Which rock is changed by heat and pressure?

Metamorphic rock is formed through the alteration of existing rocks—either igneous or sedimentary—under conditions of intense heat and pressure. This process changes the mineral composition and physical structure of the rock without it melting. For example, limestone can become marble, and shale can transform into slate under the right conditions. The key characteristic of metamorphic rocks is that their transformation is a direct result of these geological processes, differentiating them from igneous rocks, which are formed from the solidification of molten material, and sedimentary rocks, which are created through the accumulation and compaction of sediment. Crystalline rock is a broader term that can refer to rocks with a crystalline structure, including both igneous and metamorphic, but it does not specifically address the changes from heat and pressure. Thus, metamorphic rock is the correct choice in identifying rocks altered by these geological factors.