Category Archives: Tectonics

Ancient oceanic lithosphere beneath the eastern Mediterranean

The extensive active subduction zones around the Pacific ocean are responsible for a dearth of oceanic lithosphere older than about 200 Ma that still remains where it formed. Trying to get an idea of pre-Mesozoic ocean-floor processes depends almost entirely on fragmented ophiolites thrust or obducted onto continent at destructive plate margins. Yet the characteristically striped magnetic signature above in situ oceanic lithosphere offers a good chance of spotting any old oceanic areas, provided the stripes are not imperceptible because of thick sediment cover.  One of the most intriguing areas of ocean floor is that beneath the eastern Mediterranean Sea in the 3 km deep Herodotus Basin, which has long been thought to preserve a relic of old ocean floor.  Roi Granot of Ben-Gurion University of the Negev, Israel has analysed magnetic data gathered along 7 000 km of survey lines and indeed there are vague traces of stripy geomagnetic variation that has a long wavelength, to be precise there are two bands of . Mathematical analysis of the magnetic profiles suggest that they have a source  about 13 to 17 km beneath the seabed: probably crystalline crust beneath thick Mesozoic sediments (Granot, R. 2016. Palaeozoic oceanic crust preserved beneath the eastern Mediterranean. Nature Geoscience, doi:10.1038/ngeo2784).

English: Age of oceanic lithosphere Deutsch: A...

Ages of oceanic lithosphere (credit: Wikipedia)

The shape of the anomalies cannot be matched with those of younger magnetic stripes, but can be modelled to fit with a sequence of normal-reverse-normal magnetic polarity preserved in continental sequences of early Carboniferous age, about 340 Ma ago. At that age, the lithosphere would by now be old, cold and dense enough to subside to the observed depth, but the fact that it escaped subduction during amalgamation of Pangaea in the Upper Palaeozoic or when Africa collided with Eurasia in the early Cenozoic is a puzzle. Granot reckons that it most likely formed in Pangaea’s great eastern ocean embayment, known as Palaeotethys. An interesting view, but one that does not seem likely to lead any further, simply because of the great depth to which the oceanic material is buried. The deepest yet to be achieved is only 12 km in the onshore Kola Superdeep Borehole in Russia. So the changes of getting samples are slim, even if the overlying sedimentary pile proves to have hydrocarbon potential.

English: Pangea animation

Pangea break-up animation ( credit: Wikipedia)

Here is the plate tectonic forecast

As computing power and speed have grown ever more sophisticated models of dynamic phenomena have emerged, particularly those that focus on meteorology and climatology. Weather and climate models apply to the thin spherical shell that constitutes Earth’s atmosphere. They consider incoming solar radiation and longer wavelength thermal radiation emitted by the surface sources and sinks of available power, linked to the convective circulation of energy and matter, most importantly water as gas, aerosols, liquid and ice in atmosphere and oceans. Such general circulation models depend on immensely complex equations that relate to the motions of viscous media on a rotating sphere, modulated by other aspects of the outermost Earth: the absorptive and reflective properties of the materials from which it is composed – air, rocks, soils, vegetation, water in liquid, solid and gaseous forms; different means whereby energy is shifted – speeds of currents and wind, adiabatic heating and cooling, latent heat, specific heat capacity of materials and more still. The models also have to take into account the complex forms taken by circulation on account of Coriolis’ Effect, density variations in air and oceans, and the topography of land and ocean floor. The phrase ‘and much more besides’ isn’t really adequate for such an enormous turmoil, for the whole caboodle has chaotic tendencies in time as well as 3-D space. The fact that such modelling does enable weather forecasting that we can believe together with meaningful forward and backward ‘snapshots’ of overall climate depends on increasing amounts of empirical data about what is happening, where and when. Models of this kind are also increasingly able to address issues of why such and such outcomes occur, an important example being the teleconnections between major weather events around the globe and phenomena such as the El Nino-Southern Oscillation – the periodic fluctuation of ocean movements, winds and sea-surface temperatures over the tropical eastern Pacific Ocean.

The key principle of plate tectonics is that t...

The Earth’s 15 largest tectonic plates. (credit: Wikipedia)

The Earth’s lithosphere and deeper mantle in essence present much the same challenge to modellers. Silicate materials circulate convectively in a thick spherical shell so that radiogenic heat and some from core formation can escape to keep the planet in thermal balance.  There are differences, the obvious ones being sheer scale and a vastly more sluggish pace, but most important are the interactions between materials with very different viscosities; the ability of the deep mantle to move by plastic deformation while the lithosphere moves as rigid, brittle plates. For geophysicists interested in modelling there are other differences; information that bears on the system is orders of magnitude less, its precision is much poorer and all of it is based on measurement of proxies. For instance, information on temperature comes from variations in seismic wave speed given by analysis of arrival times at surface observatories of different kinds of wave emitted by individual earthquakes. That is, from seismic tomography, itself a product of immensely complex computation. Temptation by computing power and the basic equations of fluid dynamics, however, has proved hard to resist and the first results of a general circulation model for the solid Earth have emerged (Mallard, C. et al. 2016. Subduction controls the distribution and fragmentation of Earth’s tectonic plates. Nature, v. 535, p. 140-143).

