How Do Igneous Rocks Form?
ES0603  Igneous Rock Textures

The grain size of crystalline igneous rocks can be separated into three broad categories.

Coarse-grained igneous rocks have individual grains (crystals) that are visible to the naked eye. Grains can range from millimeters to centimeters in diameter. The grains form an interlocking network.

Fine-grained igneous rocks have small grains, less than a millimeter across, that are invisible to the naked eye. Hand samples of fine-grained rocks often appear homogeneous—they look the same throughout the rock. Hand lenses, magnifying glasses, or microscopes can reveal the small crystals in these rocks.

Igneous rocks with a porphyritic texture show two distinct grain sizes—coarse-grained crystals, visible to the naked eye, surrounded by fine-grained crystals.

Matthew Nyman, TERC
Coarse-grained texture
Matthew Nyman, TERC
Fine-grained texture
Matthew Nyman, TERC
Porphyritic texture.

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