What term describes pre-existing rocks that are transformed by high heat and extreme pressure?

The term that best describes pre-existing rocks transformed by high heat and extreme pressure is "metamorphic." Metamorphic rocks are formed when existing rocks, whether igneous, sedimentary, or even other metamorphic rocks, undergo significant changes due to increased temperature and pressure conditions within the Earth. This process often results in changes to the mineral structure and texture of the rocks, leading to new rock types such as schist, gneiss, and marble.

Metamorphism typically occurs in areas associated with tectonic activity, such as at the boundaries of tectonic plates, where rocks are subjected to the intense pressures of colliding plates or the heat from deep within the Earth. This transformation enhances or creates new physical properties and characteristics, making metamorphic rocks distinct from their original forms.

The other terms refer to different rock types: sedimentary rocks are formed from the accumulation of debris, organic material, or minerals, igneous rocks originate from the cooling and solidification of molten magma or lava, and fossilized refers specifically to the preservation of organic remains through mineralization or other processes, rather than a rock type formed under extreme conditions.