Final answer:
The oceanic crust at the solid red lines is made primarily of basalt. Basalt is a volcanic rock that forms from the cooling of lava. It is composed of elements such as silicon, oxygen, iron, aluminum, and magnesium.
Step-by-step explanation:
The oceanic crust at the solid red lines, which often indicate mid-ocean ridges where tectonic plates are diverging, is predominantly made of basalt. This basalt forms when mafic magma erupts onto the seafloor, creating volcanic rock due to the cooling of the magma. This crustal layer is about 6 kilometers thick, and basalt is primarily composed of the elements silicon, oxygen, iron, aluminum, and magnesium. The oceanic crust differs from continental crust, which is mainly composed of granite and other varied rock types and is much thicker in comparison.