Question

Which one of the following is an ultramafic rock?

• A. Granite
• B. Gabbro
• C. Dunite
• D. Basalt

Hint:

Remember:
- Felsic rocks (e.g., Granite) → High silica, light-colored.
- Mafic rocks (e.g., Basalt, Gabbro) → Moderate silica, darker.
- Ultramafic rocks (e.g., Dunite, Peridotite) → Very low silica, extremely rich in Mg and Fe.

Answer 1

The Correct Option is C

Step 1: Definition of ultramafic rocks.
Ultramafic rocks are igneous rocks that contain more than 90\% mafic minerals such as olivine and pyroxene.
They are very low in silica (less than 45\%) and are rich in magnesium and iron. Step 2: Examine each option.
- Granite: A felsic rock rich in silica and quartz, not ultramafic.
- Gabbro: A mafic rock (not ultramafic), rich in pyroxene and plagioclase.
- Dunite: Composed almost entirely of olivine (more than 90\%), making it an ultramafic rock.
- Basalt: A mafic volcanic rock, not ultramafic. Step 3: Conclusion.
Since dunite is made up predominantly of olivine (an ultramafic mineral), the correct answer is: \[ \boxed{\text{Dunite}} \]