Description
Basalt is a mafic volcanic igneous rock formed by the solidification of lava. Basalt solidification occurs at the Earth's surface, making it an effusive rock. Due to the rapid cooling, no large crystals are formed. The most common minerals in basalt are amphibole, pyroxene-olivine, and ilmenite. Other minerals, such as magnetite, can also form in basalt. Basalt is usually black and consists of small crystals. The intrusive variety of basalt is called gabbro. The shrinkage that occurs during the solidification of basalt lava leads to typical hexagonal structures (basalt columns). Basalt is an effusive rock and forms at the surface in areas of volcanic activity. It is typically a product of low-viscosity, fast-flowing lavas, such as those in Hawaii. Basalt effusives can also occur beneath the (sea) surface, such as at mid-ocean ridges. This is where pillow basalts are formed. Because the magma solidifies and reaches a temperature below the Curie point, the Earth's magnetic field is then recorded in the rock. The magnetic field changes constantly, and along the mid-ocean ridges, a pattern of successive magnetic periods, known as magnetic polarity zones, develops. This is used in paleogeographic reconstructions and rock dating. It is believed that the name basalt is a corruption of the name of the Egyptian landscape of Bashan, where the rock also occurs.
