(left - click to enlarge image - a plagioclase feldspar phenocryst, Lambert Dome, Yosemite, courtesy Daniel Mayer)
Porphyrys are formed by a two-stage cooling of rising magma. First, deep crustal magma cools slowly, allowing formation of large phenocrysts (diameter 2 mm or more). Second, the magma cools rapidly at shallower depths having been injected upward or extruded by a volcano, allowing for formation of small crystals in the groundmass.
[images : phenocrysts of feldspar in matrix of quartz, feldspar and mica; large feldspathic phenocrysts in granite; large zoned plagioclase phenocryst; andesites with phenocrysts; formation: Death Valley vitrophyre, a phenocryst-bearing obsidian; thin-section feldspar and clinopyroxene phenocrysts in Mt. Fuji basalt; thin-section clinopyroxene phenocrysts, ppl; thin-section augite phenocryst, same augite phenocryst in cross-polarized light; resorbed quartz phenocryst, in PPL; thin-section anhedral amphibole phenocryst; igneous rocks in thin section]