The oceanic crust is relatively thin compared to the continental crust. While the continental crust can reach an average thickness of 35 kilometers, the oceanic crust is typically 5 to 10 kilometers thick. This difference is due to the distinct geological processes that form the two types of crust.

The oceanic crust is formed at spreading centers, where new crustal material rises from the Earth's mantle. As these new oceanic crustal plates move away from the spreading center, they cool, contract, and become denser, sinking into the underlying mantle. This process of ocean floor spreading leads to the formation of the deep seafloor and ocean basins.

On the other hand, the continental crust is formed through various geological events such as volcanic eruptions, mountain-building processes, and the accretion of crustal materials. Over millions of years, the continental crust has experienced subduction, collision, and differentiation, leading to the thickening and stabilization of its structure.

Therefore, the oceanic crust is not a thick layer compared to the continental crust due to differences in their formation and evolution.