Final answer:
The rate at which magma cools determines the size of the crystals in the new rock, with slower cooling allowing for the development of larger crystals, contributing to coarse-grained igneous rocks such as granite. The correct answer is option a.
Step-by-step explanation:
The rate at which magma cools determines the size of the crystals in the new rock. Slow cooling of magma allows individual crystals more time and space to grow larger. This process often occurs deep below the earth's surface, and these larger crystals contribute to the formation of coarse-grained igneous rocks such as granite. In contrast, rapid cooling results in the formation of fine-grained rocks, such as basalt, with smaller mineral crystals due to the limited time for crystal growth.
As the magma cools, different minerals crystallize at different temperatures. For instance, feldspar crystallizes early in the cooling process at high temperatures, which allows it to form large, well-defined crystals with straight sides. Later in the cooling process, quartz crystallizes and fills in the remaining spaces.
Overall, igneous rocks can be classified by their texture, which is determined by factors such as the cooling rate and the size of the mineral grains present. Intrusive or plutonic igneous rocks exemplify a slower cooling process that permits the development of large, easily seen mineral grains, often giving the rock a sugary texture when light is reflected off the flat crystal faces.