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Visual Glossary
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.
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Close-up photo (photomicrograph) of a very thin slice of basalt. Click here to see entire slide.
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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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