Which statement best describes the formation of metamorphic rocks?

• A. They form by deposition of sediments.
• B. They form from cooling of magma.
• C. They form by weathering of rocks.
• D. They form from existing rocks through heat and pressure.

Answer: (D)

Metamorphic rocks form when existing rocks are altered by heat and pressure deep in the Earth's crust, while staying solid. The heat and pressure cause minerals to recrystallize and realign, often producing new textures such as foliation and new mineral combinations, with fluids aiding chemical changes. This is different from sedimentary rocks, which form from the deposition and lithification of sediments, and from igneous rocks, which form by cooling from a melt, or from weathering, which breaks rocks down at the surface. For example, shale can become slate, limestone can become marble, and granite can become gneiss under the right heat and pressure conditions.