3.4.2 Igneous Rocks: Aphanitic Texture, Phaneritic Texture, and Vesicular Texture

Aphanitic texture is fine grained. Igneous rocks that form at the surface or as small masses within the upper crust where cooling is relatively rapid possess a very fine-grained texture termed aphanitic. The etymology of this word: “a” means not , “phaner” means visible. By definition, the crystals that make up a aphanitic rocks are so small that individual minerals can only be distinguished with the aid of a microscope. Because mineral identification is not possible, we commonly characterized fine-grained rocks as being light, intermediate, or dark in color. Using this system of grouping, light-colored aphanitic rocks are those containing primarily light-colored non ferromagnesian silicate minerals and so forth.

Playlist of Geology and Earth Science Videos from Snow Mountain
https://www.youtube.com/watch?v=owlPSr39Nn8&list=PL6taRb3I0WLhYc8g1cuBk5crqUtLs3oXZ

Commonly seen in many aphanitic rocks are the voids left by gas bubbles that escape as lava solidifies. These spherical or elongated openings are called vesicles, and the rocks that contain them are said to have a vesicular texture. Rocks that exhibit a vesicular texture usually form in the upper zone of a lava flow, where cooling occurs rapidly enough to freeze the lava, thereby preserving the openings produced by the expanding gas bubbles.

Phaneritic texture is coarse-grained. When large masses of magma slowly solidify far below the surface, they form igneous rocks that exhibit a coarse grained texture described as phaneritic. These coarse grained rocks consists of massive intergrown crystals which are roughly equal in size and large enough so that the individual minerals can be identified without a microscope. Geologists often use a small magnifying lens to aid in identifying coarse-grained minerals. Because phaneritic rocks form deep within the Earth’s crust, their exposure at Earth’s surface results only after erosion removes the overlying rocks that once surrounded the magma chamber.