Which statement describes the age relationship of an intrusion to the surrounding rock?

• A. It is younger than surrounding rock.
• B. It is older than surrounding rock.
• C. It is the same age as surrounding rock.
• D. It formed after erosion.

Answer: (B)

An intrusion forms when magma pushes into pre-existing rock and then cools to become solid. Because the surrounding rock has to exist first for the magma to intrude into it, the intrusion must be younger than the rock it intrudes. So the correct description is that the intrusion is younger than the surrounding rock. The other ideas don’t fit: an intrusion can’t be older than the rock it cuts into, they aren’t the same age since they form at different times, and forming after erosion doesn’t directly address the essential timing between intrusion and the host rock.