Compromising Plate Tectonics without compromising the Magnetic Field?

[willbell]

Is it possible that a planet's core could cool just enough to have the plate tectonics stop working without compromising the ability of the magnetic field to block off cosmic rays? I apologize if this question is in the wrong category, specifically I'm looking for the answer based on Earth or an Earth-like world.

By plate tectonics stopped working I mean that the tectonic plates stop drifting because the currents in the mantle are no longer as fluid as they are. I want to know specifically if the magnetic field in this scenario would still be strong enough that Earth or Earth-like planet would be able to support life.

 

[Andre]

Are you a writer?

I guess any answer is highly speculative.

 

[davenn]

of course... this scenario was what the movie "The Core" was based around

Dave

 

[billiards]

Most of the heat that powers mantle convection is derived from the decay of radiogenic isotopes in the Earth's mantle. Mantle convection, and ergo plate tectonics, shouldn't really care too much about heat from the core.

Anyway, you could look at another planet like Mars and see that it does have a magnetic field, and that it doesn't have plate tectonics. Sure, the magnetic field is not very strong, but it wouldn't be a great stretch to imagine it could be stronger on a different planet.

No one really knows the thermal evolution of Earth, and until they do you won't get a satifactory answer. But bear in mind: plate tectonics is more than just mantle convection, it is a special type of convection, that has a surface layer that simulataneously acts as the upper thermal boundary layer (conduction) to the convecting mantle, and is itself actively involved in the convection process (e.g. subduction). That mode of mantle convection could be shut off if the oceans were to suddenly disappear (for example), because it relies on the significant influence of water to aid in subduction. This in principle could happen without interfering with the core convection (at least for 50-100 million years when the absence of subducted slab material might be felt at the core-mantle boundary). So the mantle could still convect, and the core too, but there would be no plate tectonics. Of course, without water life would not survive. But the water example serves as an example that it could be done, there might be other less catastrophic ways of doing it that would be more congenial to life.

 

[willbell]

Are you a writer?

I guess any answer is highly speculative.

Yes, I guess I should explain in more detail exactly what I am aiming for. I want a world (or at least a secluded continent) that more or less is a giant sand bar, like a beach, surrounded by ocean. I know that without plate tectonics that might be possible, given that everything erodes over time and that rock formation occurs mostly because of plate tectonics (igneous rock, of course there is still sedimentary rock but that isn't going to be everywhere).

So yes I am a writer, but however I do not associate myself with something like "The Core", I have a totally different aim with this project.

 

[willbell]

Most of the heat that powers mantle convection is derived from the decay of radiogenic isotopes in the Earth's mantle. Mantle convection, and ergo plate tectonics, shouldn't really care too much about heat from the core.

Anyway, you could look at another planet like Mars and see that it does have a magnetic field, and that it doesn't have plate tectonics. Sure, the magnetic field is not very strong, but it wouldn't be a great stretch to imagine it could be stronger on a different planet.

No one really knows the thermal evolution of Earth, and until they do you won't get a satifactory answer. But bear in mind: plate tectonics is more than just mantle convection, it is a special type of convection, that has a surface layer that simulataneously acts as the upper thermal boundary layer (conduction) to the convecting mantle, and is itself actively involved in the convection process (e.g. subduction). That mode of mantle convection could be shut off if the oceans were to suddenly disappear (for example), because it relies on the significant influence of water to aid in subduction. This in principle could happen without interfering with the core convection (at least for 50-100 million years when the absence of subducted slab material might be felt at the core-mantle boundary). So the mantle could still convect, and the core too, but there would be no plate tectonics. Of course, without water life would not survive. But the water example serves as an example that it could be done, there might be other less catastrophic ways of doing it that would be more congenial to life.

I don't need it to happen quickly so that will be fine, thank you for this comprehensive answer, I'm glad to know the physics of this world might be possible.

 

[billiards]

Do you want the world to be Earth? Or could it be another planet in another solar system?

Do you want human life, or could it be a specially evolved form?

 

[willbell]

Do you want the world to be Earth? Or could it be another planet in another solar system?

Do you want human life, or could it be a specially evolved form?

A different planet with a lot in common with Earth, I figure realistically a yellow star such as our own would have melted Earth before it had a shot at cooling down this much, so an old red dwarf is my best bet. I want human life, but I think it isn't mandatory.

 

[Andre]

There are more problems. If plate tectonics stops an important part of the carbon cycle is blocked. Volcanoes would no longer return the carbon that was locked in limestone and sediments. Eventually all useful carbon for the biomass will disappear due to rock weathering, coral reef and foraminafera shells building, etc.

