The Sierra Nevada pluton in California is composed primarily of granite and granodiorite. What are the extrusive compositional equivalents of these rocks?

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

Answer: (A)

Intrusive felsic rocks and their surface counterparts share the same chemical makeup; the difference is texture from cooling rate. Granite and granodiorite are felsic plutonic rocks, so their volcanic (extrusive) equivalents are rhyolite and dacite, respectively. Rhyolite matches granite’s high silica and light minerals but cools quickly to a fine-grained or glassy texture, while dacite matches granodiorite’s composition and silica level in a finer-grained volcanic form. The other options don’t fit because they either stay with intrusive rocks, pair with rocks of different composition (diorite and gabbro), or present extrusive rocks that don’t align one-to-one with granite and granodiorite.