'porphyritic-rhyolitic' originates as a compound of the English adjective 'porphyritic' and 'rhyolitic'. 'Porphyritic' ultimately comes from Greek, specifically the word 'porphyra', where 'porphyra' meant 'purple'. 'Rhyolitic' derives from the noun 'rhyolite', which originates via German and Latin from Greek elements 'rhyax' (stream) and 'lithos' (stone).

'porphyritic' changed from Greek 'porphyra' through Latin/medieval usages and Middle English forms to become the modern English 'porphyritic'; 'rhyolitic' developed from New Latin/German 'Rhyolith' (the rock name 'rhyolite') and was adjectivalized in English as 'rhyolitic'.

Initially, related roots referred to color ('porphyra' = purple) and rock/flow ('rhyax' + 'lithos' = stream-stone); over time the compound came to mean specifically a textural description of rhyolite (i.e., rhyolite that is porphyritic).