Basalt is a very common dark-colored volcanic rock composed of calcic plagioclase (usually labradorite), clinopyroxene (augite) and iron ore (titaniferous magnetite). Basalt may also contain olivine, quartz, hornblende, nepheline, orthopyroxene, etc. Basalt is a volcanic equivalent of gabbro.

Basalt is usually black or dark gray and relatively featureless. It is composed of mineral grains which are mostly indistinguishable to the naked eye. Basalt may also contain volcanic glass. Basalt may contain phenocrysts (larger crystals within fine-grained groundmass) and vesicules (holes that were filled by volcanic gases).

Black color is given to basalt by pyroxene and magnetite. Both of them contain iron and this is the reason why they are black. So this is iron again which is responsible for the coloration of basalt. Plagioclase, volumetrically usually the most important constituent, is mostly pale gray in color.

Basalt is a major rock type that occurs in virtually every tectonic setting. Basalt is clearly the most common volcanic rock on Earth and basaltic rocks (including gabbro, diabase and their metamorphosed equivalents) are the most common rocks in the crust². Basalt is also common on the Moon and other rocky planets of the Solar System.

What makes basalt so common? Basalt is the original constituent of the crust from which almost all other rock types have evolved. Basalt forms when mantle rocks (peridotite) start to melt. Rocks melt incongruently. It basically means that melt that forms has a different composition from the source rocks. Of course, it can only happen if rocks melt only partially, but this is exactly what happens in the upper mantle. It melts partially to yield basaltic magma which is less dense and rises upward to form new oceanic crust in mid-ocean ridges or volcanoes and intrusives (dikes, sills) in many other tectonic regimes. Basalt is the source rock of other more evolved volcanic rocks like dacite, rhyolite, etc.

Basaltic rocks may carry xenoliths from the mantle. Here is a bright green dunite xenolith inside basalt from Hawai’i. Width of sample 8 cm.

Classification

Neighboring rock types like basaltic andesite, basanite, picrite (picrobasalt), trachybasalt and even more distant rocks like phonotephrite or andesite may have very similar look and can be easily mistaken for basalt in many cases.

Basalt is widespread in many tectonic regimes, but there are slight variations in chemical composition which allow more precise classification. MORB is an acronym for “mid-ocean ridge basalt” and OIB for “oceanic island basalt”. MORB is a result of partial melting of the upper mantle which is already recycled many times while OIB is at least partly from more deeper part of the mantle (deep-sourced mantle plumes that feed hot spots like Hawai’i or the Canary Islands) and is therefore less depleted in incompatible chemical elements.

Composition

Average chemical composition of basalt determined by 3594 chemical analyses of basaltic rocks² (numbers are mass percents, recalculated volatile-free to total 100%):

SiO₂ — 49.97
TiO₂ — 1.87
Al₂O₃ — 15.99
Fe₂O₃ — 3.85
FeO — 7.24
MnO — 0.20
MgO — 6.84
CaO — 9.62
Na₂O — 2.96
K₂O — 1.12
P₂O₅ — 0.35

Minerals that host these chemical elements (chemical composition of igneous rocks is traditionally expressed in oxides) are augite, plagioclase and titaniferous magnetite. These minerals are difficult to demonstrate because they are too small to be seen in typical basalt, but some basaltic rocks are porphyritic (lots of porphyritic rocks can be seen here: porphyry) and show some of these minerals nicely (unfortunately not magnetite, though).

Basalt in the field

Subaerial basalt forms lava flows or pyroclastic fields and cones. Two main types of basaltic lava flows are aa lava and pahoehoe lava.

Aa lava has rough rubbly irregular crust while pahoehoe is smooth. Lava crust of aa type is broken into pieces while pahoehoe retains its continuity. Both lava flow types are massive beneath the crust and this massive interior may be columnar. Columns are separated from each other by narrow cracks which form because cooling basaltic magma contracts. Cracks start to form at the surface and propagate deeper as lava cools. Submarine basalt usually forms pillows. Pillow basalt forms as a result of very rapid cooling. Outer part of forming pillow cools very quickly in contact with cold seawater while the interior still fills with molten lava.

Basalt mostly forms lava flows because it is among the least viscous magma types and therefore does not generate explosive volcanic eruptions, but sometimes pyroclastic material is formed when magma contains more volcanic gases. Basaltic rocks can be thrown out of volcanic vents as lapilli (singular: lapillus) and volcanic bombs. Basaltic volcanoes are fed by dikes (planar intrusive rock bodies when solidified that cut through other rocks) and sills (similar to dike but generally parallel to preexisting bedding planes).

Basaltic lava flows of Kilauea volcano in Hawai’i.

Metamorphism and weathering

Basalt is largely composed of minerals with little resistance to weathering. Hence, basalt as a whole also tends to disintegrate faster than granite and other felsic rock types. Magnetite is one of the most resistant common minerals in basalt and forms the bulk of heavy mineral sands. Other minerals disintegrate and release their components to water as ions or form clay minerals. Iron and aluminum are among the least mobile ions and therefore tend to form laterite deposits enriched in these elements.

Basalt metamorphoses to a number of different rock types, depending on pressure, temperature, and the nature of volatile compounds that react with minerals in basalt. Most common metamorphic rocks with basaltic protolith are chlorite schist, amphibolite, blueschist, and eclogite.

Etymology

The term “basanite” was already used in antiquity and “basalt” is probably a faulty transcription of basanite. It was German scholar Agricola (Georg Bauer) who first mentioned “basalt” in 1546. He referred to black columnar rocks from Stolpen (near Dresden in Germany) which is indeed basalt even according to modern classification principles¹.

References

1. Tomkeieff, S. I. (1983). Dictionary of Petrology. John Wiley & Sons.
2. Best, Myron G. (2002). Igneous and Metamorphic Petrology, 2nd Edition. Wiley-Blackwell.
3. Le Maitre, R. W. (2005). Igneous Rocks: A Classification and Glossary of Terms: Recommendations of the International Union of Geological Sciences Subcommission on the Systematics of Igneous Rocks, 2nd Edition. Cambridge University Press.