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Basalt is a type of igneous rock that shows rapid cooling, e.g., lava from a volcano, and is common in volcanic islands, ocean ridges, and the tectonic-plate material formed from the upwelling mantle. The rocks brought back from the Moon are basalts, and they are presumed to exist on other planets and moons that have had geologically active periods. Constituents include silica (SiO2) and feldspar.