A rock with large crystals scattered on a background of much smaller crystals is called a porphyritic rock.

Here's why:

* Porphyritic Texture: This refers to the specific texture of the rock where larger crystals (called phenocrysts) are embedded within a finer-grained matrix of smaller crystals (called groundmass).

* Formation: This texture develops when magma or lava cools slowly, allowing large crystals to form first. Then, the remaining magma cools more rapidly, creating the smaller crystals.

Examples of Porphyritic Rocks:

* Porphyritic Granite: Contains large feldspar crystals in a fine-grained quartz and feldspar matrix.

* Porphyritic Basalt: Contains large plagioclase feldspar crystals in a fine-grained matrix of pyroxene and olivine.

Let me know if you have any other questions!