Final answer:
Basalt and gabbro are considered compositional equivalents because they are both mafic rocks with similar mineral content but different textures due to the cooling rates of their respective formation processes.
Step-by-step explanation:
When we say that basalt and gabbro are compositional equivalents, we are indicating that they are both mafic rocks with similar mineral content, typically including pyroxene, plagioclase, and sometimes olivine or amphibole. However, they differ in their texture due to their formation processes. Basalt is an extrusive igneous rock, meaning it cooled from lava quickly at the Earth's surface, typically resulting in a finer-grained texture. On the other hand, gabbro is an intrusive igneous rock, which means it cooled slowly from magma beneath the Earth's surface, leading to a coarser-grained texture. Both of these rock types are significant because they make up large parts of Earth's crust; basalt is predominant in the oceanic crust, whereas gabbro forms similar compositions beneath the seafloor.