In igneous petrology, the size of mineral crystals in igneous rocks is primarily determined by which controlling factor during solidification of magma or lava?

Introduction / Context:
Igneous rocks form when molten material, either magma underground or lava at the surface, cools and solidifies. One of the key observable features of igneous rocks is the size of their mineral crystals. This question asks what main factor controls crystal size in igneous rocks.

Given Data / Assumptions:

• Igneous rocks can be coarse grained or fine grained.
• Cooling can occur slowly deep underground or rapidly at the surface.
• The options mention cooling rate, depth, color, and dissolved gases.
• We assume standard geological processes of crystallization from molten rock.

Concept / Approach:
The fundamental principle is that slow cooling gives crystals more time to grow larger, while rapid cooling produces many small crystals or even glassy textures. Deep seated intrusive rocks like granite cool slowly and have large crystals, while extrusive rocks like basalt cool quickly at the surface and have small crystals. Cooling rate is therefore the primary factor controlling crystal size. Other factors like composition and gas content can influence texture but are not the main control on crystal size.

Step-by-Step Solution:

Verification / Alternative check:
Geology textbooks show diagrams comparing slow cooled granite and fast cooled basalt and explicitly state that crystal size increases with slower cooling. Laboratory crystallization experiments also demonstrate that solutions cooled slowly form larger crystals than those cooled quickly. These observations from both natural rocks and experiments confirm that cooling rate is the primary factor controlling crystal size in igneous rocks.

Why Other Options Are Wrong:

• Depth from the surface is wrong as a direct factor; it influences cooling rate but is not the fundamental parameter in itself.
• Color of minerals is wrong because color depends on chemical composition and oxidation state, not mostly on crystal size.
• Amount of dissolved gases is wrong because it mainly affects vesicle formation and explosiveness, not the basic relationship between cooling rate and crystal growth.

Common Pitfalls:
Students sometimes overemphasize the role of depth or gas content because these factors are discussed in the context of volcanoes. The key is to focus on the process of crystallization. If atoms have more time to migrate and attach to crystal faces, larger crystals grow. When cooling is rapid, there is not enough time, and only small crystals or glass form. Remember: slow cooling equals large crystals, fast cooling equals small crystals.

Final Answer:
The rate at which the molten magma or lava cools and solidifies