As the title suggests, the authors’ main objective was understanding what controls the variety of lithospheric tectonic plates, particularly how strain becomes localised at plate boundaries. They used a circulation model for an idealised planet and examined several levels of a plastic limit at which the rigidity of the lithosphere drops to localise strain. At low levels the lithosphere develops many plate boundaries, and as the plastic limit increases so the lithosphere ends up with increasingly fewer plates and eventually a rigid ‘lid’. The modelling also identified divergent and convergent margins, i.e. mid-ocean ridges and subduction zones. The splitting in two of a single plate must form two triple junctions, whose type is defined by the kinds of plate boundary that meet: ridges; subduction zones; transform faults. Both the Earth and the models show significantly more triple junctions associated with subduction than with extension, despite the fact that ridges extend further than do subduction zones. And these trench-associated triple junctions are mainly those dividing smaller plates. This suggests that it is subduction that focuses fragmentation of the lithosphere, and the degree of fragmentation is controlled by the lithosphere’s strength. There is probably a feedback between mantle convection and lithosphere strength, suggesting that an earlier, hotter Earth had more plates but operated with fewer, larger plates as it cooled to the present. But that idea is not new at all, although the modelling gives support to what was once mere conjecture.

So, when did plate tectonics start up?

Tiny, 4.4 billion year old zircon grains extracted from much younger sandstones in Western Australia are the oldest known relics of the Earth system. But they don’t say much about early tectonic processes. For that, substantial exposures of rock are needed, of which the undisputedly oldest are the Acasta gneisses 300 km north of Yellowknife in Canada’s North West Territories, which have an age of slightly more than 4 Ga. The ‘world’s oldest rock’ has been something of a grail for geologists and isotope geochemists who have combed the ancient Archaean cratons for 5 decades. But since the discovery of metasediments with an age of 3.8 Ga in West Greenland during the 1970s they haven’t made much headway into the huge time gap between Earth’s accretion at 4.54 Ga and the oldest known rocks (the Hadean Eon).

The Deccan Traps shown as dark purple spot on ...

Continental cratons (orange) where very-old rocks are likely to lurk. (credit: Wikipedia)

There have been more vibrant research themes about the Archaean Earth system, specifically the issue of when our planet settled into its modern plate tectonic phase A sprinkling of work on reconstructing the deep structural framework of Archaean relics has convinced some that opposed motion of rigid, brittle plates was responsible for their geological architecture, whereas others have claimed signs of a more plastic and chaotic kind of deformation of the outer Earth. More effort has been devoted to using the geochemistry of all the dominant rocks found in the ancient cratons, seeking similarities with and differences from those of more recent vintage. There can be little doubt that the earliest processes did form crust whose density prevented or delayed it from being absorbed into the mantle. Even the 4.4 Ga zircons probably crystallized from magma that was felsic in composition. Once trapped by buoyancy at the surface and subsequently wrapped around by similarly low density materials continental crust formed as a more or less permanent rider on the Earth’s deeper dynamics. But did it all form by the same kinds of process that we know to be operating today?

Plate tectonics involves the perpetual creation of rigid slabs of basalt-capped oceanic lithosphere at oceanic rift systems and their motions and interactions, including those with continental crust. Ocean floor cools as it ages and becomes hydrated by seawater that enters it. The bulk of it is destined eventually to oppose, head-to-head, the motions of other such plates and to deform in some way. The main driving force for global tectonics begins when an old, cold plate does deform, breaks, bends and drives downwards. Increasing pressure on its cold, wet basaltic top transforms it into a denser form: from a wet basaltic mineralogy (feldspar+pyroxene+amphibole) to one consisting of anhydrous pyroxene and garnet (eclogite) from which watery fluid is expelled upwards. Eclogite’s density exceeds that of mantle peridotite and compels the whole slab of oceanic lithosphere to sink or subduct into the mantle, dragging the younger parts with it. This gravity-induced ‘slab pull’ sustains the sum total of all tectonic motion. The water rising from it induces the wedge of upper mantle above to melt partially, the resulting magma evolves to produce new felsic crust in island arcs whose destiny is to be plastered on to and enlarge older continental masses.

Relics of eclogites and other high-pressure, low-temperature versions of hydrated basalts incorporated into continents bear direct and unchallengeable witness to plate tectonics having operated back to about 800 Ma ago. Before that, evidence for plate tectonics is circumstantial and in need of special pleading. Adversarial to-ing and fro-ing seems to be perpetual, between geoscientists who see no reason to doubt that Earth has always behaved in this general fashion and others who see room for very different scenarios in the distant past. The non-Huttonian tendency suggests an early, more ductile phase when greater radioactive heat production in the mantle produced oceanic crust so fast that when it interacted with other slabs it was hot enough to resist metamorphic densification wherever it was forced down. Faster production of magma by the mantle without slab-pull could have produced a variety of ‘recycling’ turnover mechanisms that were not plate-tectonic.

