basalt
Basalts, extrusive igneous (volcanic) rocks that are low in silica content, dark in color, and relatively rich in iron and magnesium.
Some basalts are completely vitreous (tachylytes), and many are fine-grained and compact. However, it is usual for them to exhibit a porphyritic structure, with larger crystals (phenocrystals) of olivine, augite, or feldspar in a fine-crystalline matrix (ground mass).
Olivine and augite are the most common porphyrite minerals in basalts. Porphyry plagioclase feldspar is also found. Basaltic lava is most often spongy or pumice; The vapor cavities are filled with secondary minerals such as calcite, chlorite, and zeolite.
Basalts can be broadly classified on a chemical and petrographic basis into two main groups: tholitic and alkali basalts. Theolithic basaltic lavas feature calcareous plagioclase with augite, dove, or hypersthene, and (rarely) olivine as the dominant mafic minerals; Basalts without olivine are also well represented.
Thuliite basalts, which contain 45 to 63 percent silica, are rich in iron and include tholiites (basalts with calcium-poor pyroxene). They prevail among the lavas of the mountain belts. Its flows may build enormous plateaus, as in the northwestern United States, the Deccan of India, and the Parana Basin of South America. The active volcanoes of Mauna Loa and Kilauea in Hawaii erupt tholithic lava.
Natural alkali basalts contain olivine and, most commonly, augite diopside or titaniferos. Alkaline basalts predominate among the lavas of the ocean basins and are common among mafic lavas in the frontal and backlands of mountain belts. In the Anglo-Icelandic province, the Paleogene and Neogene lava flows of the Inner Hebrides, Antrim and Faroe Islands include large successions of tholitic and alkaline basalts.
Minerals of the feldspathoid group occur in a large number of basaltic rocks belonging to the alkali group; nepheline, analcime, and leucite are the most common, but haüynite is occasionally found. If nepheline completely replaces feldspar, the rock is known as nepheline basalt; If the replacement is only partial, the term nepheline-basanite is used. Likewise, there is analcime-, lucite-basalt, and lucite-pasanite.
Most nepheline basalts are fine-grained rocks of very dark color and date from the early Cenozoic era (65.5 million years ago to the present). It is fairly common in some parts of Germany and also occurs in the United States (as in New Mexico), and in Libya, Turkey, and elsewhere. Lucite basalts are found primarily in Italy, Germany, East Africa, Australia, and in the United States in Montana, Wyoming, and Arizona.
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