Here's why:

* Porphyritic refers to an igneous rock texture where large crystals (called phenocrysts) are embedded within a matrix of smaller crystals (called groundmass).

* The phenocrysts form first as the magma cools slowly, allowing large crystals to grow. The groundmass forms later when the magma cools more quickly, resulting in smaller crystals.

Let me know if you'd like more information about igneous rocks!