ANDESITE
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 I’ve 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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