Here's why:
* Slow cooling: When magma cools slowly, it gives crystals more time to grow, resulting in large crystals (phaneritic texture). This is common in intrusive rocks that cool deep underground.
* Fast cooling: Rapid cooling doesn't allow crystals to grow large. Instead, you get small crystals (aphanitic texture). This is characteristic of extrusive rocks that cool quickly at the Earth's surface.
Other factors that can influence crystal size:
* Composition of the magma: Some minerals crystallize more readily than others.
* Amount of dissolved gases: Gases can inhibit crystal growth.
* Presence of existing crystals: Existing crystals can act as nucleation sites for new crystals, increasing the number of crystals and potentially decreasing their size.
Examples:
* Granite: Large, visible crystals (phaneritic texture) indicating slow cooling.
* Basalt: Small, microscopic crystals (aphanitic texture) indicating rapid cooling.
* Porphyritic rocks: Contain a mix of large and small crystals, suggesting a two-stage cooling process.
Therefore, the crystal size within an igneous rock is a direct reflection of the cooling rate, and a valuable indicator of the rock's formation history.
