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However, because argon‐40 is a Noble element it generally occurs as a gas and therefore rarely occurs in minerals at the time they form. Therefore, argon‐40 that exists in minerals is likely the product of the radioactive decay of the potassium‐40. Assuming that there is no loss of this argon‐40 from the mineral and no addition from other sources, the ratio of⁴⁰K/⁴⁰Ar in the mineral should increase over time and yield reliable ages for the minerals and/or rocks in which it occurs. This is especially true for volcanic rocks because, at high temperatures, argon is a gas that escapes easily from the lava into the atmosphere (where it is the third most abundant gas, after nitrogen and oxygen). On the other hand, when the lava crystallizes to form potassium‐bearing minerals, argon‐40 produced by the decay of potassium‐40 tends to be trapped in the crystal lattice because its radius is larger than the spacing between atoms. Ideally, this sets the stage for using⁴⁰K/⁴⁰Ar to date such rocks, but as we shall see, many challenges remain. This ratio is most useful for dating samples that formed more than 100 Ka in which enough time has elapsed for accurately measurable argon‐40 to accumulate, although some dates as young as 25 Ka have been reported.