The extrusive compositional equivalents of granite and granodiorite are which rocks?

• A. Rhyolite and dacite
• B. Diorite and gabbro
• C. Granite and granodiorite
• D. Obsidian and pumice

Answer: (A)

When magma that is rich in silica and light-colored minerals erupts and cools at the surface, it forms fine-grained volcanic rocks that carry the same chemical makeup as their plutonic counterparts. Granite and granodiorite are felsic intrusive rocks, so their extrusive counterparts—those that crystallize rapidly at the surface—are rhyolite and dacite, respectively. Rhyolite mirrors granite’s high silica and alkali content with a fine texture, while dacite mirrors granodiorite’s composition in a finer-grained volcanic form.

The other options don’t fit as cleanly. Diorite and gabbro are intrusive equivalents of less felsic, more mafic rocks, not the granite/granodiorite pair. Obsidian and pumice are glassy or highly vesicular products of rapid cooling, not the standard fine-grained compositional equivalents.