Category Archives: Tectonics

Mega-impacts and tectonics

Because they are fast as well as weighty, destination-Earth asteroids and comets pack quite a punch. That is because their kinetic energy is proportional to the square of their speed (at least 13 km s-1) as well as half their mass. So, even all one half a kilometre across carries an energy a hundred times the solar energy received by Earth in a year, and a great deal more when compared with geothermal heat production. Much of the focus on the effects of impact events has dwelled on the upper crust, the oceans and atmosphere. Yet they also have huge seismic effects, with a proportion of their shock effect being dissipated throughout the entire planet. One obvious consequence would be a thermal anomaly directly beneath the crater as well as some thinning of the lithosphere and body waves affecting the rest of the solid Earth.

Thermal and mechanical processes lie at the core of tectonics, so a big question has been ‘Could impacts create mantle plumes or set new tectonic processes in motion?’ There has been speculation of diverse kinds since impacts became popular following the link between the Chicxulub crater and the end-Cretaceous mass extinction, discovered in 1980. But ‘educated guesses’ have generated more hot air than clear conclusions. Much as most of us are modelling-averse, a mathematical approach is the only option in the welcome absence of any severe extraterrestrial battering to which scientists have borne witness. With refined algorithms that cover most of the nuances of projectiles and targets – conservation of mass, energy and momentum in the context of the solid Earth behaving as a viscous medium –  Craig O’Neill and colleagues at Macquairie University, Australia, and the Southwest Research Centre in Boulder, CO USA, have simulated possible tectonic outcomes during plausible bombardment scenarios during the Hadean (O’Neil, C. et al. 2017. Impact-driven subduction on the Hadean Earth. Nature Geoscience, v. 10, p. 793-797; DOI: 10.1038/NGEO3029).

It appears that truly gargantuan objects – radius >500 km – are required to stimulate sufficient thermal anomalies that would lead to mantle upwellings whose evolution might lead to subduction at their margins. One at the limit posed by lunar cratering history (~1700 km radius) could have resulted in wholesale subduction of the entire lithosphere present at the time about 4 Ma after the impact. In the Hadean, it is likely that the lithosphere would have had a roughly mantle composition, so that the density excess needed for slab descent would have been merely temperature dependent. Note: after the onset of a basalt-capped lithosphere heat flow would have needed to be below the limit at which basalt converts to eclogite at high pressures, and thus to a density greater than that of the mantle, for continuing subduction. The authors’ Hadean scenario is one of episodic subduction dependent on the projectile flux and magnitude; i.e. with an early Hadean with stop-start subduction waning to tectonic stagnation and then a restart during the Late Heavy Bombardment after 4.1 Ga. Evidence for this is clearly scanty, except for Hadean zircons, whose presence indicates differentiation of early magmas with a peak between 4.0 to 4.2 Ga, in which magnetic intensities are preserved that are roughly as predicted by the scenario.

No impacts preserved in Precambrian to Recent times suggest extraterrestrial objects with the power to induce significant changes to global tectonics.

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Recycling of continental crust through time

Because continental crust is so light – an average density of 2700 kg m-3 compared with the mantles’ value of 3300 – it has been widely believed that continents cannot be subducted en masse. Yet it is conceivable that sial can be ‘shaved’ from below during subduction and from above by erosion and added to subductable sediment on the ocean floor. Certainly, there is overwhelming evidence for the net growth of continents through time and plenty for periods of increased and dwindling growth in the past. In some ancient orogens there are substantial slabs of continental composition whose mineralogy bears witness to ultra-high pressure metamorphism at depths greater than that of the base of continents. These slabs had been caught-up in subduction but never reached sufficiently high density to be retained by the mantle; they eventually ‘bobbed up’ again. On the other hand, if early continents were less silica rich through incorporation of substantial proportions of rock with basaltic composition parts of them could founder if subjected to high-pressure, low-temperature metamorphism. But not all crustal recycling to the mantle is through subduction. Some abnormally highly elevated parts of the continents that rose quickly in geological terms, such as the Tibetan Plateau, may have formed by lower crustal slabs becoming detached or delaminated from their base. Again modelling can help assess the past magnitude of continental recycling (Chowdhury, P. et al. 2017. Emergence of silicic continents as the lower crust peels off on a hot plate-tectonic Earth. Nature Geoscience, v. 10, p. 698-703; DOI: 10.1038/NGEO3010).