One thing that geochemists have discovered is that the composition of Archaean continental crust is very different from that produced in later times. In 1985 Ross Taylor and Scott McLennan, then of the Australian National University, hit on the idea of using shales of different ages as proxies for the preceding continental crust from which they had been derived by long erosion. Archaean and younger shales differed in such a way that suggests that after 2.5 Ga (the end of the Archaean) vast amounts of feldspar were extracted from the continent-forming magmas. This left the later Precambrian and Phanerozoic upper crust depleted in the rare-earth element europium, which ended up in a mafic, feldspar-rich lower crust. On the other hand, no such mass fractionation had left such a signature before 2.5 Ga. Another ANU geochemist, now at the University of Maryland, Roberta Rudnick has subsequently carried this approach further, culminating in a recent paper (Tang, M., Chen, K and Rudnick, R.L. 2016. Archean upper crust transition from mafic to felsic marks the onset of plate tectonics. Science, v. 351, p. 372-375). This uses nickel, chromium and zinc concentrations in ancient igneous and sedimentary rocks to track the contribution of magnesium (the ‘ma’ in ‘mafic’) to the early continents. The authors found that between 3.0 to 2.5 Ga continental additions shifted from a dominant more mafic composition to one similar to that of later times by the end of the Archaean. Moreover, this accompanied a fivefold increase in the pace of continental growth. Such a spurt has long been suspected and widely suggested to mark to start of true plate tectonics: but an hypothesis bereft of evidence.

A better clue, in my opinion, came 30 years ago from a study of the geochemistry of actual crustal rocks that formed before and after 2.5 Ga (Martin, H. 1986. Effect of steeper Archean geothermal gradient on geochemistry of subduction-zone magmas. Geology, v. 14, p. 753-756). Martin showed that plutonic Archaean and post-Archaean felsic rocks of the continental crust lie in distinctly different fields on plots of their rare-earth element (REE) abundances. Archaean felsic plutonic rocks show a distinct trend of enrichment in light REE relative to heavy REE as measures of the degree of partial melting decreases, whereas the younger crustal rocks show almost constant, low values of heavy REE/light REE whatever the degree of melting. The conclusion he reached was that while in the post Archaean the source was consistent with modern subduction processes – i.e. partial melting of hydrated peridotite in the mantle wedge above subduction zones – but during the Archaean the source was hydrated, garnet-bearing amphibolite of basaltic composition, in the descending slab of subducted oceanic crust. Together with Taylor and McLennan’s lack of evidence for any fractional crystallization in Archaean continental growth, in contrast to that implicated in Post-Archaean times.

The geochemistry forces geologists to accept that a fundamental change took place in the generation and speed of continental growth at the end of the Archaean, marking a shift from a dominance of melting of oceanic, mafic crust to one where the upper mantle was the main source of felsic, low-density magmas. Yet, no matter how much we might speculate on indirect evidence, whether or not subduction, slab-pull and therefore plate tectonics dominated the Archaean remains an open question.

More on continental growth and plate tectonics

Global Tectonics Centenary: Any Inspiring Papers?

Although Alfred Wegener first began to present his ideas on Continental Drift in 1912 his publication in 1915 of The Origin of Continents and Oceans (Die Entstehung der Kontinente und Ozeane) is generally taken as the global launch of his hypothesis. Apart from support from Alexander du Toit and Arthur Holmes, geoluminaries of the day panned it unmercifully because, in the absence of evidence for a driving mechanism, he speculated that his proposed ‘urkontinent’ (primal continent) Pangaea had been split apart by a centrifugal mechanism connected to the precession of Earth’s rotational axis. This ‘polflucht’ (flight from the poles) is in fact far too weak to have any such influence. Wegener’s masterly assembly of geological evidence for former links between the major continents was ignored by the critics, suggesting that their motive for excoriation of his suggested mechanism was as much spite against an ‘outsider’ as a full consideration of his hypothesis. It must have been hurtful in the extreme, yet Wegener defended himself with a series of revised editions that amassed yet more concrete evidence. What is often overlooked, even now that his ideas have become part of the geoscientific canon, is that in his initial Geologische Rundschau paper in 1912 he mused that the floor of the Atlantic is continuously spreading by tearing apart at the mid-Atlantic Ridge where ‘relatively fluid and hot sima’ rises. Strangely, he dropped that idea in later works. Anyhow, neither 2012 nor 2015 was celebrated in the manner of the centenary-and-a-half of Darwin’s On the Origin of Species: 2009 was marked by palaeobiologists and geneticists metaphorically dancing in the streets, if not foaming at the mouth. There have been a few paragraphs, and some minor symposia about Wegener’s dragging geology out of the 18th century and into the 20th, but that’s about it. The best centenary item I have seen is by Marco Romano and Richard Cifelli (Romano, M. & Cifelli, R.L. 2015. 100 years of continental drift. Science, v. 350, p. 916-916).

