Britannica Sources used in this answer

How did radioactive decay help add argon to Earth’s atmosphere?

Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

Wispy cirrus clouds stretch across a bright blue sky above trees and a river at Pinawa Dam Provincial Park in Manitoba, Canada, with sunlight illuminating the scene. — Atmosphere The atmospheres of planets in the solar system are composed of various gases, particulates, and liquids. They are also dynamic places that redistribute heat and other forms of energy. On Earth, the atmosphere provides critical ingredients for living things. Here, feathery cirrus clouds drift across the deep blue sky over Pinawa Dam Provincial Park, near Pinawa, Manitoba, Canada.

Argon-40 ( $^{40} Ar$ ) is the most abundant noble gas in Earth’s atmosphere, a result of the radioactive decay of potassium-40 ( $^{40} K$ ), which is a common element in the planet's crustal rocks.123

Unlike primordial noble gases that were present during the Earth's formation, $^{40} Ar$ is radiogenic. The process of its addition to the atmosphere occurs in several stages:13

Production: As radioactive $^{40} K$ decays within rock-forming minerals, it produces $^{40} Ar$ as a daughter product, which becomes trapped within the mineral crystal structures.123
Release: Through igneous activity, these rocks are melted, allowing the trapped argon gas to escape from the mineral lattice.3
Outgassing: The released gas eventually reaches the surface and is added to the atmosphere through the process of outgassing.3

Because the Earth trapped relatively little primordial argon during its accretion, this continuous accumulation of radiogenic $^{40} Ar$ over geological time has caused it to dominate the isotopic composition of atmospheric argon, making it significantly more abundant on Earth than in the rest of the solar system.143