Gneiss is obtained from
🪨 Understanding Gneiss: A Metamorphic Rock
The key to this question is understanding what Gneiss is and how it forms. Gneiss is a high-grade metamorphic rock, which means it has been subjected to intense heat and pressure deep within the Earth's crust.
Gneissic Banding: The defining characteristic of Gneiss is its distinct banding. During metamorphosis, the intense pressure causes the light-colored minerals (like quartz and feldspar) and dark-colored minerals (like mica and hornblende) to separate into alternating layers or bands.
🔬 The Dual Origin of Gneiss
This is an excellent question because Gneiss doesn't come from just one type of parent rock. It can be formed from the metamorphism of both igneous and sedimentary rocks.
From Igneous Rocks (Orthogneiss)
When an igneous rock, most commonly Granite, undergoes intense metamorphism, it transforms into Gneiss. This type of Gneiss is specifically called Orthogneiss. This is a classic and frequently cited example of Gneiss formation.
From Sedimentary Rocks (Paragneiss)
When a fine-grained sedimentary rock, such as Shale or Mudstone, is subjected to high-grade metamorphism, it can also transform into Gneiss. This type is called Paragneiss.
Conclusion on the Options
While options A and C are individually correct, option D is the best and most complete answer because it correctly identifies that Gneiss can be obtained from both igneous and sedimentary parent rocks. This demonstrates a more thorough understanding of the rock cycle.
📊 Orthogneiss vs. Paragneiss
| Type of Gneiss | Parent Rock Type | Common Parent Rock Example |
|---|---|---|
| Orthogneiss | Igneous | Granite |
| Paragneiss | Sedimentary | Shale / Mudstone |
💡 Study Tips
- Gneiss has a "Nice" layered look: Remember the characteristic banding of Gneiss.
- Remember the Prefixes: This is the best way to avoid confusion.
- Ortho- (from Greek 'orthos' meaning 'straight' or 'true') is for Gneiss from a true Igneous origin.
- Para- (from Greek 'para' meaning 'beside' or 'derived from') is for Gneiss derived from a Sedimentary origin.
- Granite -> Gneiss: This is the most famous rock transformation. If you can only remember one, remember this one. It will help you identify that Gneiss can come from an igneous rock.