What term best describes rocks that formed by relatively fast cooling?

The term that best describes rocks formed by relatively fast cooling is "aphanitic." This type of rock texture is characterized by a fine-grained appearance, where individual mineral crystals are typically too small to be seen without magnification. This rapid cooling usually occurs when molten rock, or magma, is extruded onto the Earth's surface during a volcanic eruption and solidifies quickly, leading to the formation of volcanic rocks.

In contrast, phaneritic textures indicate slower cooling rates, allowing larger crystals to develop, which are observable without a microscope. Volcanic is a broader category that encompasses a variety of rocks formed from lava, not all of which are aphanitic. Granitoid refers to a group of coarse-grained igneous rocks, typically formed from a slower cooling process in the Earth's crust, resulting in a phaneritic texture. Hence, aphanitic is specifically the term that accurately describes the fast cooling process and resulting texture.