Detailed Instructions: Given a scientific passage and an answer, generate a question for the given answer.
Q: Passage: Glaciers are melting back in many locations around the world. When a glacier no longer moves, it is called an ice sheet. This usually happens when it is less than 0.1 km2 in area and 50 m thick. Answer: an ice sheet
A:
When a glacier no longer moves, what is it called?