When igneous rocks are exposed to great heat and pressure, they can undergo metamorphism. This means they transform into a new type of rock, a metamorphic rock, with different mineral composition, texture, and often, a different appearance.

Here's what happens:

* Heat: The intense heat causes the existing minerals in the igneous rock to become unstable. They start to break down and rearrange their atoms to form new minerals that are stable at higher temperatures.

* Pressure: The pressure compresses the rock, squeezing out fluids and rearranging the minerals. This can cause the rock to become denser and more compact.

* Fluids: The heat and pressure can also drive fluids through the rock, dissolving and transporting minerals, leading to further changes in composition.

The type of metamorphic rock that forms depends on the original igneous rock, the intensity of heat and pressure, and the presence of fluids. Here are some examples:

* Basalt, a dark-colored igneous rock, can transform into schist, a metamorphic rock with a layered texture, or gneiss, a metamorphic rock with a banded texture.

* Granite, a light-colored igneous rock, can transform into gneiss or migmatite, a mixture of metamorphic and igneous rock.

Metamorphism is a continuous process, meaning the more intense the heat and pressure, the more drastic the changes in the rock. Eventually, if the temperature reaches the melting point of the rock, it will melt and turn into magma, starting the igneous rock cycle again.