Here's why:

* Igneous rocks are formed from the cooling and solidification of magma (molten rock) or lava.

* Extrusive igneous rocks form when lava erupts onto the Earth's surface and cools quickly.

* Intrusive igneous rocks form when magma cools and solidifies beneath the Earth's surface.

Since lava flow is the result of molten rock (lava) cooling on the surface, it falls under the category of extrusive igneous rocks.