Basalt is a hard, black volcanic rock that constitutes the most abundant rock type at the surface of the Earth. Layers of basaltic lava floor the ocean basins and form the largest terrestrial volcanoes. Gases emanating from vast outpouring of basalt onto the continents could have contributed to the demise of the dinosaurs. Basalt is a rock type not confined to Earth: A large fraction of the meteorites that strike the Earth are basaltic. The circular impact basins (mare) on the Moon are filled with basalt. Olympus Mons, the most massive volcano in the solar system, is thought to be the product of sustained eruption of basalt from the Martian interior.

As a material that emanates from depths that are inaccessible by direct sampling, basalt has provided scientists with an invaluable probe to elucidate the nature of the Earth's interior.