Why Plate Tectonics?

How did plate tectonics start? Why has Earth got them and none of the rocky planets have, at least as far as we know? In my ebook “Planetary Formation and Biogenesis” my explanation as to one of the reasons for why plate tectonics are absent on Mars is that the Martian basaltic mantle appears to have about 17% iron oxide whle Earth has 7 – 11%. This means it cannot make eclogite whereas Earth’s basalt can. Eclogite is a particularly dense silicate and it is only made under serious pressure. 

To see the significance, we have to ask ourselves how plate tectonics works. The core generates hot spots, and hotter mantle material rises and has to push aside other rock, and we get what we call seafloor spreading, although it does not have to be underwater. The African rift valley is an example, in this case a relatively new example where the African plate is dividing, and eventually will have sea between Somalia and the Nubian zone. Similarly, the Icelandic volcanoes are due to “seafloor spreading”. Thus matter coming up pushes the surface plates aside, but then what? On Mars, the cold basalt has nowhere to go so it forms what is called a “stagnant lid”, and heat can only escape through volcanism. On Mars, this resulted in quite significant volcanism about three and a half billion years ago, then this more or less stopped, although not as much as some think because there is evidence of volcanic eruptions around Elysium within the last two million years. The net result is the “lid” gradually gets thicker, and stronger, which means the heat loss of the Martian mantle is actually much less than that of Earth.

On Earth, what happens is that as the basaltic plates get pushed aside, one goes under another, and this is where then eclogite becomes relevant. As the plate goes down, the increased pressure causes the basalt to form eclogite, and because it is denser than its surroundings, gravity makes it go deeper. It is this pull subduction that keeps plate tectonics going.

So, what about Venus? The usual answer is that Venus had a stagnant lid, but at certain intervals the internal heat is so great there is a general overturn and there is a general resurfacing. However, maybe that is not exactly correct. Our problem with Venus is we cannot see the surface thanks to the clouds. The best we can manage is through radar, and recent (June, 2021) information has provided some surprises (Byrne, et al.,   https://doi.org/10.1073/pnas.2025919118).  Basically, what was found was evidence that many of the lowlands had broken into crustal blocks and these blocks are moving relative to each other, in the same way as pack ice moves. The cause would be mantle convection that stresses the crust. The Venusian crust has many landforms, including thin belts where crust has been pushed together to form ridges, or pulled apart to form troughs. However, these ones tend to encompass low-lying regions that are not deformed, but rather appear to be individual blocks that shift, rotate and slide past each other. The authors suggest this what Earth was like before plate tectonics got going.

As to why they started here and not there has no obvious answer. The fact that Earth rotates far more quickly will generate much stronger Coriolis forces. It may be that the absence of water on Venus removes a potential lubricant, but that seems unlikely if blocks of crust are moving. My personal view is that one key point is it needs something to force the crust downwards. Eclogite may pull it down, but something has to push the basalt down to force it to make eclogite. My guess here is that Earth has one thing the other rocky planets do not have: granitic continents. Granite floats on basalt, so if a basaltic mass was pushed against a significant granitic mass, the granite would slide over the top and its weight would push the basalt down. When it made eclogite, the denser basalt would continue its downward motion, pulling a plate with it. Is that right? Who knows, but at least it looks plausible to me.

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