Argon is produced in Earth's crust via?
Decay of K-40
Argon, particularly its dominant isotope Argon-40 (⁴⁰Ar), is produced in Earth's crust primarily through the radioactive decay of Potassium-40 (⁴⁰K).
About 11% of ⁴⁰K decays by electron capture into ⁴⁰Ar (the rest decays by β⁻ emission into ⁴⁰Ca). Over geological time-scales, this decay has accumulated huge amounts of argon in crustal rocks, which slowly outgases into the atmosphere — explaining why argon makes up about 0.93% of the Earth's atmosphere.
This radioactive-decay origin is also the basis of the K–Ar dating method in geochronology.
Volcanoes only release argon already present in rocks; photosynthesis produces O₂; and cosmic rays contribute negligibly.
Hence, the correct answer is Decay of K-40.
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