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Basalt

Pictures of Igneous Rock Types

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Basalt is an extrusive or intrusive rock that makes up most of the world's oceanic crust. This specimen was erupted from Kilauea volcano in 1960. (more below)
Makes up the oceanic crust
Photo (c) 2006 Andrew Alden, licensed to About.com (fair use policy)
Basalt is fine-grained so that the individual minerals are not visible, but they include pyroxene, plagioclase feldspar and olivine. These minerals are visible in the coarse-grained, plutonic version of basalt called gabbro.

This specimen shows bubbles made by carbon dioxide and water vapor that came out of the molten rock as it approached the surface. During its long period of storage beneath the volcano, green grains of olivine came out of solution as well. So the bubbles, or vesicles, and the grains, or phenocrysts, represent two different events in the history of this basalt.

A big close-up picture of basalt is available as a free wallpaper image. See more basalts in the Basalt Gallery and learn much more in "Introducing Basalt."

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