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Visual Glossary

Vessicular (bubbly) texture in basalt This photo was taken on one of the lava flows at Cinder Cones National Natural Landmark in Mojave National Preserve.

Basalt

A dark, fine-grained, extrusive (volcanic) igneous rock with a low silica content (40% to 50%). Rich in iron, magnesium, and calcium. Most of the ocean floor is made up of basalt and it is the most abundant volcanic rock in the Earth's crust.

Photomicrograph of basalt in thin section, cross-polarized light
Close-up photo (photomicrograph) of a very thin slice of basalt. Click here to see entire slide.

The image at left shows a typical vesicular basalt. Vesicles form when gas bubbles are trapped in the rock as it solidifies from molten basalt.

This close-up photo (photomicrograph) of a very thin slice of basalt was taken through a microscope with polarized light. Although basalt is a fairly homely rock to the naked eye, the beauty of its perfectly-formed microscopic crystals is revealed under the microscope.

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