Potassium-argon is one of the simplest dating methods and has a halflife of 1.3 billion years. It can be used on rocks within a wide range of ages—a few thousand years to billions of years. Potassium is present in most rock-forming minerals, and in samples that have Potassium-40 in them, there is generally enough Argon-40 present that the rock can be accurately dated, even if there is only a small amount of Argon-40 present. Potassium is a common element found in clay minerals, tephra, micas, and evaporates. Ages can be measured very accurately with this technique. Usually, when geologists date rocks, they run more than one method of analysis if possible in order to confirm the integrity of the results.

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