Basalt oceanic crust is mainly composed of basic volcanic rocks. It is the most important component of the earth's oceanic crust and lunar sea, and also the important component of the earth's continental crust and lunar land. In 1546, G. Agricola first used the word basalt in geological literature to describe the black rock of Saxony, Germany. The Chinese basalt is derived from Japanese. Japan has discovered the black olive basalt in Xuanwu cave, Hyogo Prefecture, hence its name.
Basalt is a kind of basic extrusive rock. Its chemical composition is similar to gabbro or diabase. The content of SiO2 varies from 45% to 52%, the content of K2O + Na2O is slightly higher than that of intrusive rock, and the content of CaO, Fe2O3 + FeO and MgO is slightly lower than that of intrusive rock. Minerals are mainly composed of basic feldspar and pyroxene. Secondary minerals are olivine, hornblende and biotite. Rocks are dark, generally black, sometimes grey-green and dark purple. A porphyritic structure. Stomatal structure and almond structure are common.
The volume density of the basalt is 2.8-3.3 g/cm 3, and the compressive strength of the compacted basalt is very high, up to 300 MPa, sometimes higher. The strength decreases with the presence of vitreous and stomata. Basalt has high durability, many joints, and most of the joints are pentagonal or hexagonal, forming columnar joints. Because of its brittleness, it is difficult to extract large blocks of stone. Because of the common stomatal and almond structures, although basalt is widely distributed on the surface, it can not be used as a decorative stone.
The main components of basalt are silicon dioxide, aluminum oxide, iron oxide, calcium oxide, magnesium oxide (and a small amount of potassium oxide, sodium oxide), of which silicon dioxide content is the most, accounting for about 45-50%.
The color of the basalt is usually black, dark brown or dark green; the basalt near the Tengchong Volcanic Group is mostly cyan-grey, but also dark red, orange, yellow. Because of its dense texture, its specific gravity is heavier than that of ordinary granite, limestone, Sha Yan and shale. However, some basalts also reduce their weight and even float in water because of their special stomata. Therefore, the light porous basalt is called "pumice". In villages near the foot of the Mazhan Volcanic Group in Tengchong, Yunnan Province, the light porous basalt is called "pumice".
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