Plate tectonics’ basic tenet is that discrete segments of the lithosphere behave as rigid bodies, whose motion is accommodated by extensional, overriding and strike-slip faulting at equally discrete boundaries. That is true to a first approximation for the parts of plates made up from oceanic lithosphere, which is rheologically strong because of its mineralogical composition. Continental lithosphere is weakened by its quartz-rich crust, which tends to behave plastically at high temperatures deep within it. So it is no surprise that opposed motions of plates induce large-scale shortening and thickening of continental lithosphere that they carry, but there are no orogens in the ocean basins. The largest site of active continental shortening and thickening is, of course, the Alpine-Carpathian-Himalayan orogen. The Tibetan Plateau is underpinned by continental crust that is in the process of being thickened as India drives north-eastwards into Asia, at about 4 to 5 cm per year. Consequently it the largest area of high-elevations on the planet. In the 1973 John Dewey and Kevin Burke speculated that forces involved in continent-continent collisions with irregular margins might expel thickened continental lithosphere sideways, at right angles to the opposed plate motions. Peter Molnar of the University of Colorado in Boulder and Paul Tapponnier of the Institute of Global Physics in Paris applied this on a grand scale to the neotectonics of the Tibetan Plateau and East Asia in 1975. They considered that south-eastward expulsion was channelled by the many enormous strike-slip faults in the region. In a sense, this notion considers the continental tectonics to be akin to the rigid-body behaviour of oceanic parts of plates. If the overall motions involving Tibet and continental lithosphere to the east was dominated by plastic deformation in the deep crust and mantle, the motion would be taken up by a host of smaller faults in the brittle upper crust. Geodetic measurements using GPS over the last 17 years do conflict with the movement of discrete blocks of East Asian crust (see Quantifying motions inside continents, March 2004 EPN). Two papers published in July 2004 also lean towards plastic behaviour of the bulk continental lithosphere. One uses data from surface seismic waves to show about 30% ductile thinning in the middle and lower crust beneath Tibet (Shapiro, N.M. et al. 2004. Thinning and flow of Tibetan crust constrained by seismic anisotropy. Science, v. 305, p. 233-236). The other is based on interferometric analysis of radar data from satellites, which involves measuring signal differences between radar data captured on different dates, in this case between 1992 and 1999 (Wright, T.J. et al. 2004. InSAR observations of low slip rates on the major faults of western Tibet. Science, v. 305, p. 236-239). The technique has mainly been used to look for vertical displacements associated with earthquakes and volcanoes. By eliminating the effects on signals by terrain, using an accurate digital elevation model, InSAR results can estimate the motion of the surface along and opposite to the illumination direction of the radar pulses, thereby detecting horizontal ground movements over a period of several years with sub-centimetre precision. Rather than revealing large movements in the two opposed directions that are expected on either side of large strike-slip faults, such as the Karakorum and Altyn Tagh Faults, there was none. In a zone crossing western Tibet from NNE to SSW, much of the orogen appears to be moving slowly eastwards, irrespective of the large faults. Tapponnier still maintains the importance of the big faults, and perhaps the InSAR survey coincided with a period of tectonic quiescence.
Early Earth’s Nemesis
William Hartmann’s proposal that, shortly after it formed, the Earth suffered impact by a planet about as big as Mars has become a central feature on ideas about our planet’s evolution and the origin of the Moon. The problem with the theory is a conundrum that lies in quite esoteric geochemistry. Studies of meteorites show that the oxygen isotopes in them vary considerably, and that almost certainly resulted from their forming at varying distances from the Sun where fractionation among oxygen’s stable isotopes had different effects on their proportions. So it is possible to judge the original orbits in the solar system of meteorites’ parent bodies. Martian meteorites are identified on this basis. The difficulty with Hartmann’s idea is that rocks from the Earth and Moon have nearly identical oxygen isotope proportions. There seems no way that an errant planet that crashed into the Earth could not have left its mark in oxygen isotopes, particularly in those of the Moon, for debris flung off from the Earth would have mixed with that from the colliding body. It turns out that there is a possible explanation (Chown, M. 2004. The planet that stalked the Earth. New Scientist, 14 August 2004, p. 26-30). The Earth’s orbit could have involved the accretion of more than one planet from interstellar dust. This can happen once a planet has grown until it has sufficient gravitational potential to interact with solar gravity. The result is a series of points in the orbit (Lagrange points) where the two gravitational fields exactly balance. Matter that drifts into Lagrange points accumulates there rather than being swept up by the growing, larger planet. So considerable mass can build up, even enough to make a small companion planet. While all the main planets were growing, gravitational fields were continually changing, so the Lagrange points would not remain as stable as they are today. A small planet formed at one of them would begin to move erratically within the Earth’s orbit. Eventually it would be caught up by mutual attraction between the two, and then would collide with the Earth, but not at immense speed. So far, the hypothesis based on complex modelling of Lagrange accretion seems plausible. Geochemists will be pleased because it resolves their fundamental conundrum about the similar chemistries of the Earth and its Moon.
Uranium in the core?
The constant, but complex circulation in the Earth’s liquid outer core almost certainly results in the self-exciting dynamo believed to be responsible for the geomagnetic field and its periodic reversals in polarity. If an electrical conductor moves in a magnetic field a current is generated in it, which in turn creates a magnetic field, hence self-excitation. The outer core’s convective motion requires a heat supply of some kind. There are three general possibilities: the heat is left over from the Earth’s energy of accretion; it is generated from latent heat released as the solid inner core grows slowly by crystallization of iron-nickel alloy; or there is significant radioactive decay in the core. Compared with estimates for the Earth’s overall radioactive heat production, based on the composition of the primitive meteorites (ordinary chondrites) from which it is thought to have formed, there is excess heat flowing through the surface. This is believed to emanate from the core. Separating the three possible heat sources is not yet possible, but it is possible to rule the generation of enough to account for excess heat flow by one of the possible mechanisms. If the inner core has been crystallising out since the core formed around 4500 Ma ago, the latent heat being released is too small. Much attention has focussed on a possible radioactive source, for which the unstable natural isotope of potassium (40K) is a plausible candidate. The sulphide phases of metal and chondritic meteorites do contain potassium, so the element has affinities for sulphur as well as its dominant tendency to enter silicate melts and minerals. The core almost certainly contains a sizeable proportion of Fe-Ni sulphides. One geoscientist, Marvin Herndon based in San Diego, California, reckons there is another possibility (Battersby, S. 2004. Fire down below. New Scientist, 7 August 2004 issue, p. 26-29); uranium. To most geochemists, the idea is implausible, because uranium has such a strong affinity for silicates that it ought never to have entered the metallic and sulphide liquids that sank through the early Earth to form the core. Herndon bases his idea on an alternative type of meteorite from which the Earth could have formed by accretion, enstatite chondrites. They have lower oxygen contents than ordinary chondrites, and would have created strongly reducing conditions in the undifferentiated early Earth. Such planetary chemistry, claims Herndon, would induce uranium to enter dense sulphide liquids and the core. This view has not found much support, but experiments in detection of neutrinos and antineutrinos, when they are more efficient than at present, may resolve the issue of radioactivity in the core, because decay of unstable isotopes produces antineutrinos.