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

Step 1: Understanding the Question:
We need to pick the rock that is NOT formed as a volcanic, meaning extrusive, igneous rock, from among Dacite, Syenite, Trachyte and Komatiite.

Step 2: Key Concept:
Igneous rocks form from cooling magma or lava and are grouped by where that cooling happens. Volcanic (extrusive) rocks cool quickly at or near the surface, giving them a fine grained texture. Plutonic (intrusive) rocks cool slowly deep underground, giving them a coarse grained texture. Many plutonic rocks have a fine grained, chemically matching extrusive twin, for example granite pairs with rhyolite, and gabbro pairs with basalt.

Step 3: Detailed Explanation:
Dacite is a fine grained, extrusive rock of intermediate composition, chemically between andesite and rhyolite, formed from lava, so it is volcanic.
Trachyte is the fine grained, extrusive equivalent of syenite, also formed from lava at the surface, so it is volcanic too.
Komatiite is an ultramafic lava rock, mostly known from ancient Archean eruptions that were hot enough to produce this unusual, very magnesium rich lava, so it is also volcanic.
Syenite, unlike the other three, is a coarse grained rock that crystallizes slowly deep inside the crust. It is the plutonic, intrusive counterpart of trachyte, sharing a similar chemical composition but a different, slow cooling origin.

Step 4: Final Answer:
Syenite is the plutonic rock in the list, not a volcanic one, so the correct option is (B). Dacite, option (A), Trachyte, option (C), and Komatiite, option (D), are all extrusive lava rocks.