Which type of rock forms when an existing rock is changed by heat, pressure, or chemical reactions?

The correct answer is metamorphic rock. This type of rock forms when an existing rock, which can be either sedimentary, igneous, or even another metamorphic rock, undergoes significant changes due to heat, pressure, or chemical processes. This transformation can alter the mineral composition and structure of the original rock.

For instance, limestone can become marble, a common example of a metamorphic rock, under conditions of high temperature and pressure. These changes do not occur through melting but rather through solid-state processes that alter the physical and chemical properties of the rock.

The process of metamorphism is important in geology because it plays a critical role in the rock cycle and helps to recycle and transform materials within the Earth's crust. In contrast, sedimentary rocks form from the accumulation and lithification of sediments, while igneous rocks form from the cooling and solidification of molten magma or lava, neither of which involve the transformative processes characterized by metamorphic rocks.