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human
Why is argon so abundant in Earth’s atmosphere?

Looking at a pie chart of Earth’s atmospheric composition, argon gets its own “slice”, being nearly 25x more abundant than all other non-oxygen/nitrogen gases COMBINED. Why should this be the case?? I understand that lack of reactivity probably accounts for some of it, but that still doesn’t explain why argon is much more abundant than other noble gases, like neon and krypton. 

Thanks for any info you can provide!
gpt
Argon-40 is (sometimes) a breakdown product of radioactive potassium-40. Earth has a lot of potassium (its crust is about 2% potassium) and even though only like 0.01% of that is potassium-40, since argon doesn’t chemically react with anything else (and is generally too heavy to float off into space) the resultant argon from Earth’s K-40 can just keep on accumulating, forever.

With over 4 billion years of K-40 decay happening, that has eventually added up to a lot of argon.