# What properties must a rocky body possess in order to exhibit plate tectonics?

The Earth exhibits plate tectonics, but the other terrestrial planets do not (though Mars and Venus may have exhibited plate tectonics in the past).

What is "special" about Earth that allows it to have plate tectonics? Equivalently, what conditions are necessary for a rocky body to have plate-tectonic activity?

(I assume that papers like (1) and (2) are good surveys of the topic, but I lack the background to synthesize the information in these papers into a good answer.)

(1) Paula Martin et al 2008 Phys. Educ. 43 144 doi:10.1088/0031-9120/43/2/002

(2) C. O'Neill et al 2007 Earth and Planetary Science Letters 261 1-2 doi:10.1016/j.epsl.2007.05.038

Plate tectonics appears to require multiple factors all arraigned in feedback loops.

• The planet must be large enough that it's surface to volume ratios is low enough to trap enough heat from radioactive decay to power tectonic motion. Convection requires a minimal threshold of energy trapped in the material before that trapped energy causes mechanical motion. Planets smaller than earth radiate internal heat away to quickly because of their larger surface to volume ratios. Thus little convection and little tectonic motion.

• The planet must have sufficient mass such that it's escape velocity exceeds that of a hydrogen ion ejected from a water molecule via UV hydrolysis. If the planet does have sufficient mass, then the hydrogen release by UV hydrolysis will be recaptured. If not, the hydrogen will escape the planet and its orbit and the planet will eventually desiccate. Earth has enough mass to retain hydrogen, Mars and Venus do not. In the past, Mars had water but it leaked hydrogen over the eons until it became a desert. Venus likely suffered the same fate.

• Oxygen producing life: Even with sufficient mass to trap hydrogen, a planet will still leak free hydrogen from having the hydrogen rise to the top of the atmosphere and there being knocked off by the solar wind. Free oxygen prevents this by a) forming ozone thus blocking the UV that breaks down water molecules and b) recombining with the free hydrogen that does form to produce water again.

• The presence of large amounts of water produce hydrated minerals which, on the size and time scales of tectonic motion, behave more like a deformable putty than solid rock. These deformable minerals allow tectonic motion. Without them, tectonic forces would cause shattering instead of drift. (See: Plate Tectonics and Water, and, Role of water in the tectonics of Earth and Venus.)

• Tectonic subduction recycles carbon trapped in carbonates by chemical weathering and organisms in the ocean. Subduction draws the carbonates down to the lower crust where they are "cooked" and the carbon converted to $\mathrm{CO}_2$ and $\mathrm{CH}_3$ which are returned to the biosphere via vulcanism. Without this recycling, all the carbon in the biosphere would end up as limestone on the ocean floor. Starved of carbon, all life would perish.

• It appears that the Earth has a much thinner and much more heterogeneous crust than either Mars or Venus. Likely, this is a result of the collision that produced the moon. The thinner crust is easier for geothermal convection to move. Also, it's possible that the collision "stirred" the distribution of heat producing radioisotopes leaving more near to the top of the mantel. On Venus and Mars, it is likely that the very heavy radio isotopes sank to the core. This would make the upper mantel on Earth hotter than the other planet and hotter than it would have been without the collision. That would drive convection faster.

Likely, the critical difference for Earth was the collision that formed the moon. That both "stirred the pot" and added more mass. Earth retains both heat and hydrogen. Retaining hydrogen means retaining water which means life and hydrated minerals. Life produces oxygen that protects the water. Hydrated minerals provide the elasticity needed for tectonic motion. Tectonic motion recycles carbon needed for life.

Take away anyone of these factors and a planets tectonic motion will eventually wind down in less than a billion years.

• -1. This answer is reasoning from a small sample size and makes a number of false claims. Many of the bullet points are either wrong or irrelevant. The Earth does lose hydrogen, currently about 3 kg/second, due to photodissociation in the upper atmosphere. The primary reason it's not more than that is because the stratosphere is currently so very dry. This may not have been the case long, long ago and most likely will not be the case a billion years from now. With regard to oxygenation, most geologists place the onset of plate tectonics well before oxygenation of the atmosphere. – David Hammen Oct 15 '14 at 15:08
• Would you consider marking the problematic claims as speculation in the answer or removing the entirely wrong ones? I would do it according your comment, but I don't have the reputation for editing. Otherwise thank you for your comment, I am very interested in this topic. – Irigi Oct 17 '14 at 21:22
• @Irigi - It's OK to edit an answer to fix typos, broken links, spelling errors, bad grammar, etc. It's not OK to change the meaning of an answer. Don't fix wrong answers. It's far better to write an alternative answer that's correct. – David Hammen Oct 18 '14 at 11:38
• @David Thank you, I didn't know. – Irigi Oct 18 '14 at 11:46
• @ David Hammen- Small sample size? Really? Thanks of the laugh. I'm working on my space probe in the back yard but until then we're stuck with the one sample. The earth does looses hydrogen largely from solar wind knock off, not escape velocity ejection by bond breaking as does Venus and Mars, although it can if there is enough UV for a long enough time, but there isn't because of the oxygen which forms ozone. I can dig up the numbers if you insist. – TechZen Oct 21 '14 at 20:02

A basic and oversimplified answer would be that Earth has the right amount of heat, both to begin with and a continued supply from radioactive elements. Venus has too much (surface temp. is around 750 K) and therefore the planet doesn't have the same stratification (i.e. cold-hard crust, mantle, core) as Earth. Mars on the other hand it too cold. It doesn't have the same size as Earth resulting in less thermal energy which is the driving force behind plate tectonics. This lack of heat results in very little force trying to move the crust around.

The answer to what makes Earth so special is really it has the 'right' amount of heat. Keeping in mind that the 'goal' is to get rid of the heat and plate tectonics and all that it entails (e.g. mantle plumes, mid-ocean rifts, volcanoes) are ways to release that heat from the Earth. Eventually all these mechanisms will slow down and stop and Earth will be in a similar state to Mars.

As for your main question, I am no physicist, but I would speculate that formation by acceration would be required so that the body was stratified into some form of cold-hard crust, mantle, and core, and there would be some minimum limits on the size of the body (possible maximum limits but I have no idea about that) because anything too small would solidify pretty quickly (or just become a blob of molten magma, would depend on how hot it was and what kind of environment it was in i.e. has an atmosphere or no atmosphere). But as I previously stated the key factors would be the right amount of initial heat (from the accreation of the body) and the correct amount of insulation. Too much of both and you get Venus, too little Mars.

As a side point this article makes the good point that it does depend on what you define as plate tectonics and what we have on Earth may not be the norm... And who knows Venus might have cooled down enough in a few hundred million years that it present Earth-like plate tectonics.