Texture
Textures of Igneous Rock
You will learn to observe and recognize the particular texture of an unknown sample of an igneous rock because texture is one of the two factors in the simplified classification table.
These textures are features developed and controlled by the environment and process of solidification.
There are four (4) primary textures that develop during solidification of molten rock.
crystalline
fragmental
glassy
vesicular
One example of each type is provided below. Study each image and refer to your lab book or other references for additional detail. Then view instructional videos in each section.
Click any image to enlarge.
Crystalline texture is recognized by the presence of mineral crystals that interlock like pieces of a jigsaw puzzle - 1, 2, and 3 in the image. Crystals may be large enough to be visible (as shown here), or microscopic. Crystalline texture forms by the cooling of magma and the formation of minerals during crystallization.
Watch the crystalline instructional videos for further details.
Image credit: Scott Brande
Fragmental texture is recognized by the presence of a mixture of debris including fragments of broken rock (1) and fine-grained volcanic ash (2) produced during volcanic eruptions.
Watch the fragmental instructional videos for further details.
Image credit: Scott Brande
Glassy texture is recognized by the presence of one or more of the following features: (1) smoothly curved fractures, (2) smoothly concave (or scalloped) indentations bounded by sharp ridges, (3) closely spaced shatter marks, (4) homogeneous appearance without other features, such as grains, crystals, vesicles, etc. Glass texture forms by the cooling of lava at such a high rate as to prevent the formation of mineral crystals.
Watch the glassy instructional video for further details.
Image credit: Scott Brande
Vesicular texture is typically recognized by the presence of smoothly rounded holes (cavities) resulting from the trapping of bubbles of gas (vapor) during the solidification of lava. Cavities do not have to be spherical, because they are deformed as the lava flows during solidification. Holes do not have to be the same size.
Watch the vesicular instructional video for further details.
Image credit: Scott Brande