Basalt

Basalt is an extrusive dark-colored igneous rock, with a mafic composition —rich in magnesium and iron silicates and silica—, which constitutes one of the most abundant rocks in the earth's crust

Basalts usually have a porphyritic texture, with phenocrysts of olivine, augite, plagioclase, and a fine crystalline matrix. Sometimes it can appear in the form of a glass, called sideromelano, with very few or no crystals.

Basalt is the most common volcanic rock and exceeds any other igneous rock in Earth's surface area, even combined, since it forms most of the ocean floor. Large extensions of basalt can be found on the continents which are called traps. In turn, it is common to find it in oceanic islands and continental and insular volcanic arcs.

Rocks similar to and often related to basalts include diabase, gabbro, and andesite.

It is also found on the surfaces of the Moon and Mars, as well as some meteorites.

Forms and occurrence

Castellfullit de la Roca, in Gerona (Spain), a village built on basaltic columns.

Basalt covers about 70% of the Earth's surface and is larger than all other igneous rocks combined. This rock is particularly abundant on the ocean floor as it forms the top layer of the oceanic crust (not counting the sediments that partly cover it). In scientific contexts, the basalt that originates in mid-ocean ridges and composes the upper layers of oceanic crust. Apart from ordinary oceanic crust there are large expanses of predominantly basalt called traps, which can cover thousands of km², with individual lava flows with volumes of more than 2,000 km³. Some of the main traps are found in the Paraná Basin, Siberia, the Deccan Plateau, the Karoo, and the Columbia River Basin. The basalt presents is in continental and insular volcanic arcs and in oceanic islands. unique.

Basalto in the form of solid lava.

When basalt surfaces during volcanic eruptions, temperatures range from 1,100 to 1,250 °C. In the form of lava, basalt flows relatively easily and can form shield volcanoes which are mainly composed of this rock. The easy flow of basalt is due to its low silica content, which allows basalt flows to advance more than 20 km and magma gases to escape without forming eruptive columns.

Basalt can occur in a variety of forms such as lava, fiery avalanches, in mudflows, hyaloclastites, as pyroclasts, and ash. When basalt occurs as lava it can take the form of cushion lava, Aa lava, pahoehoe lava and form lava tubes.

Basaltic magma crystallizing in a dyke forms the subvolcanic equivalent of basalt, diabase, while the same magma crystallizing in a magma chamber forms gabbro, the plutonic equivalent of basalt.

Alien Basalt

Basalt also occurs on the surfaces of other bodies in the solar system, such as Mars, Venus, or the Moon, where it covers approximately 17% of the surface. Lunar basalt has some differences from terrestrial basalt, including a higher ilmenite content. Some achondrite-type meteorites are basalts, evidence of volcanic activity on the celestial body from which they originated. There are basaltic achondrites that derive from the Moon while another group of basaltic achondrites called "shergottites" from the surface of Mars.

Chemistry and mineralogy

Sunset Crater's vestibule, Arizona. Coin of US$ 25 cents of scale.

Basalt is dark in color and rich in iron and magnesium. Compared to other igneous rocks, basalt has a low silica content. Although basalt can occur in the form of glass, with little or no crystals, it often contains phenocrysts of olivine, augite, and plagioclase. Basalts often have a porphyritic texture with the aforementioned phenocrystals and a fine crystalline matrix.

Example of a chemical composition of basalt expressed as a mass percentage of oxides:

The data is the average of the analysis of 3594 basalt samples.

Origin

basalt hills in the Traps of the Decán, India.

There is a dispute about whether the basalt in the magma state is primary (it would originate directly from the fusion of rocks) or if it derives from another type of more mafic magma. In any case, there are several rocks that have the necessary elements so that, through their direct fusion or their fusion and subsequent refinement, they produce basaltic magma. These are: peridotite, pyroxenite, hornblendite, basalt itself, and other rocks from metamorphosed basalts, such as amphibolite and eclogite. For a number of reasons, several of these rocks have been disqualified as a possible source of basaltic magma., being favored the thesis that peridotites give rise to basalts, however a minority of scientists favor eclogites.

