Andesite, Vesicular Andesite & Diorite

All three of these rocks form from the same minerals: Mostly plagioclase feldspar with some quartz, biotite and amphibole (see blue zone in the diagram below). The principal difference then is not what they're made of but how and where they formed.

Both andesite and vesicular andesite formed at or near the surface (extrusive). This results in fast cooling and a fine texture (crystals are less than 1mm). The difference is that vesicular andesite formed from magma containing dissolved gas which escaped while the magma was cooling. As the gas escaped it formed bubbles or gas pockets. Most andesite is non-vesicular and contains no vessicles. Both have a fine texture.

Diorite is formed from the same minerals but at depth in the crust (intrusive). It therefore cooled more slowly giving crystals time to grow larger (1 to 10mm). Thus diorite has a coarse texture.

 

Vesicular Andesite

Andesite

Andesite

Diorite

Diorite

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