As I understand it, the Earth is radioactive solely because of radioactive materials it captured early in its formation, and that there are essentially no new radioisotopes coming in from space anymore. Given that all radioisotopes eventually decay down to stability, when will the Earth be radiologically dead? Assuming this happens before the Sun dies, will it have a major impact on our geology?
Some radioactive isotopes are being formed now, including Carbon-14.
More importantly, all radioactive substances decay with a half-life, meaning that half of the substance decays in X amount of time, or (from a different perspective) that any given atom will have a 50% chance of decay in X amount of time.
That means that
- we can only speak of probabilities that everything has decayed and
- there will be some remnants for a very, very long time, especially when you consider elements with half-lives in the billions of years.
Uranium-238 has a half-life of about 4.something billion years, so there will still be nonneglible amounts of it left when the Sun dies, but precisely because it has such a long half-life, it’s not very “hot” to begin with. How low a radioactivity level are you willing to consider “radiologically dead”?
Perhaps a defining point would be when there’s no longer sufficient decay to keep the Earth’s core molten and thus maintain the Earth’s magnetic field?
Or another, where plate tectonics grinds to a halt, which in my WAG would probably happen first.
So it’s true that radiation is a major reason we have plate tectonics? I wasn’t certain of that, although I had heard it was one of the reasons.
Anyway, yes, if plate tectonics stop due to radiation tailing off I’d certainly consider the planet to be radiologically dead.
My understanding is that the radioactivity supplies a good bit / majority of the heat (some of it is due to the fact that a molten ball as massive as the earth just takes awhile to cool off).
So, as time passes the earth is both cooling off and the heat being supplied from radioactivity is dropping as well.
Whether plate tectonics stopping is “earth” shattering or not I have no idea.
Inasmuch as the core of the planet remains hot and molten due to radioactive decay then yes, plate tectonics are a result of radiation. Once the radioactive decay stops (or slows down sufficiently) and the molten stuff down there solidifies more and more plates will no longer be “floating” and will be locked in place.