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Basalt is an igneous (volcanic) rock that is 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 typical for them to exhibit a porphyry structure, with larger crystals (phenocrystals) of olivine, augite, or feldspar in a microcrystalline (ground mass) matrix.

Olivine and augite are the most common porphyrite minerals in basalts. Plagioclase porphyry feldspar is also found. Basalt lavas are often spongy or opaque; 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 major groups: theolitic and alkali basalts. Tolitic basaltic lavas feature calcified plagioclase with augite, dove, or Hyperstein, and (rarely) olivine as the dominant mafic minerals; Basalts without olivine are also well represented.

Thulite basalts, which contain 45 to 63 percent silica, are rich in iron and include tholiites (basalts with calcium-poor pyroxene). They reign among the lavas of 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. Active volcanoes Mauna Loa and Kilauea in Hawaii are erupting lava.

Natural alkali basalts contain olivine and, more commonly, ogite diopside or titaniferos. Alkaline basalts predominate among the lavas of the ocean basins and are common among mafic lavas in the fore and backlands of mountain belts. In the British Icelandic province, Paleogene and Neogene lava flows in the Inner Hebrides, Antrim and the Faroe Islands, including a large series of tholitic and alkali 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. Similarly, there are analytical basalts, leucite and lucite basalts.

Most nepheline basalts are fine-grained rocks of very dark color and date from the Early Cenozoic (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), Libya, Turkey, and other places. Lucite basalts are found primarily in Italy, Germany, East Africa, Australia, and the United States in Montana, Wyoming, and Arizona.

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