Final answer:
Igneous rocks can be divided into two basic categories: intrusive or plutonic rocks, which form from magma that cools slowly within the Earth, and extrusive or volcanic rocks, which form from lava that cools rapidly on the Earth's surface.
Step-by-step explanation:
Igneous Rocks Categories Igneous rocks can be divided into two basic categories: intrusive (or plutonic) and extrusive (or volcanic). The classification of igneous rocks is based on chemistry and texture. The magma's chemistry and temperature determine which minerals will form within the rock. The size of the mineral grains is determined by the cooling rate of the magma. Slow cooling rates result in larger mineral grains, leading to the formation of intrusive igneous rocks, which cool slowly within the Earth's crust. Conversely, fast cooling rates produce smaller grains, leading to the formation of extrusive igneous rocks that cool quickly on the Earth's surface. The visibility of the mineral grains without magnification is a key factor in classifying the texture of igneous rocks. For example, a coarse-grained igneous rock containing 10% quartz, 10% potassium feldspar, 50% plagioclase feldspar, 20% pyroxene, and 10% amphibole would be classified as a diorite if it is intrusive. If the mineral grains were fine, indicating a fast cooling rate, the rock would be called andesite, an extrusive counterpart.