Basalt (; ) is an aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the surface of a rocky planet or moon. More than 90% of all volcanic rock on Earth is basalt. Rapid-cooling, fine-grained basalt is chemically equivalent to slow-cooling, coarse-grained gabbro. The eruption of basalt lava is observed by geologists at about 20 volcanoes per year. 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 ~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.
Molten basalt lava has a low viscosity due to its relatively low silica content (between 45% and 52%), resulting in rapidly moving lava flows that can spread over great areas before cooling and solidifying. 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.
Basaltic magmas within Earth are thought to originate from the upper mantle. The chemistry of basalts thus provides clues to processes deep in Earth's interior.
Definition and characteristics
Basalt is composed mostly of oxides of silicon, iron, magnesium, potassium, aluminum, titanium, and calcium. Geologists classify igneous rock by its mineral content whenever possible, with the relative volume percentages of quartz (crystalline silica (SiO2)), alkali feldspar, plagioclase, and feldspathoid (QAPF) being 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, with plagioclase making up at least 65% of its feldspar content. This places basalt in the basalt/andesite field of the QAPF diagram. Basalt is further distinguished from andesite by its silica content of under 52%.
It is often not practical to determine the mineral composition of volcanic rocks, due to their very fine grain size, and geologists then classify the rocks chemically, with the total content of alkali metal oxides and silica (TAS) being particularly important. Basalt is then defined as volcanic rock with a content of 45% to 52% silica and not more than 5% alkali metal oxides. This places basalt in the B field of the TAS diagram. Such a composition is described as mafic.
Basalt is usually dark grey to black in colour, due to its high content of augite or other dark-coloured pyroxene minerals, but can exhibit a wide range of shading. Some basalts are quite light-coloured due to a high content of plagioclase, and these are sometimes described as leucobasalts. Lighter basalt can be difficult to distinguish from andesite, but a common rule of thumb, used in field research, is that basalt has a color index of 35 or greater.
The physical properties of basalt reflect its relatively low silica content and typically high iron and magnesium content. The average density of basalt is 2.9 g/cm3, compared with a typical density for granite of 2.7 g/cm3. The viscosity of basaltic magma is relatively low, around 104 to 105 cP, although this is still many orders of magnitude higher than water (which has a viscosity of about 1 cP). The viscosity of basaltic magma is similar to that of ketchup.
Basalt is often porphyritic, containing larger crystals (phenocrysts) formed prior to the extrusion that brought the magma to the surface, embedded in a finer-grained matrix. These phenocrysts usually are of augite, olivine, or a calcium-rich plagioclase, which have the highest melting temperatures of the typical minerals that can crystallize from the melt and are therefore the first to form solid crystals.
Basalt often contains vesicles, formed when dissolved gases bubble out of the magma as it decompresses during its approach to the surface, and the erupted lava then solidifies before the gases can escape. When vesicles make up a substantial fraction of the volume of the rock, the rock is described as scoria.
The term basalt is at times applied to shallow intrusive rocks with a composition typical of basalt, but rocks of this composition with a phaneritic (coarser) groundmass are more properly referred to as diabase (also called dolerite) or, when more coarse-grained (crystals over 2 mm across), as gabbro. Diabase and gabbro are thus the hypabyssal and plutonic equivalents of basalt.