Rock Textures

The origin of a rock can often be detected from its texture—the sizes and orientations of its mineral or rock fragment components. Most extrusive rocks are fine grained, meaning their mineral components (grains) are less than 1 millimeter in diameter. Lava flow rocks typically have a chilled margin that is very fine grained, or aphanitic. Grain size then increases progressively toward the center of the flow. Thicker flows can have medium‐ to coarse‐grained centers.

A porphyritic rock contains coarser‐grained crystals (phenocrysts) that are supported in a matrix (groundmass) of finergrained minerals. The larger minerals had already crystallized and were extruded with the magma, which then rapidly cooled to form the groundmass. Obsidian (volcanic glass) is a hard, supercooled, very fine‐grained volcanic rock composed of silica.

Basalt flows that have a ropy surface are called pahoehoe flows and form when the lava's exterior quickly solidifies into rock. An aa (pronounced ah‐ah) flow develops a partially solidified surface as it moves forward. Continued advance breaks the solidified flow's top and sides into a rough, rubbly mass.

Magmas often contain dissolved gas because of higher pressures deep underground. When the magma is suddenly released and vents at the surface, the gas “bubbles” out of the magma, creating numerous holes, cavities, or voids called vesicles. Pumice is a volcanic rock that has so much internal void space from gas bubbles that it floats in water. Scoria is a very vesicular basalt that contains more gas space than rock and has a very rough, irregular, and pocked exterior.

The lithification of ejected rock fragments and other pyroclastic material creates a variety of fragmental textures. Dust and ash are the finest‐grained particles, followed by cinders (pea sized), lapilli (walnut sized), and bombs or blocks, which can be up to a meter across or larger. Blocks are ejected pieces of hardened lavas; bombs are semimolten pieces of lava that solidify as they fall. Small crystals (generally feldspars) that had been formed in the magma before it was ejected are also deposited with the other pyroclastics. A tuff is composed of fine‐grained pyroclastic material and is named by the most distinctive component, such as an ash tuff or crystal tuff. A welded tuff is a rock that consists of ash particles and glass shards that were hot enough to fuse together when it was deposited. The rocks that contain the larger bombs are called tuff breccias or agglomerates.

Other distinctive extrusive rock textures occur in flood basalts and submarine lava flows. Flood basalts cool and contract to form vertical, parallel, generally six‐sided columns called columnar structures or columnar jointing (Figure ). As a submarine lava flow cools, blobs of lava may break through the exterior and harden immediately in the cold water, forming small rounded shapes called pillow structures. These are especially useful to the geologist for determining that the rock was formed on the ocean floor and for indicating the base of the flow (Figure ).

Figure 1

              Columnar Jointing

Figure 2

               Pillow Structures