Igneous Rocks

Intrusive vs Extrusive

• Intrusive rocks cool very slowly below Earth’s surface, producing mineral crystals that can be seen with the naked eye.

• Extrusive rocks cooled either at the surface of the Earth(fast) or in the atmosphere (very nice). Crystals are too small to be seen.

Rock Texture

• Aphanitic - fine grained, crystals too small to be seen, can exhibit small rounded cavities

• Phaneritic - Individual crystals are visible to the naked eye and are typically of roughly uniform size.

• Glassy - extreme rapid cooling, no crystals, smooth surface

• Porphyritic - visible crystals embedded in fine grains

• Pyroclastic - shows a mixture of rock fragments, pumice, and volcanic ash

• Vesicular - many cavities on its surface and inside

Rock Names

Granite

Intrusive, Phaneritic, Felsic

Can contain feldspar, quartz, biotite

Granite Pegmatite

Intrusive, Phaneritic, Felsic

Can contain quartz, feldspar, muscovite

Diorite

Intrusive, Phaneritic, Intermediate

Can contain feldspar, biotite, hornblende

Gabbro

Intrusive, Phaneritic, Mafic

Can contain feldspar, olivine, magnetite, augite

Peridotite

Intrusive, Phaneritic, Ultramafic

Can contain olivine

Rhyolite Porphyry

Extrusive, Aphanitic, Felsic

Can contain quartz, plagioclase

Andesite Porphyry

Extrusive, Aphanitic, Intermediate

Can contain plagioclase, hornblende, magnetite, biotite

Basalt

Extrusive, Aphanitic, Mafic

Can contain plagioclase feldspar, olivine, hornblende

Basalt Porphyry

Extrusive, Pyroclastic, Mafic

Can contain plagioclase feldspar, olivine

Vesicular Basalt

Extrusive, Aphanitic, Mafic, Vesicles

Can contain hornblende, olivine, biotite

Pumice

Extrusive, Vesicular, Felsic

Can contain hornblende

Scoria

Extrusive, Vesicular, Mafic

Can contain olivine, plagioclase

Obsidian

Extrusive, Glassy, Felsic

Can include magnetite, hornblende, biotite, plagioclase