What would happen to an igneous rock exposed at earths surface?

An igneous rock can change under pressure to metamorphic.

a igneous rock can turn into another igneous rock by melting then resolidifying, it can turn into a sedimentary rock by getting weathered then recemented, or it can turn into a metamorphic rock by being subjected to great heat and pressure

It would turn into a lava rock like to give your garden cirtent types of nutrients