Classification of andesites may be refined according to the most abundant phenocrystal. Example: olivinic andesite, if olivine is the principal accessory mineral.
Andesite can be considered as the extrusive equivalent to plutonicdiorite. As diorites, they are characteristic of subduction tectonic environments in active oceanic margins, such as the western coast of South America. Due to this, the name andesite is derived from the Andes mountain range.
ANDESITE, a name first applied by C. von Buch to a series of lavas investigated by him from the Andes, which has passed into general acceptance as the designation of a great family of rocks playing an important part in the geology of most of the volcanic areas of the globe.
The andesites have mostly a porphyritic structure, and the larger felspars and ferro-magnesian minerals are often visible to the naked eye, lying in a finer groundmass, usually crystalline, but sometimes to a large extent vitreous.
Garnet and sapphire have also been found in andesites, and perhaps have the same signification; a rose-red variety of epidote (withamite) is known as a secondary product in certain andesites (Glencoe, Scotland), and the famous red porphyry (porfido rosso) of the ancients is a rock of this type.
Feeds |
Contact