Basalt is a fine-grained, extrusive igneous or volcanic rock. It is usually black and contains mainly mafic (dark coloured) minerals, although the crystals are usually too small to be seen without magnification.
Sometimes it may contain sparse crystals, or small round pockets that were gas bubbles.
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How is it formed?
When ever you see TV footage of a volcano, it is usually basalt that you see flowing out over the surface and forming the lava flow. Most of the Hawaiian Islands, for example, are made up of basalt lava flows.
Basalt is the fine-grained equivalent of gabbro. It is fine grained because the magma cools very rapidly when it flows out onto the surface. Basalt forms from magma that is rich in iron and magnesium and poor in silica (quartz).
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