Question

At a depth of about 400 km inside the Earth, which one of the following occurs?

• A. Conversion of most silicates to perovskite structure
• B. Conversion of plagioclase-peridotite to spinel-peridotite
• C. Transformation of olivine to spinel structure
• D. Conversion of spinel-peridotite to plagioclase-peridotite

Hint:

Remember: At ~400 km depth, olivine undergoes phase transition to spinel; at ~660 km, spinel transforms into perovskite.

Answer 1

The Correct Option is C

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: \[ \boxed{\text{Transformation of olivine to spinel structure}} \]