It's important to clarify that rocks don't become igneous rocks. Igneous rocks are formed directly from the cooling and solidification of molten rock. Here's the breakdown:

1. Melting: Existing rocks (igneous, sedimentary, or metamorphic) must melt deep within the Earth's crust or mantle. This can be caused by various factors like:

* Heat from the Earth's core: The Earth's core provides a constant source of heat, which can melt surrounding rocks.

* Pressure: When rocks are buried deep under the Earth's surface, the immense pressure can cause them to melt.

* Presence of water: Water can lower the melting point of rocks, making them melt at lower temperatures.

2. Cooling and Solidification: Once the molten rock (magma) reaches the surface (lava) or cools underground, it starts to solidify. The process of cooling and solidification determines the texture and mineral composition of the igneous rock.

In summary, the key processes are:

* Melting: Existing rocks transform into magma.

* Cooling and Solidification: Magma or lava cools and solidifies into igneous rock.