Final answer:
The sentence uses a metaphor to compare the earth to a shell.
Step-by-step explanation:
The sentence provided uses a metaphor to describe the earth. A metaphor is a figure of speech that directly compares two things by stating that one thing is another. In this case, the sentence is comparing the earth to a specific type of surface. Based on the sentence, the earth is being compared to a shell. The word 'shell' suggests a hard, protective covering, which gives the reader the impression that the earth is resembling something that is solid and structured.