Argon is a noble gas that is found in the atmosphere of Earth and other planets. It was formed by the radioactive decay of potassium-40 in the Earth's mantle. Potassium-40 is an isotope of potassium that decays to argon-40 and other elements. The argon-40 that is produced then enters the atmosphere through volcanic activity. Argon is also released into the atmosphere from the decay of uranium-235 and thorium-232. These elements are found in the Earth's crust and mantle and decay to produce helium and argon, which are released into the atmosphere. Argon is also produced by the decay of other elements in the atmosphere, such as nitrogen-14, oxygen-18, and neon-20. These elements are all unstable and decay to produce lighter elements, including argon.