The potassium-argon method was utilized to determine the age of volcanism at Pilot Knob. This method is based upon the decay of the radioactive isotope of potassium (potassium 40) to argon 40, an isotope of argon, an inert gas. By knowing the concentration of potassium in a rock mineral and the amount of argon gas produced by radioactive decay trapped within the minerals, an age can be assigned to the rock because the decay rate of potassium 40 to argon 40 is known from experimental work. The age of Pilot Knob volcanism dated through the potassium-argon method is 79.5 +/- 3 million years, and agrees with the age derived by correlation with fossils to other radiometrically dated deposits.