How does radiogenic argon-40 get into the atmosphere? There's an awful lot of argon in the atmosphere: about 0.93% of the atmosphere is argon[1], making it the third-most abundant gas after N2 and O2. As I understand it, most of it is produced by the following decay path: 

$$
\ce{^{40}_{19}K + ^{0}_{-1}e- -&gt; ^{40}_{18}Ar + \gamma{} + \nu_{e}}
%edit
$$

How does all this radiogenic argon make it into the atmosphere, though? Most of the decaying potassium-40 must surely be at some depth beneath the surface, thus trapping the argon-40 in the interior of the Earth, right?
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.