Michigan Test for Teacher Certification (MTTC) Secondary Integrated Science Practice test

What distinguishes volcanic igneous rocks from plutonic igneous rocks?

1. Volcanic rocks come from underground sources

2. Plutonic rocks form above ground

3. Volcanic rocks cool quickly at the surface

4. Plutonic rocks are always finer-grained

The correct answer is: Volcanic rocks cool quickly at the surface

Volcanic igneous rocks are formed from the rapid cooling and solidification of molten lava or magma at or near the Earth's surface. Because of this quick cooling process, the minerals within volcanic rocks have less time to grow, resulting in a fine-grained texture. This characteristic is essential for distinguishing volcanic rocks from plutonic rocks, which form from magma that cools slowly beneath the Earth's surface, allowing for the growth of larger, coarser-grained crystals. In contrast, plutonic rocks, formed in deeper crustal environments, typically exhibit a much coarser texture because the slower cooling process allows crystals to develop fully. This difference in cooling rates and resulting textures is fundamental in classifying igneous rocks based on their formation environments.