Various lines of evidence suggest that between 65 to 70% of the present continental volume existed by 3 billion years ago, yet that does not manifest itself in the rock record; perhaps a sign that some has returned to the mantle. It is also widely suggested that plate tectonics in the modern style began at about that time. Pryadarshi Chowdhury and colleagues simulate what may happen at depth in continent-continent collision zones – the classic site of orogenies –at different times in the past. Under the hotter conditions in the early Archaean mantle delamination would have been more likely than it has been during the Phanerozoic; i.e. the peeling off and sinking of the denser, more mafic lower crust and the attached upper mantle. The authors show that increased mantle temperature further back in time increases the likelihood and extent of such delamination. It also encourages partial melting of the descending continental material so creating rising bodies of more silicic magma that add to the remaining continent at the surface. Together with the lower crust’s attachment of to a mantle slab, this ensures that the peeled off material is able to descend under its own load. Once below a depth of 250 km felsic rocks are doomed to further descent. Waning of radiogenic mantle heat production encourages descending slabs to fail and break from the connection with lithosphere at higher levels so that a smaller proportion of the lower crust becomes detached and recycled. This evolution suggests that less and less continental crust is recycled with time. This broadly fits with current geochemical ideas based on the record of radiogenic Nd-, Sr- and Pb-isotopes in rocks ranging from early Archaean to Phanerozoic age.

Plate tectonic graveyard

Where do old plates go to die? For the most part, down subduction zones to mix with their original source, the mantle. Earth-Pages has covered evidence for quite a few of the dead plates, which emerges from a geophysical technique known as seismic tomography – analogous to X-ray or magnetic resonance scans of the whole human body. For 20 years geophysicists have been analysing seismograms from many stations across the globe for every digitally recorded earthquake, i.e. virtually all of those since the 1970s. This form of depth sounding goes far beyond early deep-Earth seismometry that discovered the inner and outer core, various transition zones in the mantle and measured the average variation with depth of mantle properties. Tomography relies on complex models of the paths taken by seismic body waves and very powerful computing to assess variations in the speed of P- and S-waves as they travelled through the Earth: the more rigid/cool the mantle is the faster waves travel through it and vice versa. The result is images of deep structure in 2-D slices, but the quality of such sections depends, ironically, on plate tectonics. Most earthquakes occur at plate boundaries. Such linearly distributed, one-dimensional sources inevitably leave the bulk of the mantle as a blur. Around 20 different methodologies have been developed by the many teams working on seismic tomography. So sometimes conflicting images of the deep Earth have been produced.

Results of seismic tomography across Central America showing anomalously fast (in blue) P- (top) and S-wave (bottom) speeds in map view at a fixed mantle depth (1290 km, left) and as vertical sections (right). The blue zones at right are interpreted to show a steeply dipping slab that represents subduction of the eastern Pacific Cocos plate since about 175 Ma ago (credit: van der Meer, D.G et al. ‘Atlas of the Underworld)

The technique has come of age now that superfast computing and use of multiple models have begun to resolve some of tomography’s early problems. The latest outcome is astonishing: ‘The Atlas of the Underworld’ catalogues 94 2-D sections from surface to the core-mantle boundary each of which spans 40° or arc – about a ninth of the Earth’s circumference (see: van der Meer, D.G., van Hinsbergen, D.J.J., and Spakman, W., 2017, Atlas of the Underworld: slab remnants in the mantle, their sinking history, and a new outlook on lower mantle viscosity, Tectonophysics online; doi.org/10.1016/j.tecto.2017.10.004). Specifically, the Atlas locates remnants of relatively cold slabs in the mantle that are suspected to be remnants of former subduction zones, or those that connect to active subduction. The upper parts of active slabs are revealed by the earthquakes generate along them. At deeper levels they are too ductile to have seismicity, so what form they take has long been a mystery. Once subduction stops, so do the telltale earthquakes and the slabs ‘disappear’.

