Igneous petrology — plutonic vs. volcanic examples Which of the following is a plutonic (intrusive) igneous rock formed by slow cooling at depth?

Introduction / Context:Igneous rocks are categorized by cooling environment: intrusive (plutonic) rocks crystallize slowly underground, producing coarse grains; extrusive (volcanic) rocks cool rapidly at the surface, yielding fine grains or glassy textures. Recognizing these helps predict engineering behavior.

Given Data / Assumptions:

• Plutonic examples: granite, gabbro.
• Hypabyssal (shallow intrusive): dolerite/diabase, with medium grain.
• Extrusive examples: basalt, rhyolite.

Concept / Approach:Granite is the classic plutonic rock: coarse-grained quartz-feldspar-mica texture from slow cooling at depth. Dolerite is typically hypabyssal and medium-grained; basalt is fine-grained extrusive.

Step-by-Step Solution:Identify cooling environment: deep and slow → plutonic.Match example: granite fits plutonic; dolerite is subvolcanic; basalt is volcanic.Therefore, the plutonic rock among options is granite.

Verification / Alternative check:Hand specimen reveals coarse interlocking grains in granite; basalt shows fine matrix; dolerite displays medium ophitic textures.

Why Other Options Are Wrong:Dolerite is not fully plutonic; it is hypabyssal.

Basalt is extrusive; rapid cooling at the surface.

“All of the above” cannot be correct because basalt is not plutonic.

Rhyolite is extrusive, felsic equivalent of granite.

Common Pitfalls:Grouping all igneous rocks without regard to cooling depth; assuming grain size solely indicates mineral composition rather than cooling rate.

Final Answer:Granite