In the shape of plate tectonics the Earth sciences hosted what was truly a revolution in science, albeit 50 years on from its discoverer’s announcement. It was through the persistent agitation by his tiny band of supporters, that the upheaval was unleashed when the revelations from palaeomagnetism, seismology and many other lines of evidence were resolved as plate tectonics by the discovery of ocean-floor magnetic stripes by Vine, Matthews and Morley in 1963. Despite an explosion of papers that followed, elaborating onthe new theory and showing examples of its influence on ‘big’ geology , counter-revolutionary resistance lasted almost to the first years of the new century. By then so much evidence had emerged from every geological Eon that opponents looked truly stupid. Even so, the skepticism among those sub-disciplines that were ‘left out’ of geodynamic thought continued to blurt out with the emergence of other exciting aspects of the Earth’s history. I remember that, when three of us in the Open University’s Department of Earth Sciences proposed in 1994 that the influence of impacts by extraterrestrial objects ought to figure in a new course on the evolution of Earth and Life we were sneered at as ‘whizz-bang kids’ by those more earth-bound. Trying belatedly in 1996 to introduce students to another revolutionary development – the use of sedimentary and glacial oxygen isotopes in unraveling past climate change – became a huge struggle in the OU’s Faculty of Science. It went to the press eventually and for 2 years our students had the benefit. But the murmuration of dissent ended with a force-majeur re-edit of the course, by someone who had played no role in its development, expunged the lot and changed the ‘offending’ section back to the way it had been a decade before.  As they say: ho hum!

Oddly, in the last 15 years or so of trying to follow in Earth-Pages what I considered to be the most exciting developments in the geosciences, it has become increasing difficult to find papers in the top journals that are truly ground-breaking. Of course that may just be ageing and a certain cynicism that often companies it. From being spoiled for choice week after week it has become increasingly difficult from month to month to maintain the standards that I have set for new work. Has Earth science entered the fifth phase of a ‘paradigm shift’ predicted by philosopher Thomas Kuhn in his 1962 book The Structure of Scientific Revolutions? According to him once a science has entered a period when there is little consensus on the theories that might lie at the root of natural processes there is a drift in opinion to a few conceptual frameworks that seem to work, albeit leaving a lot to be desired. Weaknesses at the frontier between theory and empirical knowledge become increasingly burdensome as a result of the steady plod of ‘normal science’ until the science in question reaches a crisis. If existing paradigms fail repeatedly, science is ripe for the metaphorical equivalent of a ‘Big Bang’: maybe an entirely new discovery or hypothesis, or an idea that has been suppressed which new data fits better than any others that have been common currency. Plate Tectonics is the second kind. After the revolution much is reexamined and new lines of work emerge, until in Kuhn’s 5th phase scientists return to ‘normal science’. That looks like a pretty good story, on paper, but other forces are at work in science; external to scientific objectives. Most of these are a blend of economics, political ideologies and managerial ‘practicalities’. If the Earth sciences have entered the doldrums of novelty, I suspect it is these forces that are bearing some kind of glum fruit.

The old concept of academic freedom has gone by the board. Institutions demand that research is externally funded – the more the better as the institution, at least in the UK, demands a kind of tax (40% of that proposed) supposedly to cover corporate overheads including salaries of support staff. If an academic doesn’t pull in the dosh, she is not much favoured. If the individual doesn’t publish regularly either, there is a weasel sanction: Josephine Soap is declared ‘research inactive’. Consortia of researchers are more and more in vogue: managers and funders like ‘team players’, so individuals who are bright and confident enough to ‘stick their necks out’ cannot do that in a consortium publication and as often as not are ‘left on the bench’. Risk taking is more dangerous now and to stay ‘research active’, and in many cases of non-tenured posts getting a salary, an individual, even a few like-minded colleagues have to publish 2 or 3 papers a year.

It’s worth mentioning that open access publishing is not just all the rage, it has become more or less compulsory. Of course, it has some benefits for scientists in less well-heeled countries, but there is a downside. You have to raise the cash demanded by journals for the privilege or potentially universal access – at least US$1000 a pop, depending on a journals Impact Factor, and that of course is an odiously essential corporate consideration – and having done that woe betide those who do not publish and spend it. Academic publishing is the most profitable sector of the trade, the more so as print is supplanted by electronic delivery – the 50 free reprints is a thing of the past. So there are more and more journals and each of them strives to get out more issues per year, and of course those have to be filled. To me, this all adds up to more and more ‘pot-boiler’ articles and a tendency to maximise the flesh rendered from the body of research work and into the pot. Taken together with the stresses of commodification in higher education and the now vertical corporate structures from which it is constituted, it shouldn’t be a surprise that excitement and inspiration are at a premium in the weekly and monthly output of such a marginal science as that concerned with how the world works.

