Category: Geophysics

Advances in seismic tomography of the mantle, greater knowledge of mineralogical phase changes right down to its base and modelling of processes within the core have revolutionised ideas on the physical aspects of deep mantle processes that contribute to convection and magmatism. The thermal features of the deep Earth are of crucial importance, so it is excellent to see a timely review of how heat moves at and around the core-mantle boundary (CMB) (Lay, T. et al. 2008. Core-mantle boundary heat flow. Nature Geoscience, v. 1, p. 25-32). The review gives a readable means of catching up with developments, using simple and not too speculative diagrams. You can find plenty about temperatures and physical properties at the CMB, the various contributions to heat flow and their magnitudes, and the significance of the newly discovered transformation of the deep mantle ‘catch-all’ mineral perovskite to another phase, post-perovskite. Heat that flows from the core into the lower mantle, as much as a third of the total current surface flux of about 45 terawatts, must make a profound contribution to convection in the core and thus to the geomagnetic dynamo. But there is a temperature contrast at the CMB of 500 to 1800 degrees that surely must affect physical processes in the deepest mantle, such as the initiation of mantle plumes. A puzzling new discovery is of ultra-low seismic velocities in the bottom few tens of kilometres of mantle, which  Thorne Lay, John Herlund and Bruce Buffett discuss. Finally, the whole of Earth history encapsulates the evolution of heat flow, which underpins the dynamics of our planet. The historically complex interplay between evolving sources of heat – inherited from Earth accretion and Moon formation; radiogenic sources, and physical and chemical phenomena that are played out as the core evolves – should be curricular issues for all Earth scientists.

Judging by the growing procession of research grant proposals aimed at studying the inner workings of the Earth’s core through computer modelling, it would be easy to assume that a major breakthrough was just over the horizon. What you need is some kind of supercomputer to handle the massive complexity of core fluid dynamics and then channel that through one of several concepts of a geodynamo, first towards simulating the present field and then to how the geomagnetic field swirls and occasionally flips. The fourth biggest there is belongs to the Japanese geophysical community; the Earth Simulator, which is certainly well ahead, in terms of power and speed, of facilities available to less endowed scientists. Recently, about 10% of its power was let loose for a 9 month modelling run that focussed on complex motion in the liquid outer core that theory should generate (Takahashi, F. et al. 2005. Simulations of a quasi-Taylor state geomagnetic field including polarity reversals on the Earth Simulator. Science, v. 309, p. 459-461). Hitherto, modelling had produced pictures of varying magnetic intensity that bore some resemblance to the real magnetic field at the Earth’s surface, and did indeed come up with reversals. Yet a variety of models all produced similarly plausible patterns in space and time. The snag was the limit to matching the viscosity of liquid iron with spin rate. Geomagnetists suspect that the Ekman number, which represents that relationship, is very low in the Earth’s core, i.e. there is very low drag in core circulation, and that adds to complexity. Until the Earth Simulator was built, no power on Earth could deal with the high spatial resolution needed to simulate properly motions at low Ekman numbers. Takahashi and colleagues were able to drop the Ekman number 10 times below any previous simulation.

Real-looking features did begin to emerge in the time sequence for the field at the core’s surface. The most interesting was the formation of zones of opposed polarity at high latitudes, soon (in about 1000 years of simulated time) to be followed by a reversal. The zones move progressively polewards to coalesce, when the overall magnetic polarity all but disappears, and then a reversed field becomes established. However, this is not real but a model dependant phenomenon, even though it is possible to see patterns akin to those observed today – many geophysicists believe the Earth is on a magnetic cusp before a reversal. Will it ever be real is an obvious question, in the same way that related climate simulations may flatter to deceive. The problem is not a lack of models, nor conceivably computing power, but a lack of real data. The ocean floor contains masses of information on past reversals, and cunning analyses of palaeomagnetism in lavas that cooled slowly through the Curie point at the time of a reversal show astonishing things that happened. Excellent maps of the modern field are available, but reality in a reversal is a time series of that mapped field. Without such data, and the time to collect it (the modelling simulates evolution over 5200 years) before the next order-of-magnitude jump in computing power (perhaps 10 years off), it is very difficult to see a justification for this kind of modelling, as opposed to that for climate, which does have a more rapid response time.

See also: Kerr, R.A. 2005. Threshold crossed on the way to a geodynamo in a computer. Science, v. 309, p. 364-365.

