 Glassy non-vesicular (obsidian) |
 Glassy vesicular (pumice) |
 Fine non-vesicular (basalt) |
 Fine vesicular (vesicular andesite) |
 Coarse (diorite) |
 Very coarse (pegmatite) |
Texture
Texture is one characteristic (along with mineral content) used to identify igneous rocks.
Extrusive rocks may cool so fast that no crystals
grow. These have glassy texture.
Rocks with glassy texture may be non-vesicular (obsidian and basaltic glass) or,
if they formed from gassy magma, (pumice and scoria) they will have small
bubbles or vesicles giving them a rough, vesicular appearance.
Extrusive igneous rocks which cooled somewhat more slowly will contain small (less than 1mm) crystals. These include rhyolite, andesite, basalt, and some diabase. Most have a fine texture but again, if any of these formed from gassy magma they will have a vesicular texture. They will be fine/vesicular in texture.
Intrusive igneous rocks cool slowly underground. The crystals in them grow to larger sizes giving them a coarse texture. Because they cool deep in the earth under great pressure, any gas dissolved in the magma cannot escape. Thus, intrusive rocks are never vesicular.
|
Glassy non-vesicular (obsidian) |
Glassy vesicular (pumice) |
|
Fine non-vesicular (basalt) |
Fine vesicular (vesicular andesite) |
|
Coarse (diorite) |
Very coarse (pegmatite) |