Some cunning radiometric dating

At the end of the 1970’s I was invited by the Deputy Director of the Geological Survey of India (Southern Region) to participate in the Great Postal Symposium on the Cuddapah Basin: a sort of harbinger of the Internet and Skype, but using snail-mail. Feeling pretty honoured and most intrigued I accepted; not that I knew the first thing about the subject. A regular stream of foolscap mimeographed contributions kept me nipping out of my office to check my pigeon hole for about 6 months. I learned a lot, but felt unable to comment. Four years on I was taken across the Cuddapahs by my first research student – a budding moto-cross driver with a morbid fear of bullock carts – en route from the Archaean low-grade greenstone-granite terrains of Karnataka for a peek at the fabled charnockites near Chennai (then Madras). A bit of a round-about route but spurred by my memories of the Great Postal Symposium. Sadly, the detour was marred for me by a severe case of sciatica brought on by manic driving, the state of the trans-Cuddapah highway and a misplaced gamma-globulin shot to ward off several varieties of hepatitis: I mainly blamed the nurse who demanded that I drop my drawers and bravely take the huge needle in a buttock – they do these things more humanely these days. Anyhow, apart from seeing many dusty villages build of slates perfect enough to make a full-size snooker table, my mind was elsewhere and I have long regretted that.

Landsat image mosaic showing part of the Cuddapah Basin.

Landsat image mosaic showing part of the Cuddapah Basin.

Hosting possibly the world’s only diamondiferous Precambrian conglomerate, the Cuddapah Basin contains a 5 km thickness of diverse sedimentary strata, but no tangible fossils. It rests unconformably on the Archaean greenstone-granite terrain of the Dharwar Craton and so is Proterozoic in age; an Eon that spans 2 billion years. The middle of the lowest sedimentary formations (the Papaghni and Chitravati Groups) contains volcanic rocks dated at ~1.9 Ga; another group is cut by a ~1.5 Ga granite, and hitherto the youngest dateable event is the emplacement of 1.1 Ga kimberlites that sourced the diamonds in the conglomerate. Until recently the stratigraphy has been known in some detail, but how to partition it in Proterozoic time is barely conceivable with just three dates in the middle parts that span 800 Ma. All that can be said about the base of the Cuddapah sediments is that they are younger than the 3.1 to 2.6 Ga Archaean rocks beneath. Since the uppermost beds are truncated by a huge thrust system that shoved deep crustal granulites over them their minimum age is equally vague.

Structurally, the Basin began to form on a stable continent underpinned by the Dharwar Craton, but when that collided with Enderbyland in Antarctica, as part of the accretion of the Gondwana supercontinent, sedimentation may have been in an entirely different setting. Indeed, some of the sediments have been carried over the undisturbed part of the basin by a major thrust system. To explore both sedimentary and tectonic evolution Australian, Indian and Canadian geoscientists combined to sample and radiometrically date the entire pile (Collins, A.S. and 13 others 2015. Detrital mineral age, radiogenic isotopic stratigraphy and tectonic significance of the Cuddapah Basin, India. Gondwana Research, v. 28, p. 1294-1309). By precisely dating detrital micas and zircons from the sediments the team was able to check the source region of sedimentary grains as well as to establish a maximum age for each major stratigraphic unit. This helped establish a 3-part sedimentary and tectonic history. The earliest sediments came from the cratonic area to the west, but there are signs that collisional orogeny between 1590 and 1659 Ma produced a new sedimentary source in metamorphic rocks forming to the east. A return to westward provenance marked the youngest sedimentary setting. This enabled the team to suggest a dual evolution of the Basin, first as an extensional rift opening at the east of what is now the Dharwar craton followed by collisional orogeny that transformed the setting to that of a foreland basin, analogous to the Molasse basin in front of the Alps during Cenozoic times, ending with tectonic inversion when extension changed to compression and thrusting.

But to what extent did the work improve the age subdivision of the Cuddapah Basin? Apparently very little, which may be down to a problem with dating detrital minerals. If magmatic and metamorphic evolution was continuous in the areas from which sediments moved, then the youngest grain is a good guide to the maximum age of the sediment being analysed. The more strata are analysed in this way the better the detail of sedimentary timing. But two tectonic terrains are unlikely to produce zircons time and time again during a period approaching a billion years. The data indicate only 3 or 4 episodes of ‘zirconogenesis’ in the sedimentary hinterlands, between about 900 to 1940 Ma. Apart from helping correlate sedimentary formations that were previously deemed stratigraphically different – which did help in tectonically unravelling this complex major feature – several hundred isotopic analyses of zircons and micas have give much the same timing as was known already in more precise terms from stratigraphy assisted by a few dozen conventional radiometric dates.

The core’s influence on geology: how does it do it?

Although no one can be sure about the details of processes in the Earth’s core what is accepted by all is that changes in core dynamics cause the geomagnetic field to change in strength and polarity, probably through some kind of physical interaction between core and deep mantle at the core-mantle boundary (CMB). Throughout the last 73 Ma and especially during the Cenozoic Era geomagnetism has been more fickle than at any time since a more or less continuous record began to be preserved in the Jurassic to Recent magnetic ‘stripes’ of the world ocean floor. Moreover, they came in bursts: 5 in a million years at around 72 Ma; 10 in 4 Ma centred on 54 Ma; 17 over 3 Ma around 42 Ma; 13 in 3 Ma at ~24 Ma; 51 over a period of 12 Ma centring on 15 Ma. During the Late Jurassic and Early Cretaceous the core was similarly ‘busy’, the two time spans of frequent reversals being preceded by quiet ‘superchrons’ dominated by the same normal polarity as we have today i.e. magnetic north being roughly around the north geographic pole.

