Final answer:
Crystal formation size in igneous rocks is determined by the rate of cooling; slow cooling allows for large crystals to develop, while fast cooling generally results in fine-grained or glassy textures.
Step-by-step explanation:
The size of crystal formation during the cooling of magma or lava is significantly influenced by the rate at which the material cools. Slow cooling, typically occurring beneath the Earth's surface, allows individual crystals to grow large, clearly seen in coarse-grained textures where crystals are large enough to distinguish between different minerals. For instance, feldspar, which crystallizes early at high temperatures, tends to have straight crystal sides due to the available space and time for growth.
In contrast, extrusive igneous rocks, which cool quickly, often lack significant crystal growth, resulting in a fine-grained texture. An example of this would be a basaltic lava flow, which cools quickly after erupting. Conditions under rapid cooling may also lead to the formation of glass, such as in silica, where SiO4 tetrahedra do not arrange in a crystalline structure but rather form a glass with a random arrangement of molecules.