The slabs covered by the ‘Atlas’ only go back as far as the end of the Permian, when the current round of plate tectonics began as Pangaea started to break-up. It takes 250 Ma for slabs to reach the base of the mantle and beyond that time they will have heated up and begun to be mixed into the lower mantle and invisible. Nevertheless, the rich resource allows models of vanished Mesozoic to Recent plates and the tectonics in which they participated, based on geological information, to be evaluated and enriched. Just as important, the project opens up the possibility of finding out how the mantle ‘worked’ since Pangaea broke up, in 3-D; a key to more than plate tectonics, including the mantle’s chemical heterogeneity. Already it has been used to estimate changes in the total length of subduction zones since 250 Ma ago, and thus arc volcanism and CO2 emissions, which correlates with estimates of past atmospheric CO2 levels, climate and even sea levels.

See also:  Voosen, P. 2016. ‘Atlas of the Underworld’ reveals oceans and mountains lost to Earth’s history. Science; doi:10.1126/science.aal0411.

Lee, H. 2017. The Earth’s interior is teeming with dead plates. Ars Technica UK, 18 October 2017.

Zealandia: a hitherto undiscovered continent?

Mid-February 2017 saw the announcement in the world’s media of what was made out to be a previously unsuspected, drowned continent. No, not in the Atlantic, but surrounding New Zealand. For geoscientists this was not ‘fake news’, but neither was it a surprise. Precise bathymetry based on satellite data rather than more conventional soundings from ships had long shown a substantial area (4.9 million km2) of the Coral and Tasman Seas between Australia and New Zealand and the Pacific to the immediate south-east of New Zealand was considerably less deep than the mean for the world’s ocean floors. It shows up clearly on Google Earth.  The name ‘Zealandia’ had been suggested in 1995. The media flurry emerged from a paper published in the March/April 2017 issue of the Geological Society of America’s on-line newsletter (Mortimer, N. and 10 others 2017. Zealandia: Earth’s hidden continent. GSA Today, v. 27(3 March April 2107); doi:10.1130/GSATG321A.1), the phrase ‘hidden continent’ no doubt pulling in the hacks like mackerel to a piece of tin foil.

The extent of Zealandia shown by Google Earth – the paler the blue coloration the shallower the ocean floor

The 10 New Zealander authors with one Australian, based their definition of the anomalously shallow ocean floor as a continent on data accumulated over many years from geophysical surveys and spot sampling of rocks from dredging, drilling and field work on the area’s few islands as well as New Zealand itself. As well as being at a relatively high elevation – a mean of -1100 m compared with -3700 for the oceans as a whole – samples are  predominantly those expected from continental crust. In fact orogenic belts exposed in New Zealand can be traced lithologically and topographically on several large submerged ridges. Samples of its underlying mantle found as xenoliths in igneous rocks yield radiometric dates of 2.7 billion years. So it is an ancient entity, unlike oceanic crust none of which exceeds about 200 Ma. The ocean floor also exhibits a number of sedimentary basins dating back to the Cretaceous, which contain terrigenous clastic rocks and limestones that reach thicknesses of 2 to 10 km. Seismic surveys give an average P-wave speed of 6.5 km s-1 through the underlying crust, a density of 2830 kg m-3 and a crustal thickness between 30 and 46 km, none of which apply to mafic oceanic crust.