While most geoscientists are well aware that past changes in the geomagnetic field are useful as a means of timing sea-floor spreading and stratigraphic correlation, and that records of the direction of palaeomagnetism are keys to ancient plate movements.  Most, however, understand only vaguely why Earth has a magnetic field that flips polarity from time to time: there is some kind of self-sustaining dynamo due to motion in the liquid-metal outer core.  That aspect of geomagnetism involves tough theory and maths.  So for Scientific American to present an up-to-date review of how that dynamo might work is both surprising and welcome (Glatzmaier, G.A. & Olson, P. 2005.  Probing the geodynamo.  Scientific American April 2005, p. 33-39).  The review covers what is currently known about convective motion in the outer core, both laminar and turbulent, and how the simpler laminar convection has been used in computer modelling that simulates how the geodynamo works.  It is complex even at that level of simplification, because thermal convection is affected by the Coriolis effect: much like that in the atmosphere.  Even though the idea of a dynamo inducing magnetic flux is a basic principle of physics, one based on fluid circulation is in constant motion and change.  Surface monitoring of shifts in the magnetic field help chart that aspect.  The issue of reversal is, literally, the knottiest problem for geomagnetists, and they have to resort to the old idea of lines of flux and the effect of contortions by motion at the core-mantle boundary to grapple with how polarity flips might occur.  Computer simulations show the development of what can only be described as chaos in the geomagnetic field at the core-mantle boundary, and much smoothed, but nonetheless odd variability at the surface, as the poles prepare to reverse.  For a period of around 6 000 years the field wobbles like a massive jelly as it lurches across the planet, sometimes splitting into several “blobs” of different polarity.  Eventually it settles down into its new configuration.  To some extent this strange behaviour is matched by what little is known in detail about the progress of reversals from the geological record (see Magnetic polarity reversals in May 2004 issue of EPN).

Over the last 150 years, the Earth’s dipolar magnetic field has been declining so fast that it will vanish in around a thousand years.  Breakdown of the dipole is known to have characterized past reversals in magnetic polarity, together with a decrease in the field to very low values. That is a worrying prospect, because the strength and polarity of the Earth’s magnetic field serves to deflect the flux of energetic particles from the Sun, which would otherwise bombard the surface with potentially disastrous effects.

The likely source of planetary magnetic fields is turbulent circulation of a liquid iron core.  Movement of such an electrical conductor is bound to generate such a field, in the manner of a self-sustaining dynamo – movement of a conductor in a magnetic field that the motion itself generates results in current flow that sustains the magnetic field. Perturbation of core motion would give rise to continual deviations from a perfect dipole.  Charting such deviations is therefore a means of sensing how the core’s circulation behaves.  There have been two satellites devoted to monitoring the global magnetic field – The US Magsat in 1978 to 1980 and the Danish Oersted launched in 2000.  Comparing results from the two reveals a remarkable patchiness, the largest being one to the south of Africa in which the field points downwards, opposite to the upward-pointing field of the main dipolar field in the southern hemisphere (Hulot, G. et al. 2002.  Small-scale structure of the geodynamo inferred from Oersted and Magsat satellite data.  Nature, v. 416, p. 620-623).  The Earth contains “anti-dynamos”, and if they merged and grew, the overall polarity might flip.  Not only that, but for a while at least the poles of the reversed state need not line up with the rotational axis.

Gauthier Hulot of the Institut de Physique du Globe de Paris, with French and Danish colleagues, have modelled the generalized magnetic maps as proxies for core circulation.  The dominant features, other than a slow westward drift, are probably vortices close to the rotational poles, akin to those induced in the atmosphere by large-scale variations in air temperature.  But there are asymmetries, of which that south of Africa is the largest..  They too are probably vortices, perhaps related to convection columns.  Those showing a likely fluid motion linked to the Earth’s rotation cluster beneath the Pacific, whereas counter flows dominate the hemisphere centred on the Atlantic.

See also:  Olson, P. 2002.  The disappearing dipole.  Nature, v. 416, p. 591-594.

New Scientist’s excellent Inside Science pull-out series now includes one on the Earth’s core (Bowler, S. 2000.  Journey to the Centre of the Earth.  Inside Science #134,  New Scientist 14 October 2000).  This covers the origin and evolution of the core, how geologists can assess its composition and structure, and the link between motions in the core and the fluctuations in the Earth’s magnetic field.  Like all the Inside Science pull-outs, Sue Bowler’s treatment is at a level easily followed by non-Earth scientists but nonetheless informative and up to date.