Basalt is the dark volcanic rock that makes up most of Earth’s oceanic crust.
It forms when iron- and magnesium-rich magma erupts or cools near the surface. At mid-ocean ridges, mantle material partially melts and produces basaltic lava, creating new seafloor as tectonic plates separate. Basalt also forms at many volcanic islands and large continental lava fields.
Basalt is usually fine-grained because lava cools relatively quickly, leaving crystals too small to see easily. Its common minerals include plagioclase feldspar, pyroxene, and sometimes olivine. The rock is generally denser and darker than continental rocks such as granite.
Basalt should not be confused with gabbro. Both have broadly similar chemistry, but gabbro cools slowly underground and therefore has larger visible crystals, while basalt cools at or near the surface.