Porphyritic
andesite lava
Andesite is a volcanic rock intermediate in composition
between basic and acid. It is lighter coloured than basalt.
It is one of the typical rocks of the volcanic island arcs
surrounding the Pacific Ocean. Many andesites show
porphyritic texture, where larger, well-shaped crystals are
set in a finer matrix. Here the large crystals are of
plagioclase feldspar. They crystallized relatively slowly in
a magma chamber below the surface. When the magma was
erupted onto the surface, the rest of the melt crystallized
much more rapidly, forming tiny interlocking crystals in the
matrix that cannot be distinguished by the naked eye.