A hard, dense, dark volcanic rock composed chiefly of plagioclase, pyroxene, and olivine, and often having a glassy appearance.
Dark igneous rock that is low in silica content and comparatively rich in iron and magnesium. Some basalts are glassy (have no visible crystals), and many are very fine-grained and compact. Basaltic lavas may be spongy or pumice-like. Olivine and augite are the most common minerals in basalts; plagioclase is also present.
Basalts may be broadly classified into two main groups:
1. Calc-alkali basalts predominate among the lavas of mountain belts; the active volcanoes of Mauna Loa and Kilauea in Hawaii erupt calc-alkali lavas.
2. Alkali basalts predominate among the lavas of the ocean basins and are also common in mountain belts.
Basalt is hard, dense igneous rock that makes up much of the material in tectonic plates. The part of the Earth's crust beneath the oceans consists mainly of basalt whereas continental crust consists mainly of less dense rocks, such as granite.
Basalt is the most common rock type in the Earth's crust (the outer 10 to 50 km). In fact, most of the ocean floor is made of basalt.
Huge outpourings of lava called "flood basalts" are found on many continents. The Columbia River basalts, erupted 15 to 17 million years ago, cover most of southeastern Washington and regions of adjacent Oregon and Idaho.
Basaltic magma is commonly produced by direct melting of the Earth's mantle, the region of the Earth below the outer crust. On continents, the mantle begins at depths of 30 to 50 km.
Shield volcanoes, such as those that make up the Islands of Hawaii, are composed almost entirely of basalt.
Basalt is a hard, black volcanic rock with less than about 52 weight percent silica. Because of basalt's low silica content, it has a low viscosity (resistance to flow). Therefore, basaltic lava can flow quickly and easily move >20 km from a vent. The low viscosity typically allows volcanic gases to escape without generating enormous eruption columns. Basaltic lava fountains and fissure eruptions, however, still form explosive fountains hundreds of meters tall. Common minerals in basalt include olivine, pyroxene, and plagioclase. Basalt is erupted at temperatures between 1100 to 1250¡ã C. The mineral grains are so fine that they are impossible to distinguish with the naked eye or even a magnifying glass. They are the most widespread of all the igneous rocks. Most basalts are volcanic in origin and were formed by the rapid cooling and hardening of the lava flows. Some basalts are intrusive having cooled inside the Earth's interior.
Material Notes:
Igneous rock, with a fine texture and grain size < 1 mm (extrusive). It is a basic rock, with a total silica content of 45-55%. Predominent minerals in this generally dark colored rock are hornblende and plagioclase feldspar.
