1. Rapid Cooling: Lava exposed to the air or water cools down much faster than magma trapped deep underground. This rapid cooling leaves little time for mineral crystals to grow large.
2. Nucleation Sites: The rapid cooling allows for a large number of nucleation sites for crystal growth. This means many small crystals form simultaneously, rather than a few large crystals.
3. Limited Diffusion: The fast cooling rate restricts the diffusion of atoms and ions needed for crystal growth. This further limits the size of the crystals that can form.
4. Viscosity: Lava is typically more viscous than magma, meaning it's thicker and flows more slowly. This slows down the movement of atoms, reducing the distance they can travel before solidifying, leading to smaller crystal sizes.
In contrast:
Intrusive rocks, formed from magma that cools slowly underground, have ample time for mineral crystals to grow large, resulting in coarse-grained textures.
Therefore, the rapid cooling rate and other factors associated with extrusive rock formation lead to the formation of small grains.
