sample shows that 12.5% of its radioactive potassium-40 remain undecayed.

How old is the basalt?

Answer 1

Most of the Moon's basalts erupted between about 3 and 3.5 billion years ago, but the oldest samples are 4.2 billion years old, and the youngest flows, based on the age dating method of crater counting, are estimated to have erupted only 1.2 billion years ago.