Igneous Rocks - GEOL 1403 - Physical Geology

Which kind of rock is likely to form at a divergent boundary?

Basalt

Which rocks have porphyritic textures?

Andesite

Which rocks have fine-grained textures?

Basalt, dunite

Are rocks likely to be darker or lighter than those at the bottom of Bowen's Reaction Series?

Darker on top

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

Syenite

F,I,M,UM: Intermediate Texture: Coarse-grained Mineral composition: alkalic feldspar, hornblende/biotite Other properties: different-colored rock combinations. May be pink, white, black, gray Inferred rock origin: Intrusive

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.

Peridotite

F,I,M,UM: Ultra-mafic Texture: Coarse-grained Mineral composition: olivine, pyroxene Other properties: dark green to black color. Inferred rock origin: Intrusive. The coarse-grained version of komatiite.

Dunite

F,I,M,UM: Ultra-mafic Texture: fine-grained Mineral composition: pyroxene, plagioclase, olivine Other properties: yellow/green color due to olivine content Inferred rock origin: intrusive. Essentially it's peridotite but it consists of a lot of olivine.

Would you expect to find olivine and quartz in the same rock?

No, because quartz and olivine form at very different temperatures

Which rocks have glassy textures?

Obsidian

Which rocks have pyroclastic textures?

Rhyolite, tuff

Which kinds of rock are likely to form at a convergent boundary?

Rhyolite/granite, andesite/diorite

Which rocks have vesicular textures?

Scoria, pumice

Which rocks have coarse-grained textures?

Tuff, granite, gabbro, syenite, diorite