Question

Which of the following pairs represent correct plutonic - volcanic equivalents?

• A. Granodiorite - dacite
• B. Norite basalt
• C. Dunite - komatiite
• D. Nepheline syenite - phonolite

Answer 1

The Correct Option is A, D

To determine which pairs represent correct plutonic-volcanic equivalents, we need to understand the geological classification of rocks into two main categories: plutonic (intrusive) and volcanic (extrusive) rocks.

• Plutonic (Intrusive) Rocks: These are rocks formed from magma that cools and solidifies beneath the Earth's surface. They have a coarse-grained texture due to the slow cooling process.
• Volcanic (Extrusive) Rocks: These are rocks formed from lava that cools and solidifies on the Earth's surface. They have a fine-grained texture due to quick cooling.

For the question at hand, we need to match these pairs based on their classification:

1. Granodiorite - Dacite:
   • Granodiorite is a plutonic rock that is coarse-grained.
   • Dacite is the volcanic equivalent of granodiorite, having a similar composition but with a fine-grained texture. Thus, this is a correct pair.
2. Norite - Basalt:
   • Norite is a plutonic rock mainly composed of pyroxene and plagioclase.
   • Basalt is not the volcanic equivalent in terms of composition; its plutonic equivalent is gabbro. Thus, this is not a correct pair.
3. Dunite - Komatiite:
   • Dunite is a plutonic rock rich in olivine.
   • Komatiite is a rare volcanic rock, but not the exact volcanic equivalent of dunite. Instead, peridotite is seen as a better counterpart. Thus, this is not a correct pair.
4. Nepheline Syenite - Phonolite:
   • Nepheline syenite is a plutonic rock composed of nepheline and feldspar.
   • Phonolite is the volcanic equivalent of nepheline syenite. Thus, this is a correct pair.

Based on the above analysis, the correct plutonic-volcanic pairs are:

• Granodiorite - Dacite
• Nepheline Syenite - Phonolite

Hence, the correct answer includes these two pairs.