The Cenozoic history of magnetic reversals - black periods were when geomagnetic field polarity was normal and white when reversed. (credit: Wikipedia)

The Cenozoic history of magnetic reversals – black periods were when geomagnetic field polarity was normal and white when reversed. (credit: Wikipedia)

Until recently geomagnetic ‘flips’ between the two superchrons were regarded as random , perhaps suggesting chaotic behaviour at the CMB. But such a view depends on the statistical method used. A novel approach to calculating reversal frequency through time, however, shows peak-trough pairs recurring 5 times through the Cenozoic Era, approximately 13 Ma apart: maybe the chaos is illusory (Chane, J. et al. 2015. The 13 million year Cenozoic pulse of the Earth. Earth and Planetary Science Letters, v. 431, p. 256-263). So, here is a kind of yardstick to see if there may be any connection between core processes and those at the surface, which Chen of the Fujian Normal University, Fushou China and Canadian and Chinese colleagues compared with the very detailed Cenozoic oxygen-isotope (δ18O) record preserved by foraminifera in ocean-floor sediments, which is a well established proxy for changes in climate. Removing the broad trend of cooling through the Cenozoic resulted in a plot of more intricate climatic shifts that matches the geomagnetism record in both shape and timing of peak-trough pairs. It also turns out, or so the authors claim, that both measures correlate with changes in the rate of Cenozoic subduction of oceanic lithosphere (a measure of plate tectonic activity), albeit negative – peaks in magnetism and climate connecting with slowing in the pace of tectonics.

The analyses involved some complicated maths, but taken at face value the correlations beg the questions why and how? Long-term climate change contains an astronomical signal, encapsulated in the Milankovich hypothesis which has been tested again and again with little room for refutation. So is this all to do with gravitational influences in the Solar System. More exotic still is the possibility of 13 Ma cyclicity linking the Milankovich mechanism with the vaster scale of the Sun’s orbit oscillating through the disc of the Milky Way galaxy and theoretical hints of a mysterious role for dark matter in or near the galaxy. Or, is it a relationship in which climate and the magnetic field are modulated by plate tectonics through varying volcanic emissions of greenhouse gases and the deep effect of subduction on processes at the CMB respectively? To me that seems more plausible, but it is still as exceedingly complex as the maths used to reveal the correlations.

Continental hot-spot track in eastern Australia

cosgrove volcano track shown on map of australia

The Cosgrove volcanic track on a natural-colour image mosaic of Australia (credit: Drew Whitehouse, NCI National Facility VizLab)

It is sometimes forgotten that not only oceanic lithosphere provides evidence for hot spot tracks, probably because they are so obvious as island and seamount chains on bathymetric maps. They are not so clear on continents, either because of erosion of volcanoes or topography dominated by features that predate volcanism, but they account for about 20% of proposed tracks. Eastern Australia seems well endowed; four of them marked by a variety of volcanic structures that trend parallel to the Indo-Australian Plate’s NNE Cenozoic drift powered by the Southeast Indian Ridge that separates it from the Antarctic Plate. The timing of the volcanism along the proposed tracks is also highly persuasive. The longest of the tracks, extending about 2000 km SSW from Cape Hillsborough on the coast of central Queensland through New South Wales to Cosgrove in Victoria, is marked by sporadic volcanoes whose age decreases from Late Eocene in the north to Late Miocene in Victoria.

Unlike oceanic hot-spot tracks, those on continents are not continuous lines of volcanic occurrences. The Cosgrove track has several volcanic gaps, up to 650 km wide. This kind of patchy feature once encouraged hot-spot sceptics to question the tectonic affinities of what they regarded as fortuitous alignments. Where volcanic age trends consistent with the hypothesis emerged such doubts have faded into the background academic ‘noise’. In the case of the Cosgrove track all but one of the dates of volcanism tally quite well with the Cenozoic absolute motion of the Indo-Australian Plate and their position along the track (Davies, D.R. et al 2015. Lithospheric controls on magma composition along Earth’s longest continental hotspot track. Nature, v. 525, p. 511-514). Yet the objective of the authors, from the Australian National University and the University of Aberdeen in Britain, was not merely to establish the alignment as a hot-spot track, but to suggest what may have resulted in its marked patchiness.

