Flood basalts are thick sequences of many such flows that can cover hundreds of thousands of square kilometres and constitute the most voluminous of all volcanic formations.

Geologists classify igneous rock by its mineral content whenever possible; the relative volume percentages of quartz (crystalline silica (SiO2)), alkali feldspar, plagioclase, and feldspathoid (QAPF) are particularly important.

An aphanitic (fine-grained) igneous rock is classified as basalt when its QAPF fraction is composed of less than 10% feldspathoid and less than 20% quartz, and plagioclase makes up at least 65% of its feldspar content.

[8] Basalt is usually dark grey to black in colour, due to a high content of augite or other dark-coloured pyroxene minerals,[9][10][11] but can exhibit a wide range of shading.

[17] Basalt is often porphyritic, containing larger crystals (phenocrysts) that formed before the extrusion event that brought the magma to the surface, embedded in a finer-grained matrix.

[5][22] During the Hadean, Archean, and early Proterozoic eons of Earth's history, the chemistry of erupted magmas was significantly different from what it is today, due to immature crustal and asthenosphere differentiation.

[25] The modern petrological term basalt, describing a particular composition of lava-derived rock, became standard because of its use by Georgius Agricola in 1546, in his work De Natura Fossilium.

Agricola applied the term "basalt" to the volcanic black rock beneath the Bishop of Meissen's Stolpen castle, believing it to be the same as the "basaniten" described by Pliny the Elder in AD 77 in Naturalis Historiae.

[28] Decompression melting can occur in a variety of tectonic settings, including in continental rift zones, at mid-ocean ridges, above geological hotspots,[29][30] and in back-arc basins.

[53]: 65 Relative to most common igneous rocks, basalt compositions are rich in MgO and CaO and low in SiO2 and the alkali oxides, i.e., Na2O + K2O, consistent with their TAS classification.

[55] The abundances of the lanthanide or rare-earth elements (REE) can be a useful diagnostic tool to help explain the history of mineral crystallisation as the melt cooled.

They are tholeiitic basalts particularly low in total alkalis and in incompatible trace elements, and they have relatively flat REE patterns normalized to mantle or chondrite values.

[57] Mid-ocean ridge basalts have been subdivided into varieties such as normal (NMORB) and those slightly more enriched in incompatible elements (EMORB).

[58] Isotope ratios of elements such as strontium, neodymium, lead, hafnium, and osmium in basalts have been much studied to learn about the evolution of the Earth's mantle.

[64] Basalt in the tops of subaerial lava flows and cinder cones will often be highly vesiculated, imparting a lightweight "frothy" texture to the rock.

[66] ʻAʻā types of blocky cinder and breccia flows of thick, viscous basaltic lava are common in Hawaiʻi.

[67] Amygdaloidal structure is common in relict vesicles and beautifully crystallized species of zeolites, quartz or calcite are frequently found.

The initial phase of Surtsey's eruption was highly explosive, as the magma was quite fluid, causing the rock to be blown apart by the boiling steam to form a tuff and cinder cone.

[78][79] Volcanic glass may be present, particularly as rinds on rapidly chilled surfaces of lava flows, and is commonly (but not exclusively) associated with underwater eruptions.

The dark areas visible on Earth's moon, the lunar maria, are plains of flood basaltic lava flows.

[98] Lunar basalts differ from their Earth counterparts principally in their high iron contents, which typically range from about 17 to 22 wt% FeO.

[103] From 1972 to 1985, five Venera and two VEGA landers successfully reached the surface of Venus and carried out geochemical measurements using X-ray fluorescence and gamma-ray analysis.

[108] Evidence from Earth-based telescopes and the Dawn mission suggest that Vesta is the source of the HED meteorites, which have basaltic characteristics.

[111] Analysis of the Voyager images led scientists to believe that these flows were composed mostly of various compounds of molten sulfur.

However, subsequent Earth-based infrared studies and measurements from the Galileo spacecraft indicate that these flows are composed of basaltic lava with mafic to ultramafic compositions.

[120] Chemical weathering also releases readily water-soluble cations such as calcium, sodium and magnesium, which give basaltic areas a strong buffer capacity against acidification.

[123] Metamorphosed basalts are important hosts for a variety of hydrothermal ores, including deposits of gold, copper and volcanogenic massive sulfides.

[125] Fe- and Mn- oxidizing bacteria have been cultured from weathered submarine basalts of Kamaʻehuakanaloa Seamount (formerly Loihi).

[126] The impact of bacteria on altering the chemical composition of basaltic glass (and thus, the oceanic crust) and seawater suggest that these interactions may lead to an application of hydrothermal vents to the origin of life.

Underwater basalt deposits, scattered in seas around the globe, have the added benefit of the water serving as a barrier to the re-release of CO2 into the atmosphere.