Basalt

Basalt is an effusive rock of volcanic origin, dark or black in color with a relatively low silica (SiO2) content (45 to 52% by weight). Basalt consists primarily of calcic plagioclase and pyroxenes; some basalts may also be rich in olivine. The intrusive correspondent of basalt is gabbro. Basalt can range in appearance from porphyritic to microcrystalline to glassy. It comes from a magma solidified quickly in contact with air or water and is the main rock constituting the upper part of the oceanic crust. Basaltic magmas are formed by decompression melting of the Earth’s mantle.

The word “basalt” comes from late Latin basaltes, a rewrite of the Latin word basanites, meaning “very hard rock.” Instead, the latter derives from the ancient Greek βασανίτης (basanìtes), from βάσανος (bàsanos) perhaps coming from the Egyptian bauhun, “slate.” The modern petrological term “basalt” used to describe a particular lava rock compound originated in 1556 with Georg Agricola in his famous work on mining and mineralogy De re metallica, libri XII. Agricola used the word “basalt” to denote the black volcanic rock of the Schloßberg castle hill near Stolpen, believing it to be the same “very hard rock” described by Pliny the Elder in his Naturalis historia.

Areas of diffusion

Basalts are the most widespread effusive lithological type on Earth: in general, but not absolute, there is a predominance of olivine basalts in oceanic areas and tholeiitic basalts in continental regions. Basalts are always laid down in large expansions and flows; in a submarine environment they give rise to pillow lava and hyaloclastites.

The basalts form the base of most of the oceanic bottoms, while in continental environment form tabular structures, known as plateau; they constitute, moreover, volcanic buildings always of considerable size and modest slope (shield volcanoes), because of the extreme fluidity of lava, flows of various sizes, strands, sills, etc..

Typical basaltic plateaux are those of the Deccan (India), Parana (Brazil), Columbia and Snake River (USA). In Italy, basaltic lavas are quite common in the Veneto area (Lessini, Berici, Monte Baldo) and, above all, in Sicily (Iblei and Etna), in Sardinia (Montiferru) and in the Aeolian Islands. Lunar basalt, rock that forms the bottom of the lunar seas.

Lunar basalts

Lunar basalts sampled by the Apollo missions have shown some differences in chemistry with those on Earth. First, they are anhydrous, that is, without water, and have very low amounts of elements with low boiling points, such as carbon, chlorine, nitrogen, lead and zinc. They are also richer in heavy elements, such as titanium, aluminum, and zirconium.

Based on their chemical composition, lunar basalts can belong to two different families, rich or poor in titanium. A further difference between lunar basalts and terrestrial basalts can be seen in the absolute age of the younger basalts. While terrestrial basalts continue to solidify even presently from lavas emitted from mid-ocean ridges, the younger lunar basalts exhibit an age of about three billion years.

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