The Incapillo ignimbrite is formed by potassium-rich and magnesium-poor rhyodacite, forming glassy and porous pumice with individual clasts of in diametre.
Rocks from Incapillo are rich in sodium and have high ratios of lanthanum and samarium to ytterbium and high ratios of barium to lanthanum, as well as high lead-206 to lead-204 and strontium-87/strontium-86 ratios [REF].