 The Incapillo ignimbrite is formed by potassium @-@ rich and magnesium @-@ poor rhyodacite , forming glassy and porous pumice with individual clasts of 5 – 20 centimetres ( 2 @.@ 0 – 7 @.@ 9 in ) in diametre . Typical pumice contains crystals of biotite , hornblende , plagioclase , quartz , and sanidine , with minor amounts of apatite , iron oxides , and titanite . The lava domes have uniform crystalline compositions that are richer in magnesium than the ignimbrite . Within the lava dome rocks lie phenocrysts of amphibole , biotite , plagioclase , quartz , and titanite with some alkali feldspar in some domes . Older domes have higher amphibole and lower quartz content than younger domes . Post @-@ caldera domes are strongly hydrothermally altered . 
