Which rock type is formed by the combination of extreme heat and pressure on existing rocks?

The formation of metamorphic rock occurs when existing rocks, either igneous or sedimentary, undergo significant changes due to extreme heat and pressure. This process can alter the mineral composition and structure of the rock without it melting. For instance, the application of high temperatures can cause minerals to recrystallize, while increased pressure can lead to foliation or layering in the rock. This transformative process distinguishes metamorphic rock from other types, which have different formation processes.

Sedimentary rocks are formed from the accumulation of sediments, while igneous rocks originate from molten magma or lava that cools and solidifies. Volcanic rock, a subtype of igneous rock, is specifically formed from lava that cools on the Earth's surface. Thus, the unique conditions that create metamorphic rock are not present in these other categories. This understanding highlights the specifics of how rocks change under varying environmental conditions, showcasing the importance of heat and pressure in the rock cycle.