What is the result of the combination of heat and pressure on existing rocks?

The combination of heat and pressure on existing rocks leads to the creation of metamorphic rocks. When tectonic processes such as subduction or continental collision occur, existing rocks (which can be either igneous, sedimentary, or other metamorphic rocks) experience significant changes. The high temperatures increase the mobility of the atoms and ions within the minerals, while the immense pressure can cause the minerals to recrystallize or align in a new structural configuration. This process transforms the original rock into a new type of rock with distinct mineral composition and texture, such as schist, gneiss, or marble, depending on the parent rock and conditions involved.

Other processes mentioned in the other choices represent different geological processes. The formation of sedimentary layers, for instance, involves the accumulation and compaction of sediments over time, which is distinctly different from the metamorphic process. Chemical weathering pertains to the breakdown of minerals at the Earth's surface into new minerals and solutes due to chemical reactions, and erosion involves the removal and transport of rock or soil by wind, water, or ice, none of which involve the heat and pressure required to create metamorphic rocks.