At a depth of about 400 km inside the Earth, which one of the following occurs?
Show Hint
- Conversion of most silicates to perovskite structure
- Conversion of plagioclase-peridotite to spinel-peridotite
- Transformation of olivine to spinel structure
- Conversion of spinel-peridotite to plagioclase-peridotite
The Correct Option is C
Solution and Explanation
Step 1: Recall Earth's mantle mineralogy. - The Earth's upper mantle mainly consists of peridotite (rich in olivine and pyroxenes). - With depth, increasing pressure leads to structural transitions of minerals.
Step 2: Depth-related phase transitions. - At shallow depth (< 50 km): plagioclase peridotite is stable. - Between 50–100 km: spinel peridotite is stable. - At ~400 km depth (transition zone): olivine (forsterite) transforms into a spinel structure (wadsleyite). - At deeper depth (~660 km): spinel transforms into perovskite structure.
Step 3: Check options. - (A) Perovskite structure occurs deeper (~660 km), not at 400 km. - (B) Plagioclase → Spinel transition occurs at ~50–100 km, not 400 km. - (C) Olivine → Spinel transition occurs at ~400 km. Correct. - (D) Spinel → Plagioclase is reverse of actual process, so incorrect.
Final Answer: \[ \boxed{\text{Transformation of olivine to spinel structure}} \]
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