Question

During contact metamorphism, with increasing temperature,

• A. the ratio of volume to surface area of mineral grains increases.
• B. the ratio of volume to surface area of mineral grains decreases.
• C. the reaction kinetics becomes slower.
• D. hydrous minerals become more stable.

Hint:

At higher temperatures, minerals break into smaller grains, increasing surface area relative to volume. Hence \(V/A\) ratio always decreases.

Answer 1

The Correct Option is B

Step 1: Recall contact metamorphism process. Contact metamorphism occurs when rocks are heated by intrusion of magma, usually at relatively shallow crustal levels. High temperature is the dominant factor.

Step 2: Effect of increasing temperature. With increasing temperature: - Grain boundaries become unstable, grains break down into smaller crystals. - As grains become finer, their surface area per unit volume increases.

Step 3: Interpret the ratio. \[ \text{Ratio} = \frac{\text{Volume}}{\text{Surface Area}} \] - For smaller grains, surface area increases faster than volume. - Therefore, the ratio \(\dfrac{V}{A}\) decreases.

Step 4: Eliminate wrong options. - (A) Increases → incorrect. - (C) Reaction kinetics become slower → wrong, they become faster at higher temperature. - (D) Hydrous minerals become more stable → wrong, they tend to dehydrate and break down.

Final Answer: \[ \boxed{\text{the ratio of volume to surface area of mineral grains decreases.}} \]