There are plenty of areas on the ocean floor that have such continental affinities, but they are small and referred to as microcontinents. To be dubbed ‘continent’ obviously involves an essence of mightiness, but for geologists the term also implies a lack of connection: hence Europe is a mere part of the Eurasian continent. The six geologically recognised continents (Africa, Eurasia, North America, South America, Antarctica, and Australia) are spatially isolated by geological and/or bathymetric features. Zealandia obeys that criterion, but only just: its NW end comes as close as 25 km to the crust of Australia, but the line of separation is a major fracture zone and 3600 m deep trough. However, Zealandia is considerably smaller than the recognised continents, but about the size of India and Arabia which some have regarded as having been a continent (India) and one in the process of formation (Arabia). Mortimer and co suggest that the size needed to be called a continent should be >1 million km2, which would clearly put New Zealand on a continent separate from Australia – long a source of irritation to Kiwis!

Setting aside any suggestion of some nationalist motives, Zealandia is very interesting. The very fact that it is uniquely drowned require some explanation. A great deal of evidence suggests that once being at the flank of Gondwanaland an extensional plate margin spalled it away around 85 Ma ago. In so doing tectonic forces substantially thinned the crust in a similar manner to what is presently happening on a smaller scale beneath the Afar Depression of Ethiopia. That would tend to result in widespread subsidence once any thermal buoyancy during rifting had cooled to increase crustal density. Such a process would explain the alternations of linear ridges and troughs that characterise this section of continental crust, but are less developed in the other continents.

More on continental growth and plate tectonics

http://www.bbc.co.uk/news/world-asia-39000936

https://www.thesun.co.uk/news/2887128/what-is-zealandia-new-continent/

Archaean continents derived from Hadean oceanic crust

As DNA is to tracing  human evolution and migration, so various isotope systems are to the evolution of the Earth. One of the most fruitful is the samarium-neodymium (Sm-Nd) system. The decay of 147Sm to 143Nd is used in dating rocks across the full range of Earth history, given coeval rocks with a suitable range of Sm/Nd ratios, because the decay has a long half life (1.06 x 1011 years). However, samarium has another radioactive isotope 147Sm with a half life that is a thousand times shorter (1.06 x 108 years). So it remains only as a minute proportion of the total Sm in rocks, most having decayed since it was formed in a pre-Solar System supernova. But its daughter isotope 142Nd is present in easily measurable quantities, having accumulated from 147Sm decay over the first few hundred million years of Earth’s history; i.e. during the Hadean and earliest Archaean Eons. It is this fact that allows geochemists to get an indirect ‘handle’ on events that took place in the Earth’s earliest, largely vanished history. The principle behind this approach is that when an ancient rock undergoes partial melting to produce a younger magma the rock that crystallizes from it inherits the relative proportions of Nd isotopes of its source and thereby carries a record of the earlier history.

English: An outcrop of metamorphosed volcanose...

Metamorphosed volcanosedimentary rocks from the Porpoise Cove locality, Nuvvuagittuq supracrustal belt, Canada. Possibly the oldest rocks on Earth. (credit: Wikipedia)

The eastern shore of Hudson Bay in Canada hosts the oldest tangible geology known, in form of some metamorphosed basaltic rocks dated at 4200 Ma old known as the Nuvvuagittuq Greenstone Belt – the only known Hadean rocks. They occur in a tiny (20 km2) patch associated with gneisses of tonalite-trondjhemits-granodiorite composition that are dated between 3760 and 3350 Ma. Engulfing both are younger (2800 to 2500 Ma) Archaean plutonic igneous rocks of felsic composition. Jonathan O’Neil and Richard Carlson of the University of Ottawa, Canada and the Carnegie Institution for Science, Washington DC, USA respectively, measured proportions of Nd isotopes in both sets of felsic igneous rocks (O’Neil, J. & Carlson, R.W. 2017. Building Archean cratons from Hadean mafic crust. Science, v. 355, p. 1199-1202; doi:10.1126/science.aah3823).

