Andesite is an intermediate, extrusive igneous rock.  Intermediate igneous rocks have 52-65% silica (SiO2 chemistry), in-between the felsic & mafic categories.  Intermediate igneous rocks are sometimes light-colored, sometimes dark-colored, and sometimes have colors of medium intensity.  They have little to no quartz.


Andesites have an aphanitic texture (finely-crystalline; all crystals <1 mm in size), although almost all andesites Ive seen are porphyritic (large phenocrysts with finely crystalline groundmass). The finely-crystalline component of andesites formed by cooling of rather viscous lava.  Volcanoes having lava of intermediate chemistries tend to erupt quite violently.  The worst volcanic disaster stories in history are from volcanoes having intermediate lavas such as andesite.


Andesite (7.3 cm across at its widest) - vesicular andesite from a 1.9 million year old lava flow (upper Upper Pliocene) on the eastern side of Sierra Grande Shield Volcano, Raton-Clayton Volcanic Field, northeastern New Mexico, USA (36 42' 24.26" W, 103 47' 46.75" W).  The rock is composed of plagioclase feldspar, augite pyroxene, hypersthene pyroxene, olivine, plus other minerals.




Andesite - this sample is somewhat porphyritic (the larger black crystals are the phenocrysts), but the grayish matrix is finely crystalline (aphanitic).



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