Final answer:
Basalt is more dense than granite due to its higher iron and magnesium content and the fine-grained structure from rapid cooling, compared to the slow-cooled, coarse-grained granite.
Step-by-step explanation:
Basalt is more dense than granite primarily because of its composition and the conditions under which it forms. Basalt is a mafic rock, meaning it has a higher content of magnesium and iron, elements that are denser than the silica and aluminum elements that are abundant in the continental crust and its predominant rock, granite. The oceanic crust, where basalt is usually found, is denser and thinner, whereas granite makes up the thicker, less dense continental crust. Basalts are formed from the rapid cooling of lava at the Earth's surface, resulting in a fine-grained structure, whereas granites are formed from slow cooling magma deep underground, creating a coarse-grained rock (plutonic or intrusive igneous rock).
The density of these rocks is an important characteristic, affecting not only the layering of the Earth's crust but also geological phenomena like volcanic activity and continental drift. The average density of crustal rocks is about 3 g/cm³, and basalt's density is consistent with oceanic crust, whereas granite aligns with the composition of continental crust.