Basalt is a common, dark-colored volcanic rock that makes up much of the Earth’s oceanic crust and surfaces of other planets. Its smooth, fine-grained appearance often leads people to question whether it contains crystals. Unlike rocks with obvious, sparkly crystals such as granite, understanding basalt requires looking beyond what the naked eye can see to clarify its fundamental crystalline structure.
Basalt’s Composition and Origin
Basalt is classified as an extrusive igneous rock, meaning it forms from molten rock (magma) that has erupted onto the Earth’s surface as lava and then solidified. The rock is rich in iron and magnesium, which gives it a dark color. Basalt’s crystalline structure is built from a specific mineral combination, primarily plagioclase feldspar and pyroxene. Small amounts of the mineral olivine are also often present.
The Direct Answer: Crystal Size and Texture
Yes, basalt is composed entirely of crystals despite its uniform appearance. The reason the rock appears smooth and non-crystalline is that the individual mineral components are typically too small to be seen without a microscope, often less than 1 millimeter in size. Geologists use the term aphanitic to describe this fine-grained texture, where the crystalline structure is present but not visible. However, some basalt samples are porphyritic, meaning they contain larger, distinct crystals called phenocrysts, embedded within the fine-grained matrix. These phenocrysts, usually olivine or plagioclase feldspar, began forming deep underground before the lava erupted.
How Rapid Cooling Affects Crystal Growth
The small size of basalt’s crystals is a direct consequence of the speed at which the lava cools and solidifies. When molten rock erupts onto the surface, it is exposed to air or water, which causes a very rapid drop in temperature, preventing the atoms within the melt from having enough time to organize into large, well-structured crystals. In the rapidly cooling environment of a lava flow, the mineral molecules quickly solidify where they are, resulting in many small crystals forming simultaneously. If the cooling is extremely fast, such as when lava meets the ocean, the rock can even solidify into a volcanic glass with virtually no crystal structure at all. This process stands in sharp contrast to how intrusive igneous rocks, like granite, are formed: granite cools very slowly deep beneath the Earth’s surface, allowing mineral molecules to migrate and assemble into large, easily visible crystals.