Here's why:

* Earth's Crust: The Earth's crust is the outermost layer, composed of rock. It's divided into two main types:

* Oceanic Crust: Thinner and denser, made primarily of basalt (an igneous rock).

* Continental Crust: Thicker and less dense, with a more complex composition but largely composed of granite (also an igneous rock).

* Igneous Rocks: These rocks form from the cooling and solidification of molten rock (magma or lava).

Therefore, three of the four layers of the Earth's crust are primarily composed of igneous material (oceanic crust, continental crust, and a thin layer of igneous rock that may be present within the continental crust). The fourth layer would be sedimentary rocks, which are formed from the accumulation and cementation of sediments.