Igneous Textures

The different textures igneous rocks can have, based on how they cool and whether they are intrusive or extrusive.

Phaneritic (crystal size, cooling rate, environment, formation, examples)

• Crystal Size: Coarse, visible

• Cooling Rate: Slow

• Environment: Intrusive (plutonic)

• Formation: Deep underground, slow cooling allows large crystals

• Examples: Granite, Gabbro

Aphanitic (crystal size, cooling rate, environment, formation, examples)

• Crystal Size: Fine, not visible to the naked eye.

• Cooling Rate: Fast

• Environment: Extrusive (volcanic)

• Formation: Near or at the surface, rapid cooling prevents large crystal growth

• Examples: Basalt, Rhyolite

Porphyritic (crystal size, cooling rate, environment, formation, examples)

• Crystal Size: Mixed – large phenocrysts + fine grained matrix (groundmass)

• Cooling Rate: Two stages; initial slow cooling followed by rapid cooling.

• Environment: Can be both intrusive and extrusive.

• Formation: Crystals form slowly, then magma erupts and cools quickly

• Examples: Porphyritic Andesite, Porphyritic Granite/basalt

Pegmatitic (crystal size, cooling rate, environment, formation, examples)

• Crystal Size: Very coarse, extremely large crystals.

• Cooling Rate: Very slow, fluid-rich

• Environment: Intrusive (plutonic), usually near edge of magma body

• Formation: Formed from water-rich magma, allowing for large crystal growth in pockets. Late stage crystallization.

• Examples: Pegmatite

Glassy (crystal size, cooling rate, environment, formation, examples)

• Crystal Size: No crystals, entirely amorphous.

• Cooling Rate: Very rapid, instantaneous.

• Environment: Extrusive (volcanic). Lava quenched in air or water

• Formation: Forms when lava cools too quickly for crystals to develop.

• Examples: Obsidian

Vesicular (crystal size, cooling rate, environment, formation, examples)

• Crystal Size: Contains small gas bubbles (vesicles), usually with a fine-grained matrix.

• Cooling Rate: Rapid cooling at low pressure with gas-rich lava.

• Environment: Extrusive (volcanic), formed from lava that traps gas. Surface eruptions

• Formation: Gas bubbles form when lava cools, trapping bubbles and leading to a porous texture.

• Examples: Pumice, scoria.

Pyroclastic (crystal size, cooling rate, environment, formation, examples)

• Crystal Size: Fragmental, rock bits and ash.

• Cooling Rate: Rapid, often associated with explosive eruptions, chaotic.

• Environment: Extrusive (volcanic). Formed from explosive volcanic activity.

• Formation: Forms from the accumulation of volcanic ash, pumice, and other volcanic debris ejected during an eruption.

• Examples: Tuff, volcanic ash, volcanic breccia.