Final answer:
The given sentence "Molten rock inside the earth is magma; magma on the earth's surface is lava" is a compound sentence.
Step-by-step explanation:
The given sentence "Molten rock inside the earth is magma; magma on the earth's surface is lava" is a compound sentence.
A compound sentence is a sentence that consists of two or more independent clauses joined by coordinating conjunctions, semicolons, or punctuation marks.
In this case, the sentence has two independent clauses: "Molten rock inside the earth is magma" and "magma on the earth's surface is lava." These two clauses are joined together using a semicolon to show a close relationship between the ideas.