What characterizes metamorphic rock?

Metamorphic rock is characterized by its formation through the transformation of existing rock types—igneous, sedimentary, or other metamorphic rocks—due to the application of significant heat, pressure, or chemically active fluids. This process, known as metamorphism, alters the mineral composition and structure of the rock without it melting completely. During metamorphism, the existing rocks experience changes that can affect their texture, mineral composition, and overall structure.

The other options describe different types of rocks or processes that do not pertain to metamorphic rocks. For example, magma cooling results in igneous rocks, while sedimentary processes involve the accumulation and compaction of sediments. Volcanic rocks primarily originate from volcanic activity, which is distinct from the metamorphic processes that involve pre-existing rocks undergoing transformation. Therefore, the defining characteristic of metamorphic rock is indeed the transformation it undergoes due to heat and pressure.