Igneous Rocks - GEOL 1114 - Physical Geology

Pumice

F,I,M,UM: Felsic Texture: Vesicular Mineral composition: silica content, iron, magnesium Other properties: extremely lightweight; long (horizontal) holes in the rock Inferred rock origin: extrusive lava flow; usually a rhyolitic explosion

Tuff

F,I,M,UM: Felsic Texture: coarse-grained Mineral composition: pyroclastic rocks Other properties: Consists of large rocks and small rocks. It has a white/gray/black color. Inferred rock origin: Intrusive

Granite

F,I,M,UM: Felsic Texture: coarse-grained Mineral composition: quartz, orthoclase feldspar, biotite, hornblende Other properties: combinations of pink, black, white; large crystals Inferred rock origin: Intrusive; single cooling; pyroclastic deposit. The coarse-grained version of rhyolite.

Rhyolite

F,I,M,UM: Felsic Texture: fine-grained or porphyritic Mineral composition: lava rock of orthoclase feldspar and quartz Other properties: pink to gray color; crystals may be all the same size (small) or a combination of large and small Inferred rock origin: Extrusive; nonviolent volcanic eruption. The fine-grained version of granite.

Obsidian

F,I,M,UM: Felsic/Intermediate Texture: glassy/no grains/fine-grained Mineral composition: silicon dioxide Other properties: extremely smooth texture, typically black color, and glassy appearance Inferred rock origin: extrusive lava flow

Andesite

F,I,M,UM: Intermediate Texture: Porphyritic Mineral composition: andesine and one or more mafic constituents Other properties: large and small crystals. Color can be gray with white to light gray phenocrysts. Inferred rock origin: Extrusive; violent volcanic eruption. The fine-grained version (?) of diorite.

Diorite

F,I,M,UM: Intermediate Texture: coarse-grained Mineral composition: sodic plagioclase, hornblende, biotite, or pyroxene Other properties: similar in appearance to granite - black and white combinations Inferred rock origin: intrusive. The coarse-grained version of andesite.

Scoria

F,I,M,UM: Mafic Texture: Vesicular Mineral composition: basalt Other properties: Has many holes in it; lightweight; reddish color Inferred rock origin: extrusive lava flow; a basaltic (very explosive) volcanic explosion

Gabbro

F,I,M,UM: Mafic Texture: coarse-grained Mineral composition: feldspar and pyroxene Other properties: dark colored typically. Larger crystals than basalt. Inferred rock origin: Intrusive. The coarse-grained version of basalt.

Basalt

F,I,M,UM: Mafic Texture: fine-grained Mineral composition: plagioclase feldspar, augite, olivine Other properties: dark appearance. Small crystals. Inferred rock origin: Extrusive. Very violent volcanic eruption. The fine-grained version of gabbro.