Final answer:
Mafic rocks like basalt and gabbro are high in iron and magnesium, predominantly make up the oceanic crust, and are found extensively in the ocean floor and volcanic island chains, rather than in the continental mountain belts or crust.
Step-by-step explanation:
Mafic rocks such as basalt and gabbro are known for their high iron and magnesium content. They are integral to the composition of the oceanic crust, which is mostly submerged under the oceans and encompasses about 55% of Earth's surface. Oceanic crust, typically about 6 kilometers thick, consists predominantly of volcanic rocks like basalt, which are produced by the cooling of volcanic lava.
Contrastingly, the continental crust is mainly composed of granite and includes a variety of igneous, metamorphic, and sedimentary rocks. It is significantly less dense than the oceanic crust, causing it to rise higher on the mantle. However, basalt and gabbro's presence is most pronounced in the ocean floors and volcanic island chains, where these mafic rocks form a large part of the geological structure, unlike the composition of continental crust or continental mountain belts.
Therefore, the correct answer to the student's question is that mafic rocks like basalt and gabbro make up most of the ocean floor and volcanic island chains.
Answer: d) make up most of the ocean floor and volcanic islands chains