1. ## Lifespan of Platetectonics?

How long will the earth have internal heat?

2. Not a very precise question.

The Earth radiates somewhere between 21x10^12 to 46x10^12 watts at the surface (NASA uses the lower figure). This is only about 0.05% of the incoming solar radiation that warms the Earth's surface.

In a couple of billion years, mantle temperatures may fall to the point that plate tectonics cease. Eath's internal heat will have to dissipate through a growing thickness of crustal rock, but the eventual loss of atmosphere will lower the average surface temperature. The sharpest thermal gradiant will remain near the surface so if you plan to tunnel and live underground, this may be viable for some 5 billion years or until the sun expands as a red giant.

3. I think this is an interesting question, the base of earthquake is relent upon the displacement of the plates, like this problem.

http://www.aps.org/publications/apsn...9910/earth.cfm

http://www.ds.mpg.de/english/researc...wax/index.html

4. kzb
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The Earth's internal heat comes from radioactive decay of long-lived radionuclides. When this was realised, it was a key point in science.

Previously, Kelvin had calculated the gravitational energy release as the earth formed, and found the Earth had to be less than 20 million years old, as the energy would have radiated away in a longer period than this.

When radioactive decay was discovered, it was realised this would keep the planet hot for billions of years. This then reconciled other geological findings by giving things time to have happened.

The most important radionuclides from this point of view are that of uranium-238 (half life 4.47 billion years), thorium-232 (half-life 14 billion y) and potassium-40 (1.28 billion y).

Since the planet formed, about half the uranium-238 has decayed, much less than half the thorium-232 has gone, and most of the potassium-40 has gone. Thorium-232 is more abundant than uranium-238. Now I'm not sure about the precise balance of heat outputs in the decay chains, but I think you can see from the half-lives there must be plenty of life left plate tectonics.

There are also less well known primordial radionuclides. For example, all platinum is radioactive as it contains a few percent platinum-190 (660 billion years). Similarly samarium-148 half-life is 7 million billion years. Neither of these will have decayed significantly since the earth formed, and they will still be there when it ends.

The earth is expected to become uninhabitable within one billion years from now, because of increasing solar output.

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The earth is expected to become uninhabitable within one billion years from now, because of increasing solar output.
Not by me. It's just another technical problem for us to solve.

6. Originally Posted by kzb
The Earth's internal heat comes from radioactive decay of long-lived radionuclides. When this was realised, it was a key point in science.
The way I heard the story in geology 101 was that most of the internal heat is from formation and formation of the inner core. The radioactive heat has a high concentration in the crust which confused Kelvin's calculations.

It's even more complicated, combining the two, but mostly that, it seems.

7. kzb
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According to this link, some of the heat comes from latent heat of crystallisation being released as the liquid core solidifies.

http://www.physorg.com/news62952904.html

However, it also says the bulk of the heat comes from radioactive decay. I guess tidal heating comes into it also. Certain people advocate the existence of a fission reactor at the Earth's core, although there seems to be a lot of skeptism about that....

I never did geology at school unfortunately so I only remember the Kelvin story as related to us in physics, and then perhaps unreliably. The story was, until radioactivity was discovered, there was a massive discrepancy between the time required for geological processes to have taken place, and the time for the Earth to cool, as calculated by Kelvin. There was supposed to have been a big argument over it, with Kelvin going to his grave refusing to accept the radioactive heating theory, but it has been accepted ever since.

8. Originally Posted by kzb
The earth is expected to become uninhabitable within one billion years from now, because of increasing solar output.
It's closer to five billion, I think.

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Originally Posted by Noclevername
It's closer to five billion, I think.
Sorry, kzb has it right at 1 billion years. Itīs not the red giant phase thatīs the problem, but just the gradual heating of the sun (moving up the HR diagram). By 1 billion AD, the mean temperature will exceed 60 - 70 C, which should mean the end of life.

Fortunately Mars might become habitable for the following billion years - or it might be possible to move the earth out as far as Mars (NASA has done the study).

10. Could a planet with more radiation have a longer plate tectonic lifespan?

My reasoning behind my questions are is to speculate on possible extrasolar planets. Say that, a world orbiting 26 Draconis, which is probably 8BY old, what types of worlds would be orbiting that system? Would mountains exist? Would the worlds be dead like Mars and Venus?

I've read that in one billion years, the Earth will start to turn into a water-green-house effect world. Like Venus, with no oceans or seas, but with thick and moist water atmosphere.

11. Originally Posted by RalofTyr
Could a planet with more radiation have a longer plate tectonic lifespan?

My reasoning behind my questions are is to speculate on possible extrasolar planets. Say that, a world orbiting 26 Draconis, which is probably 8BY old, what types of worlds would be orbiting that system? Would mountains exist? Would the worlds be dead like Mars and Venus?

I've read that in one billion years, the Earth will start to turn into a water-green-house effect world. Like Venus, with no oceans or seas, but with thick and moist water atmosphere.
The internal radiation of a planet depends mainly on its composition; the crust just doesn't absorb much heat. Even in the case of Venus, it's the massively thick atmosphere that keeps in so much heat, without it the surface would lose heat relatively quickly. For a planet like Earth, a hot water greenhouse is the most likely development; whether it would last long enough to absorb all the oceans I don't know. Venus has almost no water left in its atmosphere, but it has been baking for a long time, and may have had less to start. Earth will also move away from the Sun naturally, I'm not sure if this is enough to cancel or diminish the effects.

12. kzb
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I don't think you necessarily need plate tectonics to create mountains, there are some very high mountains on the moon for example. Or perhaps they would have eroded away on an earthlike planet by now.

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