Basalt
Basalt
Basalt is a dark-colored, fine-grained, igneous rock composed mainly of plagioclase and pyroxene minerals. It most commonly forms as an extrusive rock, such as a lava flow, but can also form in small intrusive bodies, such as an igneous dike or a thin sill. Basalt has a composition similar to gabbro, with the primary difference being that basalt is fine-grained while gabbro is coarse-grained.
Formation and Composition
Basaltic magma is commonly produced by direct melting of the Earth's mantle, the region below the outer crust. On continents, the mantle begins at depths of 30 to 50 kilometers. The chemistry of basalts provides clues to processes deep in Earth's interior. Basaltic lavas are frequently spongy or pumiceous; the steam cavities become filled with secondary minerals such as calcite, chlorite, and zeolites.
Types of Basalt
There are two main chemical subtypes of basalt: tholeiites and alkali basalts. Tholeiites are silica-saturated to oversaturated and dominate the upper layers of oceanic crust and oceanic islands. Alkali basalts are silica-undersaturated and are common on oceanic islands and in continental magmatism.
Geological Significance
Basalt underlies more of Earth's surface than any other rock type. Most areas within Earth's ocean basins are underlain by basalt. Although basalt is much less common on continents, lava flows and flood basalts underlie several percent of Earth's land surface. Basalt is a very important rock in the study of geology and volcanology.
Notable Formations
- Columbia River Basalts: These flood basalts erupted 15 to 17 million years ago and cover most of southeastern Washington and regions of adjacent Oregon and Idaho.
- Olympus Mons: This shield volcano on Mars is composed of basalt and is the largest known volcano in our solar system.
Extraterrestrial Basalt
Basalt is also an important rock type on other planetary bodies in the Solar System. For example, the bulk of the plains of Venus, which cover approximately 80% of the surface, are basaltic. The lunar maria are plains of flood-basaltic lava flows, and basalt is a common rock on the surface of Mars.
Uses of Basalt
Basalt is used in a variety of applications due to its durability and abundance. It is commonly used as an aggregate in construction projects, as a decorative stone, and in the production of basalt fiber, which is used in various industrial applications.