Which statement correctly describes the formation of the three main rock types?

• A. Igneous rocks form through lithification of pre-existing rocks.
• B. Sedimentary rocks form when magma erupts and cools above the surface.
• C. Metamorphic rocks arise from the transformation of existing rock through heat and pressure.
• D. All rocks form only through weathering and erosion.

Answer: (C)

Rocks form in three basic ways that define the main rock types: cooling and crystallization of molten material, accumulation and lithification of sediments, and transformation of existing rocks under heat and pressure. When molten rock cools, either underground (intrusive) or after erupting onto the surface as lava (extrusive), it solidifies into igneous rock. Sedimentary rocks arise from pieces of older rocks or remains of organisms that are weathered, transported, deposited, and then compacted and cemented into rock. Metamorphic rocks come from pre-existing rocks that are altered by high heat and pressure, often with fluids, causing minerals to recrystallize and textures to change without melting.

The statement that metamorphic rocks arise from the transformation of existing rock through heat and pressure directly describes metamorphism, making it the best description. The other options mix up the processes: cooling of molten material forms igneous rocks, not sedimentary; lithification is a key part of making sedimentary rocks, not igneous rocks; and weathering and erosion alone do not account for the formation of all rock types.