Also the life cycle of mountain ranges is measured in millions of years. So the lifetime of your continent may indeed be limited.

 

[willbell]

There are more problems. If plate tectonics stops an important part of the carbon cycle is blocked. Volcanoes would no longer return the carbon that was locked in limestone and sediments. Eventually all useful carbon for the biomass will disappear due to rock weathering, coral reef and foraminafera shells building, etc.

Also the life cycle of mountain ranges is measured in millions of years. So the lifetime of your continent may indeed be limited.

I realized the second point, I figure that the remaining land is probably going to be very low lying and on its way to the bottom of the sea. And on the carbon cycle, I didn't realize but I guess that my best bet is that the land is fairly barren with only the tail end of life being left. Think of it like one of those post-apocalyptic stories that imagines the Earth on its last legs as it eventually moves towards the end of life as we know it.

 

[Andre]

You can make a good point of the very short supply of carbon. Plants will stop with photosynthesis below a certain level of carbondioxide in the atmosphere. Trees will probably have a harder time than grasses. But the population has to be rather inventive to prevent wasting carbon.

 

[willbell]

You can make a good point of the very short supply of carbon. Plants will stop with photosynthesis below a certain level of carbondioxide in the atmosphere. Trees will probably have a harder time than grasses. But the population has to be rather inventive to prevent wasting carbon.

The civilization isn't really much of a civilization, no one remembers their origins or the concerns about running out of Carbon (they don't remember any chemistry), so they don't realize how close to extinction they are. Like I said the scenario is fairly post-apocalyptic/dying Earth in tone and design.

 

[Andre]

Is it possible ...without compromising the ability of the magnetic field to block off cosmic rays? ...

Maybe, just maybe the magnetic field deflecting 'cosmic rays' may be a bit overrated. There are some clues that life can handle radioactivity much better than is thought sometimes.

For instance there is no evidence whatsoever of mass extinctions due to geomagnetic reversals and magnetic excursions that came with periods of strongly reduced geomagnetic field strenghts.

And also you may want to take note of "Growing up with Chernobyl"

And we have lots of people actually surviving radiation therapy

 

[willbell]

That is possible, although there was a fairly big body count from Chernobyl (cancer, etc) and Radiation Therapy does kill the target cells so your examples aren't totally appropriate.

 

[Andre]

the number of casualties of Chernobyl is highly disputed but it seems that there are more survivors while Chesser and Baker (Growing up with Chernobyl) reported thriving wildlife on the abandoned premisses. That article is easily downloadable and I really recommend to read it.

 

[willbell]

the number of casualties of Chernobyl is highly disputed but it seems that there are more survivors while Chesser and Baker (Growing up with Chernobyl) reported thriving wildlife on the abandoned premisses. That article is easily downloadable and I really recommend to read it.

I believe it was in "The World without Us" that I read about wildlife at Chernobyl, and that for the first several years after the disaster the birds would go to Chernobyl and then none of them would survive to come back again, a different batch of birds would settle and then die until the site had sat long enough for the amount of radiation to drop considerably. So what I am saying is that there might be wildlife now but when the radiation was still at higher levels there was considerable risks associated with it. So I guess they can survive some radiation but I think that cosmic rays probably stray towards a higher level of radiation. After all in an 18 month voyage to Mars it is estimated that an astronaut would receive the maximum dosage that is recommended of radiation for their lifetime, so if the planet has no magnetic field we could see people going over the limit while they are still children, I doubt anybody would live to be an adult, unless they had immune systems cranked to eleven (to kill cancer, etc).

 

[Andre]

I believe it was in "The World without Us" that I read about wildlife at Chernobyl, and that for the first several years after the disaster the birds would go to Chernobyl and then none of them would survive to come back again, a different batch of birds would settle and then die until the site had sat long enough for the amount of radiation to drop considerably.

I think that assessment would be stronger if it was supported by an objective source like Chesser and Baker, which still should be read. Of course radioactivity is on a scale. At the absolute highest end life is not possible and there is a lower threshold below which no effects on biota can be demonstrated, in between there is a long bandwidth in which radioactivity *can* (not will) lead to mutations and defects. But I understand that readers are not interested in objectivity.

 

[willbell]

I think that assessment would be stronger if it was supported by an objective source like Chesser and Baker, which still should be read. Of course radioactivity is on a scale. At the absolute highest end life is not possible and there is a lower threshold below which no effects on biota can be demonstrated, in between there is a long bandwidth in which radioactivity *can* (not will) lead to mutations and defects. But I understand that readers are not interested in objectivity.

It *will* when you are exposed over a lifetime.