Final answer:
Very small crystals in an igneous rock indicate that it was formed quickly, whereas large crystals in an igneous rock suggest that it cooled slowly over a long period of time.
Step-by-step explanation:
Igneous rocks form from the solidification and cooling of molten magma or lava. Classified into intrusive (formed beneath the Earth's surface) and extrusive (formed on the surface), common types include basalt, granite, and obsidian. Igneous rocks play a key role in Earth's geological processes and provide valuable insights into its history.
Very small crystals in an igneous rock show that it was formed quickly. Extrusive igneous rocks naturally cool quickly, resulting in fine-grained rocks or rocks with a lack of crystal growth. On the other hand, if the magma cools slowly over thousands of years, it has time to form large crystals, resulting in coarse-grained, intrusive or plutonic rocks.