Fueling tectonic plates: heat, size?

Question

I am doing some research to write a sci-fi story in a somewhat distant future and there is something I don't quite understand about plate tectonics.

I'm not sure exactly what fuels them. I know liquid water is needed, as well as internal heat that can be derived from the core itself or radiactive material. I've heard the atmosphere has some impact too. But what puzzles me more is size. Supposedly a planet that is too small cannot maintain tectonic activity for long. What are we talking about here, mass, volume or both? And going a bit further, is there some kind of estimation on the size a planet should be to maintain activity?

Thanks very much in advance to whoever answers.

Answer 1

Tectonic activity is driven by molten rock and magma in the Earth's mantle. The heat to keep all that rock molten is the Earth's core.

For a planet to have tectonic activity, it has to have a molten core. If the planet is small but relatively young, it will still be molten inside (from the heat of formation).
So the key factor would be mass.

Mars is smaller than Earth and its core has cooled.
Venus is almost the same size as Earth, but since Venus has no plate tectonics to let off heat, it is possible that it has no solid inner core, or that its core is not currently cooling, so that the entire liquid part of the core is at approximately the same temperature. Another possibility is that its core has already completely solidified.
Even a large planet like Earth can cool off over time (its estimated that it will take the Earth another 6 billion years for its core to cool enough to stop tectonics, but by then the Earth will be dead from the red giant the sun will have become.

So each planet appears to be different - for your purposes a world between the sizes of Mars and Earth could have tectonic activity.

Answer 2

I take issue with your basic premise, "water is needed, as well as internal heat that can be derived from the core itself or radioactive material. I've heard the atmosphere has some impact too." The mantle is a plastic solid that acts like a conveyor belt, moving the crustal plates around. Water is not needed to do this. I am not saying there is no water in the mantle, for oceanic plate subduction brings water along with it, but this is a result of plate tectonics not the reason for it. Radioactive material from the core heats the core, which in turn heats the mantle, which in turn heats the crust, which in turn heats the atmosphere (albeit very little compared to the sun). However, there is also radioactive material in the mantle (which heats the mantle and the crust). The atmosphere has no impact on tectonic activity, other than maintaining a certain minimum temperature that acts as a buffer in slowing the transfer of atmospheric heat going into deep space. I am not going to talk about size for the other posters did a good job of doing that.

Answer 3

Have you ever seen a lava lamp? The viscous material in the cylinder moves due to convection within the material. Convection currents are set up because of uneven heating of the material causing the warmer, hence lighter, material to risewhile it is replaced by cooler material from above.

The mantle of the earth has similar convection currents. However because the mantle substance (really molten rock or magma) is much more viscous these convection currents are much slower. You are correct in that the heat engine for this activity is caused by radioactive decay within the core. However gravitational tides caused by the moon may also contribute to this heat engine. The plates that contain the continents as well as the ocean floors float on this viscous mantle and are driven by the convection currents. Adding to this is the fact that at the mid-ocean ridges new magma is constantly extruded out which also helps to move the plates out and away from the mid-ocean ridges. Where two plates contact each other one finds the seismic active zones characterized by volcanoes and earthquakes.

Size is not as important as once thought. Although the Moon and probably Mars are no longer geologically active some Jovian moons have been found to have tectonic activity and this activity is posited to be driven by gravitational tidal forces set up between the many moons and Jupiter itself.

Answer 4

Have a look on Google. I can give you clues, like... Earth forms from a disk of dust Earth was molten The sun started to glow when everything came together under it's own gravity and got hot enough for nuclear reactions.