The cause of the partial fusion of rocks from which basaltic magma derives, directly or indirectly, varies depending on the tectonic environment. In the mid-oceanic ridges, the successive separation of the tectonic plates causes the rise of material (peridotite) from the terrestrial mantle and its partial melting due to decompression. The basalts originated on subduction zones are produced by having partial melting in the mantle after being invaded by aqueous fluids from the subducted plate. Basalts that occur in the interior of tectonic plates and not on their edges (such as mid-ocean ridges and subduction zones) are considered by most scientists to be expressions of partial melting caused by high mantle plume temperatures.

Derived Rocks

Fiery

Basaltic magma can produce rocks other than basalt such as andesite, dacite, and rhyolite by fractional crystallization, although assimilation of crustal rocks also plays an important role in the formation of these rocks. According to some laboratory experiments, it is it could generate felsic magma directly from the partial melting of basalt. In the case of the Icelandic rhyolites there are two hypotheses and both involve basalt: one that postulates that the rhyolites come from the partial melting of basalt, and another that postulates that fractional crystallization and crustal assimilation by basaltic magma generate rhyolitic magma.

Metamorphic

Eclogita, a rock that can be formed after an extreme metamorphism of the basalt.

Basalt can be a protolith of a wide range of metamorphic rocks depending on temperature and pressure conditions. Some of the metamorphic rocks that can be derived from basalt (metabasalts) are blueschist, greenschist, amphibolite, and granulite. The various metamorphic facies are named after rocks formed from a basalt protolith.

Eclogites are rocks of basaltic composition that have been exposed to extreme pressures in the mantle or in subduction zones.

Basalts altered by hydrothermal circulation near mid-ocean ridges form spilites.

Alteration and weathering

Regarding chemical weathering, the components of basalt tend to decay in the following order: glass, olivine, plagioclase, pyroxene, and finally opaque minerals. Chemical weathering of basalt consumes carbon dioxide, and the 70% of this consumption is due to the weathering of aluminosilicates with magnesium and calcium.

Sideromelan, as basalt glass is called, alters on contact with water into a material called palagonite, before finally decaying into smectite, a mineral from the clay group.

Use and properties

Basalto (black) and limestone adoquins in Lisbon.

Throughout history, basalt has been used as a construction material by various cultures, including the Olmecs of Mexico, Ancient Egypt, and the Rapanui people, to name a few. Today artificial basalt fibers are used to reinforce concrete structures.

Despite being impermeable, its use is not advisable for certain hydraulic works due to its excessive fracturing. Another defect is that basalt surfaces tend to form small white spots where the analcime mineral has been altered, possibly as a result of of solar radiation.

Basalt has a lower coefficient of thermal expansion than granite, limestone, sandstone, quartzite, marble, or slate, so it receives little fire damage. Given the low albedo of basalts, the surfaces of this rock tend to heat up more than others, as a result of solar radiation, reaching temperatures of almost 80 °C in the Sahara. The massive basalt (without vesicles) has a density of 2.8 to 2.9 g/cm³ being more dense than granite and marble but less than gabbro. On the Mohs scale of hardness it has been estimated that basalt has a hardness that can vary from approximately 4.8 to 6.5.

History

The volcanoes of Chaîne des Puys in France were instrumental in verifying the volcanic origin of the basalt.

During the decades around the year 1800, a scientific controversy raged over the origin of basalt. Disciples and followers of the German geologist Abraham Gottlob Werner held that basalt was a sedimentary rock originating from precipitation in a great ancestral ocean. This theory is called "neptunism." This theory was opposed by two camps: followers of James Hutton, later known as "plutonists", who claimed that basalt was an intrusive rock, and "vulcanists" who considered basalt to be a volcanic rock. Some of the arguments of the Neptunists against the volcanic origin of basalt was its presence in places like the Giant's Causeway and Saxony where there are no active volcanoes, as well as presumed fossil finds in basalt. The confusion caused by the finding of basalt without apparent volcanoes was also it occurred in America, where Juan Ignacio Molina noticed the basalts of Chiloé where there are currently no volcanoes, thus ruling out a volcanic origin. Against the formation of basalt in volcanic eruptions, the Neptunists argued that this was due rather to the melting of Neptunian basalt under volcanoes. By 1830 the Neptunist camp had disintegrated, losing most of its followers, who The volcanic origin of the basalt was recognized, in some cases thanks to visits to the volcanoes and basalts of the Chaîne des Puys, in France.