Here's why:

* The Earth's crust is not composed of four layers. The Earth's crust is a single layer, though it can be divided into two main types:

* Oceanic crust: Thinner, denser, and composed primarily of basalt and gabbro (igneous rocks).

* Continental crust: Thicker, less dense, and composed of a variety of igneous, metamorphic, and sedimentary rocks.

* Igneous rocks are indeed a significant component of the Earth's crust. They are formed from the cooling and solidification of magma (molten rock) or lava.

Let me know if you have any other questions about the Earth's structure!