What type of rock is formed through the alteration of existing rock by heat and pressure?

Metamorphic rock is formed through the alteration of existing rock, which can be either igneous, sedimentary, or even other metamorphic rocks, under conditions of high heat and pressure. This process is known as metamorphism, and it leads to changes in the mineral composition and structure of the rock without the rock melting.

During metamorphism, the minerals within the parent rock may become unstable due to the increased temperatures and pressures. As a result, they can either recrystallize into new minerals or realign in a different structure, often resulting in foliation or banding. Common examples of metamorphic rocks include schist and gneiss, which originate from sedimentary rocks and granite, respectively.

In contrast, igneous rocks form from the solidification of molten rock (magma or lava), while sedimentary rocks are created from the accumulation and compaction of sediments. Volcanic rocks are a subset of igneous rocks that specifically form from lava that cools and solidifies on the Earth's surface. Each of these rock types has distinct origins and processes of formation, which is why metamorphic rock stands out as the correct answer regarding alteration by heat and pressure.