Match the following rocks to their equivalent aphanitic or phaneritic igneous rocks. A. andesite 39) 40) 41) B. gabbro C. rhyolite granite basalt diorite 42) Which of the following igneous rocks exhibit aphanitic texture? A) rhyolite; gabbro B) granite; gabbro C) andesite; diorite D) andesite; rhyolite
The Correct Answer and Explanation is :
Here are the matches for the given rocks based on their equivalent aphanitic (fine-grained) or phaneritic (coarse-grained) igneous textures:
- A. Andesite → Diorite (Aphanitic and Phaneritic equivalents, respectively)
- B. Gabbro → Basalt (Phaneritic and Aphanitic equivalents, respectively)
- C. Rhyolite → Granite (Aphanitic and Phaneritic equivalents, respectively)
Answer to Question 42:
D) Andesite; Rhyolite
These are the rocks that exhibit an aphanitic texture.
Explanation:
Igneous rocks are categorized based on their texture and composition. The terms aphanitic and phaneritic describe the texture of the rock, which is determined by the size of the mineral crystals and the cooling history of the magma:
- Aphanitic Texture:
- Aphanitic rocks have fine-grained textures, meaning the mineral crystals are too small to see with the naked eye.
- This texture forms when lava cools quickly at or near the Earth’s surface, not giving crystals enough time to grow.
- Examples: Andesite, Basalt, Rhyolite.
- Phaneritic Texture:
- Phaneritic rocks are coarse-grained, meaning the crystals are large enough to see without magnification.
- This texture forms when magma cools slowly beneath the Earth’s surface, allowing ample time for crystal growth.
- Examples: Diorite, Gabbro, Granite.
The relationship between these pairs arises from their composition:
- Rhyolite and Granite are both silica-rich rocks with a felsic composition.
- Andesite and Diorite are intermediate in composition, between felsic and mafic.
- Basalt and Gabbro are mafic rocks, rich in iron and magnesium.
By understanding the cooling process and mineral composition, these equivalencies are straightforward. Thus, for rocks exhibiting aphanitic textures, the correct answer is andesite and rhyolite.