Bazalt is a type of rock found in the United Kingdom and the United States. It is an aphanitic extrusive igneous rock rich in magnesium and iron (mafic lava) exposed at or very close to a rocky planet’s or moon’s surface. Bazalt makes up more than 90% of the planet’s volcanic rock. Bazalt that cools quickly and has fine grains is chemically comparable to gabbro that cools slowly. About 20 volcanoes produce basalt lava each year, according to experts. Other planetary bodies in the Solar System also include it as a major rock type. The plains of Venus, for example, cover 80% of the planet’s surface; the lunar maria are plains of flood basaltic lava flows.
Due to its comparatively low silica content (between 45 and 52 percent), molten basalt lava has a low viscosity, resulting in fast-moving lava flows that can spread across large distances before cooling and solidifying. Flood basalts are thick sequences of numerous such flows that can cover hundreds of thousands of square kilometers, making them the most massive volcanic structures on the planet.
We assume that the upper mantle is the source of basaltic magmas on Earth. Basalt chemistry thus reveals circumstances deep within the Earth’s interior.