Final answer:
Igneous rocks are classified into intrusive (plutonic) or extrusive (volcanic) based on their texture, which is determined by the cooling rate. A coarse-grained igneous rock formed from slow cooling beneath the Earth's surface is categorized as intrusive and could be called diorite, while a fine-grained version of the same composition is called andesite.
Step-by-step explanation:
Igneous Rock Classification and Formation
The rock described in the question is an igneous rock, which is a type of rock formed from the solidification of molten material called magma (below the Earth's surface) or lava (on the Earth's surface). The cooling process significantly influences the texture of the igneous rock. Intrusive or plutonic rocks form when magma cools slowly beneath the Earth's surface, resulting in coarse-grained textures because the slow cooling allows large crystals to form. Conversely, extrusive or volcanic rocks form when lava cools quickly at the surface, leading to fine-grained textures with smaller crystals, often not visible without magnification.
If a rock has larger grains, indicating that it cooled slowly under the Earth's surface, it belongs to the group of intrusive igneous rocks. A rock composed of 10% quartz, 10% potassium feldspar, 50% plagioclase feldspar, 20% pyroxene, and 10% amphibole with a coarse-grained texture is called a diorite. However, if it had a fine-grained texture, the same composition would lead to the rock being classified as andesite.
The texture and mineral composition provide crucial insight into the history of an igneous rock, including where and how it formed. This process of classification based on the original composition of the molten material and the cooling history is vital to understanding the diverse range of igneous rocks found on Earth.