Question

What does crystal size generally indicate about the cooling of a melt?

a. Larger crystals indicate the melt cooled on the surface of the Earth
b. Larger crystals indicate the melt cooled at a shallower depth
c. Larger crystals indicate that the melt cooled rapidly
d. Larger crystals indicated that the melt cooled slowly, deep within the earth

Answer 1

Larger crystals indicate that the melt cooled slowly, deep within the earth. Option D is the answer.

Step-by-step explanation:

The size of crystals in igneous rocks provides valuable information about the cooling history of the molten rock (magma or lava). Larger crystals generally indicate a slower cooling process, allowing sufficient time for crystals to grow. In this context, option d is correct; larger crystals suggest that the melt cooled slowly, deep within the Earth's crust.

This phenomenon occurs because slower cooling allows for the formation of larger crystals, while rapid cooling, as in the case of volcanic activity or surface cooling, results in smaller crystals or even an amorphous (glassy) texture. Therefore, by examining crystal size, geologists can infer the conditions and depth at which the rock cooled.

Option D is the answer.