why is there still radioactive material on earth? Why hasn’t it all decayed?
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Some elements have enormously long half-lives. A half life is the time it takes a given quantity of a radioactive element to decay. The half life of the most common form of uranium is about 4.5 billion years which is coincidentally about the age of the Earth. So half of the uranium in the original Earth has decayed. Of the remaining half of the original uranium, half will decay in the next 4.5 billion years – and so on.
Some elements with much shorter half lives, like potassium 40 (half life 1.9 billion years) are so common that even though they’ve been through multiple half lives, there are still huge amounts of radioactive potassium in the Earth (and in bananas).
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