Most basalt forms when mafic lava flows cool at Earth's surface. Picture 1 shows a handsample of basalt, which is usually black in color. Basalt erupts on the ocean floor at ocean ridges. Here mafic lava flows from long cracks in the crust called fissures. Picture 2 shows the Mid-Atlantic Ocean ridge where it rises above sea level in Iceland.
The islands of Hawaii are mostly composed of layer upon layer of basalt which crystallized from flows initially erupting from submarine volcanos. Eventually, enough basalt accumulated to the point where the volcanos rose above sea level. The Big Island, Hawaii, is composed of several huge volcanos which have the classic, rounded shield shape of volcanos composed of basalt. Picture 3 shows one of the active shield volcanos of Hawaii, along with the pahoehoe texture that results as highly-fluid (low-viscosity) lava cools. See card 7 for a more detailed discussion of this texture.
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| basalt handsample | Mid-Atlantic Ridge | Hawaiian shield volcano |
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