Obsidian is an igneous rock that comes in a variety of colors and has an aphanitic and glassy texture. Due to its rapid cooling in explosive eruptions and its consistency high in silica, it's also called "volcanic glass." What is the composition of obsidian?

Question

Obsidian is an igneous rock that comes in a variety of colors and has an aphanitic and glassy texture. Due to its rapid cooling in explosive eruptions and its consistency high in silica, it's also called "volcanic glass."  What is the composition of obsidian?

BlackTom AI · Accepted Answer

Obsidian is a high-silica volcanic glass, which indicates a felsic (rhyolitic) composition.

1. Relevant knowledge points and concepts
- Composition categories (mafic, intermediate, felsic) are defined largely by silica (SiO2) content and associated minerals: mafic (~45–52% SiO2, rich in Fe/Mg, dark minerals), intermediate (~52–63% SiO2), felsic (>63% SiO2, rich in quartz and feldspar, lighter in color).
- Texture terms: phaneritic and aphanitic refer to crystal size (phaneritic = coarse-grained intrusive; aphanitic = fine-grained extrusive). Obsidian is glassy (no crystals) because of extremely rapid cooling.
- Obsidian typically forms from silica-rich (rhyolitic) magmas that cool too fast to crystallize, producing volcanic glass.

2. Step-by-step reasoning
- The question states obsidian is “high in silica” and forms from rapid cooling in explosive eruptions.
- High silica content corresponds to the felsic compositional range (>63% SiO2), i.e., rhyolitic chemistry.
- Rapid cooling yields glassy texture (obsidian), but composition remains felsic even though crystals are absent.

3. Calculations / intermediate steps
- No numeric calculation required, but using silica ranges clarifies classification:
  - Mafic ≈ 45–52% SiO2 → not matching “high in silica.”
  - Intermediate ≈ 52–63% SiO2 → still lower than typical obsidian.
  - Felsic >63% SiO2 → matches “high in silica” description.

4. Why the correct answer is correct
- Obsidian’s chemical signature is essentially rhyolitic (felsic): very high SiO2, abundant silica-rich glass matrix, and association with explosive, high-viscosity felsic eruptions. Those facts directly indicate a felsic composition.

5. Why the other options are incorrect
- Phaneritic: this is a texture (coarse-grained intrusive) not a composition; obsidian is glassy/aphanitic, so phaneritic is wrong.
- Mafic: denotes low-silica, iron- and magnesium-rich rocks (e.g., basalt), inconsistent with obsidian’s high silica.
- Intermediate: denotes moderate silica; obsidian is higher in silica than typical intermediate compositions.

6. Conclusion
- Therefore the correct composition classification for obsidian is felsic, which matches the provided answer: felsic.

Obsidian is an igneous rock that comes in a variety of colors and has an aphanitic and glassy texture. Due to its rapid cooling in explosive eruptions and its consistency high in silica, it's also called "volcanic glass." What is the composition of obsidian?单项选择题

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