Final Answer:
Igneous rocks with both large and small mineral crystals typically form through a process called porphyritic crystallization.
Step-by-step explanation:
Porphyritic Crystallization: The key process is porphyritic crystallization, where magma initially cools slowly beneath the Earth's surface, allowing larger crystals to form. This phase is known as the "phenocryst" stage.
Later Stage Cooling: The magma then rises or experiences a change in its environment, causing a faster cooling rate. During this rapid cooling, smaller crystals or a fine-grained matrix form around the pre-existing larger crystals.
Two Crystal Sizes: The result is an igneous rock with two distinct crystal sizes - larger phenocrysts embedded in a groundmass of smaller crystals. This phenomenon is common in rocks like porphyritic basalt or andesite.