A granite dike cuts across a shale. The dike gives a radiometric date of 84 million years old. The shale was therefore deposited more than 84 million years ago.

Question

Quizplus · Accepted Answer

This is because the granite dike must be younger than the shale in which it cuts across. Therefore, the shale must have been deposited before the granite dike was intruded, and since the dike has a radiometric date of 84 million years old, the shale must be older than that.

A Granite Dike Cuts Across a Shale