Setting up subduction

Although they have roughly the same size and overall density, and probably very similar bulk compositions, Earth and Venus behave in very different ways.  The Earth has plate tectonics, whereas radar images how that Venus has no such phenomenon.  For the most part, Earth loses its internal heat production steadily and plate movements are intimately bound up with that generalised convective heat transfer.  The surface of Venus has seen no significant deformation in half a billion years.  In fact, that surface was probably formed by a massive blurt of magma around late Cambrian times.  In some respects that is similar to the roughly 30 Ma appearance of flood-basalt volcanism on Earth, but on a scale that dwarfs large igneous provinces such as the Deccan and Siberian Traps.  Quite probably, Venus builds up thermal energy in its mantle, until its release by massive partial melting.  The key to Earth’s behaviour seems to be the fact that its oceanic lithosphere is able to break and descend into the mantle.  The gravitational force down a subduction zone is sufficient to keep plate tectonics going.  But why does it start?  Oceanic lithosphere is as strong as that beneath continents, and the other main force involved in plate tectonics, due to the gravitational effect of deepening sea floor as it cools away from constructive margins, is so low that it is unlikely to result in lithospheric failure.  This vital, but often overlooked topic is nicely reviewed by Stephen Battersby, a consultant to New Scientist (Battersby, S. 2003.  Eat your crusts.  New Scientist, 30 August 2003, p. 30-33).

A possible explanation lies in the way in which the strength of the main mantle mineral, olivine, varies with the presence of water.  Even minute amounts of water allow hydrogen ions to enter the olivine molecular lattice, thereby creating defects that can migrate and result in softening of the mineral.  Experimental deformation under mantle conditions, carried out at the University of Minnesota, show ten-fold decrease in olivine’s strength with as little as 20 parts per million of available water.  Subduction at continental margins might therefore be set in motion by the weight of sediments accumulating on the ocean floor, and with time that weight increases as the continents are eroded.  The other factor, perhaps bearing on the start of intra-oceanic subduction that forms island arcs, is the effect of transform faults and fracture zones that separate segments of different age and therefore density.  Maybe that sets up forces that stress the oceanic lithosphere.  The big problem is that the bulk of the oceanic lithosphere, is mantle rock, and when it has been left as a residue by the basalt melting at constructive margins, it is well-nigh anhydrous.  To soften it demands a source of water that permeates the peridotite.  An obvious source is seawater penetration, but at the depths involved any pathways seal up tightly.  Possibly there are wet masses in the deeper mantle, either as a result of earlier subduction or dating back to Earth’s origin.  Slow convection in the deep mantle could bring these into contact with the base of the oceanic lithosphere, where their water could permeate and weaken it to the point of failure.  Just an idea, maybe.  However, seismic tomography, so effective at charting the distribution of hot and cold (low- and high-velocity) mantle rocks, is also able to suggest places where damp, weak rock occurs in the deep mantle.  One such low-velocity blob occurs beneath the eastern seaboard of North America (maybe a relic of the Palaeozoic Iapetus subduction zone that runs parallel to the present margin), where there is, as yet, no sign of subduction.  But there is little sign that the blob is abnormally hot, and in all probability it is damp.  The history of tectonics suggests that no ocean remains with passive margins forever, and inevitably subduction ends up devouring it, in 200 Ma at most (the greatest age of today’s ocean floor).  Given time the eastern USA  may rank with the Andes!

So why does Venus behave so differently?  Although we cannot yet analyse any Venus rock (there are no accredited Venusian meteorites!) there is a plausible scenario.  Venus is the greenhouse planet.  It is highly unlikely that it ever harboured life, particularly of a photosynthetic kind which could have produced free oxygen.  In the Earth’s atmosphere, it is the presence of ozone in the stratosphere that gives the atmosphere its peculiar thermal structure, especially the tropopause.  That marks a sudden cooling that limits the height to which water vapour can rise before freezing out.  In the stratosphere temperature warms up with height, due to the minor “greenhouse” effect of ozone.  Venus probably never has a tropopause, so that clouds of water vapour could rise to the outer limits of the atmosphere warmed by high CO­2 levels.  In contact with ultraviolet light, water dissociates to hydrogen and oxygen, and at high levels the hydrogen leaks away to space.  Any oxygen is quickly drawn down by oxidation of iron at its surface.  So Venus has progressively lost all its water and as a result is a tough nut to crack, as regards forces in its interior.  Earth on the other hand is a bit like a fondant chocolate…

Wandering hot spots

It was once an axiom of plate tectonics that volcanic-island and seamount chains provided robust evidence for sea-floor spreading.  Jason Morgan in 1971 developed the notion, based on a pre-plate tectonic idea by John Tuzo Wilson, that within-plate oceanic volcanic islands derived their magma from upward moving plumes in the mantle below the lithosphere.  Many of them in the Pacific have extinct volcanic islands and seamounts arranged in straight chains that parallel the direction of sea-floor spreading shown by magnetic stripes.  He likened their formation to the burn mark on a sheet of paper passed slowly over a candle flame.  The Hawaii-Emperor chain bucks this hypothesis, by being profoundly bent from a WNW trend in its youngest part to north for ages greater than about 50 Ma.  The problem is that neither leg is at right angles to the magnetic stripes, which does rather suggest that hot spots move.  Hot spots have long been used as a frame of reference for absolute plate motions, but if one has moved then so might all the rest, and how they have moved would probably be independently of one another.  Absolute motions then are hard to judge.  The key to checking on the suspected hot-spot drift is to look at the palaeolatitude of differently aged volcanic rock samples along a chain.  This has been achieved using palaeomagnetic measurements from the S-N Emperor chain (Tarduno, J.A. et al. 2003.  The Emperor seamounts: southward motion of the Hawaiian hotspot plume in Earth’s mantle.  Science, v. 301, p. 1064-1069).  The test proved positive; the hotspot itself moved southwards between 81 to 47 Ma, while the Pacific plate was itself moving.  Other tests suggest that hotspots in the Indian and Atlantic Oceans were indeed fixed for long periods, but the Pacific ones seem to have had a tendency to wander.  Why that has happened is possibly connected to deep mantle flow, which might bend the plumes to which the hot spots owe their magmatic activity.  Maybe their source region in the mantle shifts for entirely different reasons.  Seismic tomography of the mantle has had some success in tracking the shapes of plumes, but not for relatively small ones because of its present poor resolution.  One large plume that has an enormous tilt in the vertical dimension starts near the core-mantle boundary beneath the South Atlantic and hits the lithosphere in the Red Sea.  No-one knows why, but its magmatic expression in the volcanic rocks of east Africa suggest that it too has moved from beneath Kenya about 50 Ma ago, across Ethiopia to its present position that fuels active volcanoes in the Afar Depression of NE Ethiopia, Djibouti and Eritrea.

See also: Stock, J. 2003.  Hotspots come unstuck.  Science, v. 301, p. 1059-1060.

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