Part of this layer has a relatively homogeneous composition, while the other part is less uniform in composition

Oceanic part of this layer has a relatively homogeneous composition, while the other part is less uniform in composition

Explanation:

Your question is base on the earth crust which is either oceanic crust or continental.

The earth Lithosperic layer is divided into two part namely the oceanic crust and the continental crust.

The oceanic crust is more uniform in composition than the continental crust because of the kind  of earth events that is predominant here. For example sea floor spreading  . The oceanic crust is composed dominantly of Mafic rocks. The Mafic rocks are extrusive igneous rocks. This rocks are formed when magma intrudes into the ocean surface, due to rapid cooling this rocks solidified quickly to form these rocks that are mainly basaltic. The oceanic rocks are dominantly basaltic. The basaltic dominance makes the oceanic crust relatively homogeneous in composition.

Events like sea floor spreading whereby the oceanic floor diverge forming two parts are one factors that causes mantle materials(molten magma) to reach the surface to form this basaltic rocks. The separating  point, the mid oceanic ridges is where new crust are formed from the up-welling of magma.  As you move away from the mid oceanic ridges the rocks become older.  

atomic = no of protons.

mass number-atomic number = neutron number

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