Andesite is an extrusive rock intermediate in composition between rhyolite and basalt. Andesite lava is of moderate viscosity and forms thick lava flows and domes. The word andesite is derived from the Andes Mountains in South America, where andesite is common. Andesite is the volcanic equivalent of diorite.


Other specimens - Click the thumbnails to enlarge

Group - volcanic.
Colour - variable, but typically bluish-grey or grey (lighter coloured than basalt).
Texture - porphyritic.
Mineral content - groundmass generally of pyroxene ( augite) and plagioclase, possibly with minor amounts of amphibole ( hornblende) and glass; phenocrysts of plagioclase and often pyroxene, occasionally olivine or amphibole.
Silica (SiO 2) content - 52%-63%.
Uses - can be used as aggregate, fill etc. in the construction and roading industries (often not ideal for concrete aggregate because of high silica content).
New Zealand occurrences - West Auckland (Waitakere Range), Coromandel Peninsula, Great and Little Barrier Islands, Western Bay of Plenty (Kaimai Range), Raglan-Kawhia area (Karioi, Pirongia), Taranaki, Central North Island (Ruapehu, Tongariro, Ngauruhoe).