Category: Geophysics

Most people are quite content with an annual holiday abroad, yet a number of geoscientists yearn for something more adventurous. The Croatian geophysicist Andrija Mohorovičić was among the first to study estimates of speeds at which seismic waves travelled through the Earth, discovering in 1909 that below a depth of about 30 km below the continental surface they moved faster than in the uppermost layer. He had discovered the boundary between the continental crust and the underlying mantle, a discontinuity that bears his name though often shortened to the ‘Moho’. Having been traced beneath most of the Earth’s surface, a group of American scientists discussed over a drink or three at a ‘wine breakfast’ in 1957 a project to drill through the Moho to find out what the mantle was made of. The brainchild of Harry Hess, one of the first to suggest plate tectonics as a driving mechanism for continental drift, was dubbed Project Mohole. With US government support, a drilling barge designed for offshore oil drilling and a system of thrusters and pre-GPS locational instrumentation to keep the barge on station the Mohole was spudded in 1961 on the seabed near Guadalupe Island off Baha California in Mexico; about the time that John F. Kennedy declared his belief that the USA could land a man on the Moon by the end of the 1960s. There was something of a thrill factor about Project Mohole, and its first attempts were reported in Life Magazine by John Steinbeck, author of The Grapes of Wrath and amateur oceanographer. It turned out that sending a drill bit to the mantle was more difficult than a manned lunar landing. Only a few metres of basaltic crust was recovered and Congress cancelled Mohole funding in 1966. Nevertheless, the project was the forerunner of the highly successful Ocean Drilling Program and its predecessors, probably the most prolific international collaboration of any kind.

Since the 1960s research into the mantle has been continued with great success by looking at upthrust masses such as those in the Alps and in ophiolite complexes, nodules in alkaline basalts and kimberlites that form below 100 km into the mantle, samples dredged from oceanic fracture zones, and indirectly from the geochemistry of basalts that are derived by partial melting of mantle materials. Yet, there is still an air of frustration about some igneous petrologists and geophysicists; they want to touch the real thing! Now, at last, they may have their chance, for improved drilling and positioning technology developed by ODP and the petroleum industry make a hole through the Moho feasible. Indeed one is planned once drill-bits and lubricants suitable for the anticipated temperatures and pressures have been finalised. Three sites are under consideration: near the original Mohole; in the Cocos Plate off Costa Rica and the Pacific Plate near Hawaii, each combining the coolest crust, thinnest sediment cover and shallowest possible water – i.e. just off a mid-ocean ridge or hot-spot. The Costa Rica site (ODP site 1256) has the thinnest crust due to rapid sea-floor spreading by the East Pacific Rise there and is the most likely to be drilled. It already has a core the penetrates to 1.5 km in oceanic crust and a current project aimed at sampling the cumulate gabbro layer of the lower oceanic crust. That will still be 3.5 km above the local Moho.

There is an obvious question; will an ocean-floor site, however favourable, and a hole drilled through it help resolve fundamental issues regarding the mantle? Well, probably for oceanic lithospheric mantle, but that has had basaltic magma removed from it to form the crust above. Also mid-ocean ridge basalts have geochemical features that suggest that their source mantle had been a melt source previously, compared with the source mantle materials for alkaline and some other types of basalt that seem to have been less depleted in certain elements. The most important question posed by the mantle in general concerns how it originally formed during the Earth’s earliest history, accretion of debris from the solar nebula, the moon-forming event and extraction of the metallic core. A Mohole can contribute little to those issues.

Source: Teagle, D.A.H. & Ildefonse B. 2011. Journey to the mantle of the Earth. Nature, v. 471, p. 437-439.

Related articles

• Scientists to Drill Earth’s Mantle, Retrieve First Sample? (news.nationalgeographic.com)
• Drilling into the planet: Why we want to sample the mantle (and why we already have) (bigthink.com)

It used to be a joke in the Geological Surveys of the Soviet Union that they employed so many thousands of geologists that the entire USSR could be mapped in a few years if they all linked hands and walked from east to west. Geophysicists are trying for something similar to map the mantle underlying the USA in 3-D. The USArray involves 400 portable seismometers, currently spread out at 10 km intervals in the western States, is intended to act like a fly’s eye in monitoring arrival times of seismic waves from worldwide earthquakes. The plan is to steadily move the array eastwards until by 2013 it has reached the Atlantic coast. From that data the geophysicist hope vastly to improve the resolution of seismic tomographic images of the deep Earth (see Kerr, R.A. 2009. Scoping out unseen forces shaping North America. Science, v. 325, p. 1620-1621). Yep, they are definitely going for a high ‘Wow factor’ rating. Yet is seems that there are other expletives floating around as the strangely knobbly and discontinuous architecture that is emerging from early data processing refuses to fit many simple hypotheses being tested.

The world-wide network of seismic recording stations was originally set up partly to improve detection of underground nuclear weapons tests. It is the source for the mapping of variations in seismic-wave speeds in the mantle by seismic tomography that is revolutionising ideas about the Earth’s internal dynamics. Nowadays nuclear explosions have been miniaturised so that detecting them and their locations and distinguishing them from small natural earthquakes has become difficult. The growing concerns about nuclear weapons proliferation have spurred an upgrade and expansion of seismic monitoring, and other means of verifying that seismic signals have indeed been produced by underground nuclear explosions, such as sensitive analysis of air sample for isotopes leaking from tests (Clery, D. 2009. Test ban monitoring: no place to hide. Science, v. 325, p. 382-385). If this enhanced source of seismic data is routinely made available to tomography researchers, it should boost resolution of seismic speed anomalies and sharpen up ideas about deep tectonics.

