* Intrusive Rocks: These form when magma cools and solidifies *below* the Earth's surface. This process is slow, allowing ample time for mineral crystals to grow large and well-defined.
* Slow cooling: The surrounding rock acts as an insulator, slowing down the cooling process.
* Large crystals: With more time, mineral atoms have a chance to migrate and arrange themselves into large, organized crystal structures.
* Extrusive Rocks: These form when lava erupts onto the Earth's surface. The rapid cooling results in smaller crystals.
* Rapid cooling: Exposure to air and water rapidly cools the lava.
* Small crystals: Mineral atoms don't have enough time to arrange themselves into large crystals. They may form tiny crystals or even be glassy (amorphous) if the cooling is extremely fast.
In summary: The rate of cooling is the primary factor determining crystal size in igneous rocks. Slow cooling leads to large crystals (intrusive rocks), while rapid cooling leads to smaller crystals (extrusive rocks).
