The process that causes metamorphic rock to change into igneous rock is melting and then cooling and solidifying. Here's how it works:

1. Melting: Metamorphic rocks, under intense heat and pressure deep within the Earth, can melt. This could be due to:

* Magma intrusion: Hot magma rising from the Earth's mantle can melt surrounding metamorphic rocks.

* Subduction: When tectonic plates collide, one plate can be pushed under the other (subduction). The intense pressure and heat can melt the subducted plate, which may include metamorphic rocks.

* Geothermal activity: Hot springs and volcanic areas can provide enough heat to melt metamorphic rocks.

2. Cooling and Solidification: Once the molten rock (magma) cools, it solidifies, forming new crystals. This process creates igneous rock.

Important Note: The new igneous rock will be a completely different rock type than the original metamorphic rock. It will have a new mineral composition and texture determined by the cooling conditions of the magma.