Here's why:

* Tuff forms from volcanic eruptions: Tuff is made up of ash, pumice, and other volcanic debris that has been ejected from a volcano and then compacted and cemented together.

* Igneous rocks form from cooling magma or lava: Since tuff originates from volcanic material, it directly fits into the definition of an igneous rock.

While tuff can sometimes undergo changes due to heat and pressure (metamorphism), its primary origin is volcanic, making it an igneous rock.