Basalt has a microlitic structure because it is formed by the rapid cooling of volcanic lava, which did not allow enough time for crystals to develop into larger sizes.
When basaltic magma reaches the surface during a volcanic eruption, it comes into direct contact with cold air or water, causing it to cool extremely quickly. This rapid drop in temperature does not allow enough time for minerals to form large crystals visible to the naked eye. Instead, significant quantities of microcrystals called microlites quickly form and are trapped in an amorphous mass, known as volcanic glass. The faster the magma cools, the finer the structure becomes and the smaller the crystals are. Typically, airborne lava flows or contact with water result in this type of very rapid cooling. It is primarily this high solidification rate that allows basalts to achieve their famous texture called microlitic.
Basalts are mainly composed of minerals like pyroxene and plagioclase (a type of feldspar), with a little bit of olivine. When magma rises to the surface, its temperature drops rapidly. As a result, tiny crystals called microlites form quickly without having the time to grow properly. Basically, crystallization happens so fast that the minerals cannot become large. The result is a fine-textured rock, almost smooth at first glance, but when you zoom in, you see many small minerals trapped in an amorphous volcanic glass matrix. This very fine and homogeneous mineral mixture is precisely what we call a microlitic structure.
The microlitic structure is easily recognizable: it consists of a multitude of small crystals, aptly called microlites, immersed in a kind of poorly crystallized glassy paste known as volcanic glass. These crystals grew rapidly due to the hyper-rapid cooling of the lava at the surface: they therefore did not have much time to grow. As a result, they remain tiny and dispersed within the glass. This texture gives the rock a homogeneous, very fine, and often dark appearance. When you look at basalt quickly, you might think it is uniform, but with a magnifying glass, you can see these microlites that make basalt so distinctive.
The microlitic texture of basalt is quite different from the granular structure commonly found in plutonic rocks like granite. While basalt cools quickly with tiny crystals surrounded by volcanic glass, granite cools slowly at depth and then develops large, visible crystals. This is referred to as a granular or holocrystalline structure. In contrast, some ultra-rapid lavas like obsidian don’t even have time to crystallize and result in a completely glassy texture, with no distinct crystals. Andesites, on the other hand, cool at an intermediate speed and often exhibit a porphyritic structure: large crystals in a fine matrix, typically resulting from a two-stage cooling process. Basalt, with its microlitic structure, thus clearly stands out from granular, porphyritic, or glassy textures due to its rapid solidification and fine but partial crystallization, between fine crystals and volcanic glass paste.
The basalt of the island of La Réunion offers a typical example: during the volcanic eruptions of Piton de la Fournaise, the lava cools so quickly that the minerals do not have time to grow, resulting in a dark rock sprinkled with small crystals called microlites. The same occurs in Iceland, particularly on the slopes of the Eyjafjallajökull volcano, where the rapid cooling due to contact with air or water creates this characteristic texture, fine and almost glassy. This phenomenon is also found in the basalt of the famous basalt columns of Giant's Causeway in Northern Ireland, where the lava solidified quickly as it contracted to form these astonishing hexagonal columns with a microlitic texture.
The microlitic structure of basalt is invisible to the naked eye, but revealed under a microscope, it unveils a multitude of small crystals (microlites) combined with an amorphous volcanic glass, a direct witness to the rapid cooling of the magma.
The famous basalt columns, such as those at the Giant's Causeway in Ireland or the volcanic columns of Iceland, result from the rapid and uniform cooling of basaltic magma, thereby creating astonishing structures in hexagonal columns.
Thanks to their rapid cooling, basalts sometimes preserve valuable information about Earth's past magnetic field. By analyzing the magnetic orientation of these rocks, scientists study continental drift.
Although primarily terrestrial, basalt has also been identified on other celestial bodies such as the Moon, Mars, and even some asteroids; these discoveries help geologists better understand the geological history of the solar system.
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