* Igneous rocks form directly from the cooling and solidification of molten rock (magma or lava). This is the very first step in the rock cycle.

* Sedimentary rocks form from the weathering and erosion of pre-existing rocks (including igneous rocks), which are then transported, deposited, and cemented together.

* Metamorphic rocks form when existing igneous, sedimentary, or even other metamorphic rocks are subjected to intense heat and pressure, causing them to change their mineral composition and texture.

Therefore, igneous rocks must form first because they are the source material for the other rock types.