How does radiogenic argon-40 get into the atmosphere?
Most of this 40Ar makes its way to the atmosphere in two primary ways. Firstly, since most of the K is concentrated in the continental crust, volcanic eruptions on the continents over geological time have concentrated Ar in the atmosphere. Since it is too heavy and inert to decrease in any appreciable method it has a long residence time. Secondly, volcanism at mid-oceanic ridges releases Ar into the ocean and then into the atmosphere. The current combined rate of these volcanic events is ~ 2.2×109 grams/year.