* Cooling Rate: Intrusive rocks cool slowly beneath the Earth's surface, giving crystals ample time to grow large and well-defined. This results in phaneritic textures, where individual crystals are visible to the naked eye.
* Extrusive Rocks: Extrusive rocks cool rapidly on the Earth's surface, limiting crystal growth. This results in aphanitic textures, where crystals are too small to be seen without a microscope.
Exceptions:
* Porphyritic Texture: Some extrusive rocks exhibit a porphyritic texture, which means they have larger crystals (phenocrysts) embedded in a finer-grained matrix. These larger crystals form before the lava erupts, allowing for slow cooling within the magma chamber.
* Very Slow Cooling: In rare cases, extrusive rocks can cool very slowly, allowing for larger crystals to form. This is more common in thicker lava flows or underwater eruptions where insulation slows cooling.
In summary: While there can be exceptions, intrusive rocks generally have larger, more well-defined crystals than extrusive rocks due to differences in cooling rates.
