In traditional physical-geography terminology, what are the main constituents…

Concept

In the traditional compositional terminology used in physical geography, crustal layers are identified by their dominant chemical components. The upper continental crust is associated with SIAL, meaning silica and alumina, while the denser oceanic crust is associated with SIMA, meaning silica and magnesium.

Application

This question uses the traditional SIAL–SIMA classification. Oceanic crust is mapped to SIMA, so the relevant constituent pair is silica and magnesium.

Contrast

• Silica and alumina form SIAL, the shorthand associated with the upper continental crust.

• Silica and nickel mix a crustal silicate component with nickel, which appears in the Ni–Fe description of the core.

• Silica and magnesium form SIMA, the shorthand associated with the denser mafic crust.

• The composite choices depend on whether several or none of the listed constituent pairs apply; they do not replace the specifically identified SIMA pair.

Cross-check

Oceanic crust is basaltic and denser than continental crust. Its magnesium-rich mafic character is consistent with the traditional SIMA description.

Therefore, the answer by value is silica and magnesium.