The geochemistry of lavas from the volcanoes turns out to be of two fundamentally different types: ‘common-or-garden’ basalts in the case of Queensland and peculiar potassium-rich basalts containing the K-feldspathoid leucite in New South Wales and Victoria. Why these compositional differences occur where they do emerged very clearly when their positions were plotted on a new map of the thickness variations of the eastern Australian continental lithosphere. The ordinary basalts rest on the thinnest lithosphere (£110 km), whereas the leucitites are underlain by considerably thicker lithosphere (~135 km). This suggests that the rising mantle whose partial melting produced the magmas was halted at different depths, different geochemical ‘signatures’ of basalts depending on the pressure of melting. The most interesting outcome, albeit one based on an absence of evidence, is that the very large volcanic gaps along the track are each above much thicker lithosphere (>150 km). At those depths a rising mantle plume would be much less likely to begin melting.

Hotspots and plumes

One of the pioneers of plate tectonics, W. Jason Morgan, recognised in the 1970s that chains of volcanic islands and seamounts that rise from the ocean floor may have formed as the movement of lithospheric plates passed over sources of magma that lay in the mantle beneath the plates. He suggested that such hotspots were fixed relative to plate movements at the surface and likened the formation of chains such as that to the west of the volcanically active of the Hawaiian ‘Big Island’ to linear scorching of a sheet of paper moved over a candle flame. If true, it should be possible to use hotspots as a framework for the absolute motion of lithospheric plates rather than the velocities of individual plates relative to the others. But Morgan’s hypothesis has been debated ever since he formulated it. A test would be to see whether or not plumes of rising hot material in the deep part of the mantle can be detected. This became one of the first objectives of seismic tomography when it was devised in the last decade of the 20th century: a method that uses global earthquakes records to detect parts of the mantle where seismic waves traveled faster or slower than the norm: effectively patches of hot (probably rising) and cold rock. The first such evidence was equally hotly debated, one view being that the magma sources beneath oceanic islands such as Hawaii and Iceland were actually related to plate tectonics and that the hotspot hypothesis had become a kind of belief system.

English: global distribution of 45 identified ...

Global distribution of hotspots ( credit: Wikipedia)

The problem was that mantle plumes supposedly linked to magmatic hotspots in the upper mantle would be so thin that they would be difficult to detect even with seismic tomography. Geophysicists have been trying to sharpen up seismic resolution partly by using supercomputers to analyse more and more seismic records and also by improving the theory about how seismic waves interact with 3-D mantle structure. This has culminated in more believable visualisation of mantle structure (French, S.W. & Romanowicz, B. 2015. Broad plumes rooted at the base of the Earth’s mantle beneath major hotspots). The two researchers from the University of California at Berkeley in fact showed something different, but still robust support for Morgan’s 40-year old ideas. Instead of thin plumes, they have been able to show much broader conduits beneath at least 5 and maybe more active ends of hotspot chains. The zones extend upwards from the core-mantle boundary to about 1000 km below the Earth’s surface, where some bend sideways towards hotspots, perhaps as a result of another kind of upper mantle circulation.

Whole-Earth seismic tomography cross sections beneath a variety of volcanic islands, (Credit French and Romanowicz;

Whole-Earth seismic tomography cross sections beneath a variety of volcanic islands, (Credit French and Romanowicz;

The sources of these hot columns at the core-mantle boundary appear to be zones of very low shear-wave velocities; i.e. almost, but not quite molten blobs. French and Romanowicz suggest that the columns are extremely long-lived and may even have a chemical dimension – as in the hypothesis of mantle heterogeneity. Another interesting feature of their results is that the striking vertical linearity of the columns could indicate that the overall motion of the lower mantle is extremely sluggish and punctured by discrete convection.

Thin- or thick-skinned tectonics: a test

How the continental lithosphere deforms at convergent plate margins has been a matter of opinion that depends on where observations have been made in ancient orogenic belts. One view is that arc and collisional orogens are dominated by deformation of the upper crust and especially the cover of sedimentary and volcanic rocks above deeper and older basement. This is a ‘thin-skinned’ model in which rocks of the upper crust are detached from those below and thicken more or less independently by thrust faulting, the formation of ductile nappes or a combination of the two. Mountain ranges, in this view, are the product of piling up of thrust slices or nappes, as exemplified by the Alps, Canadian Rockies and the Caledonian thrust belt of NW Scotland. Thick-skinned processes, as the name suggests, see crustal shortening and thickening as being distributed through the crust from top to bottom and even involving the lithospheric mantle. The hinterlands of both the Alps and the Scottish Caledonides show plenty of evidence for entire-crust deformation, deep crustal rocks being found sheared together with deformed rocks of the cover. It stands to reason that orogenic processes on the grand scale must involve a bit of both.

Both hypotheses stem from field work in deeply eroded, structurally complex segments of the ancient crust, and it is rarely if ever possible to say whether both operated together or one followed the other during the often lengthy periods taken by orogeny to reach completion, and the sheer scale of the process. Orogenesis is going on today, to which major seismic activity obviously bears witness. But erosion has not progress from cover through basement so, up to now, only seismicity and geodetic GPS measurements have been available to show that continental crust in general is being shortened and thickened, as well as being moved about. Potentially, a means of assessing active deformation, even in the deep crust, is to see whether or not the speeds of seismic waves at different depths are biased depending on their direction of travel. Such anisotropy would develop if the mineral grains making up rocks were deformed and rotated to preferred directions; a feature typical of metamorphic rocks. But to make such measurements on the scale of active orogens requires a dense network of seismometers and software that can tease directionality and depth out of the earthquake motions detected by it.

