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Analysis on Rock Textures
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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
arefine 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) flowdevelops 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
calledvesicles. 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,
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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
anash 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
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Figure 2
Pillow Structures
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