How are metamorphic rocks formed?

Metamorphic rocks are formed through the alteration of existing rocks, which can be either igneous, sedimentary, or other metamorphic rocks, under the influence of heat, pressure, and often chemically active fluids. This process is known as metamorphism. During metamorphism, the mineral composition and structure of the original rock change, resulting in a new type of rock with distinct characteristics.

The term "fusion" in the correct answer reflects the concept that existing rocks undergo a structural transformation rather than melting entirely, which can lead to the development of new minerals and textures. This process can occur deep within the Earth where extreme pressure and high temperatures exist, such as near tectonic plate boundaries or in regions with significant geological activity.

In contrast to the other options, the processes of crystallization from cooling magma (which describes igneous rock formation), the compression of sediment layers (which refers to sedimentary rock formation), and evaporation of water (which is related to the formation of certain types of sedimentary rocks or minerals) do not describe the conditions that lead to the formation of metamorphic rocks. Each of these geological processes leads to different rock types that do not share the same formation mechanism as metamorphic rocks.