The degree of crystallization of basalt and the size of the grain depend mainly on the cooling rate of the magma. Slow cooling (such as a few degrees of cooling per day) can produce crystals of several millimeters in size, and rapid cooling (such as 100 ° C per minute) can produce fine needle-like, plate-like crystals or amorphous glass. Therefore, under surface conditions, basalts usually have fine-grained to cryptocrystalline or vitreous structures, and a few are medium-grained structures. It often contains olivine, pyroxene and plagioclase phenocrysts to form a plaque structure. The phenocrysts can accumulate in the flowing magma, called the spotted structure. These phenocrysts are formed during the ascent of the basaltic magma through the crust (months to hours) and can also form in the large magma reservoirs prior to the eruption. The matrix structure changes greatly. With the thickness of the rock flow, the speed of cooling and the amount of volatile components, there are various transition types between the whole crystal and the vitreous, but mainly the intergranular structure, the interstitial structure and the intermittent structure. , less sublimous structure and luminescent structure.
The basalt structure is related to its consolidation environment. Basalt formed on land is often a rope-like structure, a massive structure and a columnar joint; the basalt formed under water often has a pillow-like structure. The stomatal structure and almond structure may appear in various basalts.
In explosive volcanic activity, the fiery basaltic lava spurts out the crater, forming different shapes of volcanic bombs with the difference in the degree of consolidation before landing: spindle-shaped volcanic bombs, twist-shaped volcanic bombs, irregular volcanic bombs, and Cow dung, pie, straw hat or serpentine and flat splashes of lava clusters.
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