Here's why:
* Intrusive origin: When magma cools slowly underground, it forms large crystals, resulting in a phaneritic texture (visible crystals).
* Extrusive origin: When lava erupts and cools quickly at the surface, it forms small crystals or even a glassy texture, resulting in an aphanitic texture (crystals too small to see without a microscope).
Mixed Texture: A rock formed by both intrusive and extrusive stages will exhibit a combination of these textures. You might find large crystals embedded within a finer-grained matrix.
Example:
* Porphyry: A common example of a mixed texture rock. It contains larger crystals (phenocrysts) embedded in a finer-grained groundmass. These phenocrysts likely formed during the intrusive phase, while the groundmass formed during the extrusive phase.
So, the presence of a mixed texture is a strong indicator of an igneous rock with a two-stage intrusive-extrusive origin.
