Porphyritic

[3] In extrusive rocks, the phenocrysts are surrounded by a fine-grained (aphanitic) matrix or groundmass of volcanic glass or non-visible crystals, commonly seen in porphyritic basalt.

[4] Porphyritic rocks are a product of igneous differentiation, and are generally formed when a column of rising magma is cooled in two stages: In the first stage, the magma is cooled slowly deep in the crust, creating the large crystal grains, with a diameter of 2mm or more.

In the final stage, the magma is cooled rapidly at relatively shallow depth or as it erupts from a volcano, creating small grains that are usually invisible to the unaided eye, typically referred to as the matrix or groundmass.

If these phenocrysts are different in density to the remaining melt, they usually settle out of solution, eventually creating cumulates.

[5] This can also occur when the chemical composition of the remaining melt is close to the eutectic point as it cools, resulting in multiple different minerals solidifying at once and filling the remaining space simultaneously, limiting their size and shape.

Porphyritic texture in a granite. This is an intrusive porphyritic rock. The white, square feldspar phenocrysts are much larger than crystals in the surrounding matrix; eastern Sierra Nevada , Rock Creek Canyon, California.
A porphyritic volcanic sand grain, as seen under the petrographic microscope . The large grain in the middle is of a much different size class than the small needle-like crystals around it. Scale box in millimeters.
Andesite porphyry from summit of O'Leary Peak . This is an extrusive porphyritic rock, as the pink (and black) phenocrysts are clearly visible, in contrast to the grey groundmass with its microscopic crystals.