LS-tectonite from the Paraiba do Sul Shear Zon...

Aligned minerals in a Brazilian metamorphic rock (credit: Eurico Zimbres in Wikipedia)

A joint Taiwanese-American consortium set up such a network in Taiwan, which is capable of this type of seismic tomography. Taiwan is currently taking up a strain rate of 8.2 cm per year due to motion of the Philippine Plate on whose western flank the island lies: it is part of an island arc currently colliding with the stationary Eurasian Plate and whose crust is shortening. Results of seismic anisotropy (Huang, T.-Y. et al. 2015. Layered deformation in the Taiwan orogen. Science, v. 349, p. 720-723) show that the fast direction of shear (S) waves changes abruptly at about 10 to 15 km deep in the crust. In the upper crust this lines up with the roughly N-S structural ‘grain’ of the orogen. At between 13 to 17 km down there is no discernible anisotropy, below which it changes to parallel the direction of plate motion, ESE-WNW. It seems that thin skinned tectonics is indeed taking place, although probably not above a structural detachment. Simultaneously the deep crust is being deformed but the shearing is ascribed to the descent of lithospheric mantle of the Philippine Plate beneath the Eurasian Plate, while the deep crust remains attached to the upper crust. If it were possible to examine the mineral lineations now forming in both the Taiwanese upper and lower crust where metamorphism is active, then the two directions would be apparent. Although not mentioned by the authors, perhaps the detection of different directionality of aligned metamorphic minerals in low- and high-grade metamorphic rocks might indicate such tectonic processes in the past.

When Earth got its magnetic field

For a planet to produce life it needs various attributes. Exoplanet hunters tend to focus on the ‘Goldilocks’ Zone’ where solar heating is neither so extreme nor so little that liquid water is unstable on a planet’s surface. It also needs an atmosphere that retains water. Ultraviolet radiation emitted by a planet’s star dissociates water vapour to hydrogen and oxygen and the hydrogen escapes to space. The reason Earth has not lost water in this way is that little water vapour reaches the stratosphere because it is condensed or frozen out of the air as the lower atmosphere becomes cooler with altitude. Given moist conditions survivability to the extent that exists on Earth still needs another planetary parameter: the charged particles emitted as an interplanetary ‘wind ‘by stars must not reach the surface. If they did, their potential to break complex molecules would hinder life’s formation or wipe it out if it ventured onto land. A moving current of electrical charge, which is what a stellar ‘wind’ amounts to, can be deflected by a magnetic field. This is what happens on Earth, whose magnetic field is a good reason why our planet has supported life and its continual evolution since at least about 3.5 billion years ago.

Artist's rendition of Earth's magnetosphere.

Deflection of the solar ‘wind’ by Earth’s Earth’s magnetosphere. (credit: Wikipedia)

Direct proof of the existence of a geomagnetic field is the presence of aligned particles of magnetic minerals in rocks, for instance in a lava flow, caused by their acquiring magnetisation in a prevailing magnetic field once they cooled sufficiently. The earliest such remanent magnetism was found in igneous rocks from north-eastern South Africa dated at between 3.2 to 3.45 billion years. All older rocks do not show such a feature dating back to their formation because of thermal metamorphism that resets any remanent magnetism to match the geomagnetic field prevailing at the time of reheating. There are, however, materials that formed further back in time and are also known to resist thermal resetting of any alignments of magnetic inclusion. They are zircons (ZrSiO4), originally crystallised from igneous magmas, which may have locked in minute magnetic inclusions. Zircons are among the most change-resistant materials and they can also be dated with great precision, with the advantage that the U-Pb method used can distinguish between age of formation and that of any later heating. Famously, individual grains of zircon that had accumulated in an early Archaean conglomerate outcropping in the Jack Hills of Western Australia yielded ages going back from 3.2 to 4.4 billion years; far beyond the age of any tangible rock and close to the formation age of the Earth. Quite a target for palaeomagnetic investigations once a suitable technique had been developed.

Western Australia's Jack Hills

Western Australia’s Jack Hills from Landsat (credit NASA Earth Observatory)

John Tarduno and colleagues from the Universities of Rochester and California USA and the Geological Survey of Canada report the magnetic properties of the Jack Hills zircons (Tarduno, J.A. et al. 2015. A Hadean to Paleoarchean geodynamo recorded by single zircon crystals. Science, v. 349, p. 521-524). All of the grains analysed record magnetisation spanning the period 3.2 to 4.2 billion years that indicate geomagnetic field strengths ranging from that found today at the Equator to about an eighth of the modern value. So from 4.2 Ga onwards geomagnetism probably deflected the solar wind: the early Earth was set for living processes from its earliest days. The discovery also supports the likelihood of functioning plate tectonics during the Hadean.