Igneous Textures

Geologists like igneous textures because they reveal so much about how a rock formed. The first set of textures focuses on the size of mineral crystals. Crystal size primarily reflects the rate of cooling, but is also often strongly affected by rock composition (especially water or gas content). Both intrusive and extrusive rock textures are represented.

The second set of textures is associated with volcanic rocks. Explosive volcanism creates highly distinctive features in igneous rocks.
1. Textural Terms Based On Crystal Size
Intrusive Textures Extrusive Textures
Coarse grained (phaneritic):

Slow cooling

Fine grained (aphanitic):

Fast cooling

Porphyritic (with coarse-grained groundmass):

Two phases of cooling:
one very slow, one slow

Porphyritic (with fine-grained groundmass):

Two phases of cooling:
one slow,
one fast

Pegmatitic:

Slow cooling plus high water content

Glassy:

Fast cooling plus high silica content


2. Volcanic Textures
Vesicular and Amygdaloidal

Bubble formation with relatively low gas content
Pumice and scoria:

Rock foam (lots of bubbles!)

Pyroclastic:

Explosive eruptive debris (tuffs and volcanic breccias)
Walter the Mafic/Felsic Kitty

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