Supershear earthquakes

In an analogous fashion to the sonic booms made by aircraft travelling faster than sound, it seems possible that the rupture of a fault may travel faster than the seismic waves that it generates. Evidence is accumulating that such faults produce the equivalent of a sonic boom (Fisher, R. 2009. Seismic boom. New Scientist, v. 203 (1 August 2009), p. 32-35) despite mathematical suggestions that faults cannot propagate so fast. Experiments show that there is a seismic equivalent of the Mach fronts associated with sonic booms, and they amplify the shock of earthquakes that produce them. High amplitude at the Mach front causes it to travel further away from a fault line than normal seismic surface waves – those that cause most damage, and it also gives rise to ground motions different from those normally linked with earthquakes: more like a hammer blow than shaking. The net conclusion is that these ‘supershear’ earthquakes may pose hazards beyond those involved in risk assessment near active fault zones. Field evidence for supershear events are signs of disturbance by recent earthquakes that are further from an active fault zone than existing theory predicts. So far such evidence has only turned up along active strike-slip faults on continents, such as the Kun Lun Fault in Tibet and the North Anatolian Fault in Turkey. Yet, these form the longest seismically active zones, including the infamous San Andreas Fault in California.

Global warming is intensifying cyclonic storm systems, the energy retained by the greenhouse effect being redistributed to winds and in turn to ocean waves, which even have a small effect on local gravitational potential. The effects become coupled to the solid Earth and appear as the background ‘noise’ in seismograms. So historic seismograms, both digital and in paper form, potentially supply a proxy for climate change going back as far as the 1930s when seismographic stations first began to be set up. In some instances the records are continuous, and when digitised form a unique record that integrates, but one yet to be exploited fully (Bromirski, P.D. 2009. Earth vibrations. Science, v. 324, p. 1026-1027.

For some time relics of the Farallon plate that was subducted beneath North America during its late Mesozoic and Cenozoic westward drift have been known from seismic tomography, but only in a blurred form. Advances in computation from many seismic records are steadily improving the resolution of this revolutionary technique, and a more finely tuned picture of the mantle beneath the North American continent has now emerged (Sigloch, K. et al. 2008. Two stage subduction history under North America inferred from multiple-frequency tomography. Nature Geoscience, v. 1, p. 458-462). The American-German-French team reveal several pieces of the ‘lost’ plate in an astonishingly complex 3-D representation of the North American mantle down to 1800 km. There are two main blocks: one still active and connected to the active subduction zone between British Columbia and northern California that dips steeply to about 1500 km depth, the other inactive and stranded beneath the eastern part of the continent. The authors believe that the two separated around the end of the Mesozoic. They suggest that the break coincided with the within-plate deformation and volcanism known as the Laramide era that lasted from 70-50 Ma, which probably coincided with low-angled subduction of the Farallon plate. After the break, the flat subduction ‘rolled-back’ westwards, leaving a track on volcanism across the western part of the continent. The authors also ponder on the relationship between the changed style of subduction and the thermal event that produced the Columbia River continental flood basalt event at 17 Ma.

Geomagnetic cows

Unless you are a committed ‘towny’, you may have noticed that livestock tend to face in the same direction when feeding and lying down; so much so that a herd of grazing cows can resemble a collective harvesting machine. However, few of us country folk have bothered to see if the direction in which they face varies from day to day. In fact it does; but only a bit. Thanks to the high-resolution images provided by Google Earth, a group of German and Czech scientists have measure the alignment of almost 3000 cows and wild deer that show up on images of 241 localities on 6 continents (Begall, S. et al. 2008. Magnetic alignment in grazing and resting cattle and deer. Proceedings of the National Academy of Sciences, v. 105, p. 13451–13455). In all the populations the animals roughly align themselves north-south. More to the point, they line up parallel to the local lines of magnetic force with a remarkable degree of consistency.

Now, this is not a study aimed at the annual IgNoble Awards, but a cunning check on whether herding animals have some kind of built in compass akin to those in birds. That would have an evolutionary advantage in seasonal migration – domestic cows are derived from wild bovids of the Pleistocene temperate grassland plains. I have a made a quick check of some local cattle and sheep, again using Google Earth, and I can’t say that I am convinced. But the study is based on statistical analysis of rose diagrams of the long axes of cattle, so there may be a tendency for poleward pointing. However, the herds and flock that I examined may be independent minded beasts. Yet, if Begall et al.’s stats are correct, then geophysicists have perhaps a new means of exploration for local distortions in the magnetic field as might happen near magnetite ores; incidentally sometimes rich sources of vanadium. The method may delay disoriented ramblers lacking compass or GPS receiver, and might place them at some risk. Frankly, they would be better off looking for which side of trees the moss grows on…

See also: Callaway, E. 2008. Magnetic cows in mystery alignment. New Scientist, v. 199 30 August 2008 issue, p. 10.

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.