Basalt is an extrusive igneous rock, sometimes porphyritic, and is often both fine-grained and dense. Basalt in the tops of subaerial lava flows and cinders will often be highly vesiculated, imparting a lightweight "frothy" texture to the rock. The term basalt is often casually applied to shallow intrusive rocks with a composition typical of basalt, but rocks of this composition with a phaneritic groundmass should generally be referred to as gabbro. The crustal portions of oceanic tectonic plates are predominantly made of basalt.
Unweathered basalt is frequently black to greenish-black in color, characterized by a preponderance of calcic plagioclase feldspars and pyroxene together with minor amounts of accessory minerals such as olivine. Basaltic cinders are often red. Glass may be present, particularly as rinds on rapidly chilled surfaces of lava flows, and is commonly (but not exclusively) associated with underwater eruptions. Amygdaloidal structure is common in relic vesicles and beautifully crystallized species of zeolites, quartz or calcite are frequently found.
Columnar basalt at Sheepeater Cliff in Yellowstone
The lava flows of the Deccan Traps in India, the Columbia River Plateau of Washington and Oregon states in the United States, as well as the Triassic lavas of eastern North America are basalts. Perhaps the most famous basalt flow in the world is the Giant's Causeway on the northern coast of Ireland, in which the vertical joints form hexagonal columns and give the impression of having been artificially constructed. The dark areas visible on Earth's moon, the lunar mares, are plains of basalt, and basalt Moon samples were brought to Earth by the astronauts of the Apollo program. Pliny used the word basalt and it is said to have had an Ethiopian origin, meaning a black stone.
Types of basalt
Tholeiitic basalt is relatively rich in silica and poor in sodium. Included in this category are most basalts of the ocean floor, most large oceanic islands, and continental basaltic floods such as the Columbia River Plateau. Pyroxene, plagioclase, and magnetite are common minerals.
- Alkaline basalt is relatively poor in silica and rich in sodium.
Transitional basalt has an intermediate composition. It is often rich in the minerals plagioclase and olivine.