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The primary eruption characteristic used to determine the volcanic explosivity index is the volume of pyroclastic material ejected by the volcano. Pyroclastic material includes volcanic ash, tephra, pyroclastic flows, and other types of ejecta.
Porphyritic volcanic rocks are further classified by phenocryst type using mineral name modifiers given in the order of decreasing abundance. The term phenocryst is used for a crystal that is significantly larger than the average size of the groundmass crystals; in practice, these are generally >~1 mm. Thus, olivine-plagioclase phyric basalt contains >10% phenocrysts, the dominant phenocryst being olivine, with lesser amounts of plagioclase.