Question

Which one of the following is NOT a volcanic igneous rock?

• A. Dacite
• B. Syenite
• C. Trachyte
• D. Komatiite

Hint:

Three of the four are fine grained lava rocks. One is coarse grained and forms slowly underground instead.

Answer 1

The Correct Option is B

The question is really testing whether we can tell apart volcanic (extrusive, surface cooled) rocks from plutonic (intrusive, deep cooled) rocks by name.

1. Dacite: a fine grained lava rock of intermediate composition, formed by rapid cooling at the surface, so it counts as volcanic.
2. Syenite: a coarse grained rock that forms only when magma cools slowly far underground. It is the plutonic partner of trachyte, sharing chemistry but not the eruption history, so it is NOT volcanic.
3. Trachyte: the fine grained lava equivalent of syenite, erupted and cooled quickly at the surface, so this one is volcanic.
4. Komatiite: an ultramafic lava, mostly erupted in the ancient Archean eon under unusually hot mantle conditions, still a surface cooled, volcanic rock.

Syenite is the odd one out because it crystallizes slowly underground rather than erupting as lava, so option (B) is the correct answer.

Let's summarize:

• Dacite, Trachyte and Komatiite are all fine grained rocks formed from lava at or near the surface.
• Syenite is coarse grained and plutonic, the intrusive twin of trachyte.

So Syenite is the rock that is NOT volcanic.