Which igneous rock forms when basaltic lava hardens

The answer is; Basalt rock

This type of rock is dark-colored due to the high content of magnesium and iron from the lava that forms it. Extrusive basalt rocks are fine-grained due to the rapid cooling of lava on the earth's surface. The basaltic lava is usually of low viscosity to the low silica content. Therefore, this lava flows greater distances before cooling. Basalt comprises approximately 90% of volcanic rocks on earth.