Education and Careers

From Ms Judy Judd (October 2009)

Reply by Dr Joe McCall

This is a very topical question as Professor S.R.Taylor and S.R.McLennan have bravely issued a book on 'Planetary Crusts' (Cambridge UP 2008), which I have reviewed in Geoscientist 19 (9). Their conclusion was that no other planet in the solar system nor our Moon ever experienced plate tectonics. They are all uniplate planets.

Mercury we know rather little about, no lander has ever been sent there, and the nature of the rocks is unknown: its extraordinary large density makes it anomalous; there is nothing in the imagery obtained by probes tha suggests plate tectonics, though it is now known, from imagery of greater detail to have volcanic features. Venus, has lately been suggested as having once had continents, plate tectonics and oceans, on the basis of infrared spectrometry and heat measurements on the southern highhlands (see Geonews item in the forthcoming January issue of Geoscientist), but this appears to be far-fetched, as it lost its hydrogen early because it has no magnetic shield and is bone dry, and water is essential for plate tectonics to operate. It also has a very low speed of rotation (1 V day = 243 E days) and this would inhibit an Earth-like dynamo in the liquid-solid metal core (though the rotation might have been faster in the past). The Moon certainly has never experienced plate tectonics. Mars is small, very cold and comparatively dry, and what water was there was almost certainly driven underground: I do not believe in the idea of big surface oceans on Mars, though some do! It has revealed a pattern of magnetic stripes, indicating a single reversal of polarity, and this has been seized on to suppose early plate tectonics, but this ignores the fact that that there is no evidence of continents ever existing there, and plate tectonics is initiated by a split in a supercontinent, ridge basaltic volcanism and spreading coupled with mantle convection. I have argued that in no way can these stripes reflect plate tectonics.

On the gas giants Jupiter and Saturn, and the ice giants Uranus and Neptune, there is no evidence of plate tectonics and the question hardly arises? I have in an article in the press suggested that plate tectonics on the Earth did not initiate until the end of the Archaean ~2500 Myr ago. The Earth likely was a uniplate planet up to that time, with a sialic (continental) crust and only shallow oceans, the site of greenstone belt eruptivity, dating back possibly to beyond 4000 myr. The delay in the appearance of plate tectonics was I believe because the crust had not cooled enough for the separation to occur on very deep fractures, ridges to form and the pieces of the continental crust to tear apart and move horizontally coupled with the mantle convection. There were no deep oceans before ~2500 Myr? I have worked on both the Archaean rocks and a major plate convergence zone in Iran, hence my interest in the question.