Final answer:
The size of the crystals in rock increases as the rate of cooling decreases. Slowly cooled intrusive igneous rocks, like granite, are coarse-grained with large crystals, while quickly cooled extrusive rocks, like basalt, are fine-grained or glassy with small or no crystals.
Step-by-step explanation:
When rock cools more slowly, it has a significant effect on the size of the crystals that form within the rock. The slow cooling process allows ample time for individual crystals to grow larger because they have more time and space to develop.
Intrusive igneous rocks, such as granite, are formed from magma that cools slowly and deeply below the Earth's surface, resulting in coarse-grained textures with larger mineral grains. This is distinct from extrusive igneous rocks, such as basalt, which cool rapidly on the Earth's surface and are characterized by a fine-grained or glassy texture due to the lack of crystal growth.
In the formation of intrusive igneous rocks, minerals like feldspar and quartz crystallize in sequence. Feldspar often develops early and has straight crystal sides, whereas quartz crystallizes later and fills the spaces between the other minerals. The combination of these processes contributes to the coarse, sparkly appearance of rocks like granite.
Overall, the size of the crystals is a reflection of the rate of cooling, with slower rates promoting the development of larger crystals and faster rates yielding smaller crystals. This grain size helps geologists classify rocks and infer their formation history.