The oldest gneisses contained relative proportions of 142Nd commensurate with them having been formed by partial melting of the Hadean mafic rocks about a few hundred million years after they had been erupted to form the oldest known crust; no surprise there. However, the dominant components of the local continental crust that are about a billion years younger also contain about the same relative proportions of 142Nd. A reasonable conclusion is that the Archaean continental crust of NE Canada formed by repeated melting of mafic crust of Hadean age over a period of 1.5 billion years. The modern Earth continually replenishes its oceanic crust over about 200 Ma due to plate tectonics. During the Archaean mantle dynamics would have been driven faster by much higher internal heat production. Had this involved simply faster plate tectonics the outermost skin of mafic crust would have been resorbed into the mantle even faster. By the end of the Archaean (2500 Ma) barely any Hadean crust should have been available to produce felsic magmas. But clearly at least some did linger, adding more weight to the idea that plate tectonics did not operate during the Hadean and Archaean Eons. See Formation of continents without subduction below.

Formation of continents without subduction

The formulation of the theory of plate tectonics provided plausible explanations for the growth of continental crust over time, among many other fundamental Earth processes. Briefly expressed, once basalt capped oceanic lithosphere is forced downwards at plate boundaries where plates move towards one another, beyond a certain penetration cool, moist basalt undergoes a pressure-controlled change of state. Its chemical constituents reassemble into minerals more stable under elevated pressure. In doing so, one outcome involves dehydration reactions the other being that the bulk composition is recast mainly in the form of high-pressure pyroxene and the mineral garnet: the rock eclogite. The density of the basaltic cap increases above that of the mantle. Gravity acts to pull the subducting slab downwards, this slab-pull force being the main driver of plate motions globally. Water vapour and other fluids shed by dehydration reactions rise from the subducted slab into the wedge of overlying mantle to change its conditions of partial melting and the composition of the magma so produced. This is the source of arc magmatism that persists at the destructive plate margin to increase the volcanic pile’s thickness over time. When magma is able to pond at the base of the new crust its fractional crystallisation produces dense cumulates of high-temperature mafic silicates and residual melt that is both lighter and more enriched in silica. Residual magma rises to add to the middle and upper crust while the cumulate-rich lower crust becomes less gravitationally stable, eventually to spall downwards by delamination. Such a process helps to explain the bulk low density of continental crust built up over time together with the freeboard of continents relative to the ocean floor: a unique feature of the Earth compared with all other bodies in the Solar System. It also accounts for the vast bulk of continental crust having remained at the surface since it formed: it rarely gets subducted, if at all.

One suggested model for pre-plate tectonic continent formation (credit, Robert J Stern https://speakingofgeoscience.org/2013/04/28/when-did-plate-tectonics-begin-on-earth-and-what-came-before/)

Tangible signs that such subduction was taking place in the past – eclogites and other high-pressure, low-temperature metamorphosed basalts or blueschists – are only found after 800 Ma ago. Before that time evidence for plate tectonics is circumstantial. Some geologists have argued for a different style of subduction in earlier times, plates under riding others at low angles. Others have argued for a totally different style of tectonics in Earth early history, marked by changes in bulk chemical composition of the continental crust at the Archaean-Proterozoic boundary. A new twist comes from evidence in the Archaean Pilbara Craton of Western Australia (Johnson, T.E. et al. 2017. Earth’s first stable continents did not form by subduction. Nature, v. 543, p. 239-242; doi:10.1038/nature21383). The authors found that basalts dated at about 3.5 Ga have trace-element geochemistry with affinities to the primitive basalts of island arcs. That makes them a plausible source for slightly younger felsic plutonic rocks with a tonalite-trondhjemite-granodiorite (TTG) compositional range (characteristic of Archaean continental crust). If the basalts were partially melted to yield 30% of their mass as new magma the melt composition would match that of the TTG crust. This would be feasible at only 30 km depth given a temperature increase with depth of at least 25° C per kilometre; more than the average continent geothermal gradient today but quite plausible with the then higher heat production by less decayed radioactive isotopes of uranium, thorium and potassium 3.5 Ga ago. This would have required the basalts to have formed a 30 km thick crust. However, the basalts’ geochemistry requires their generation by partial melting of earlier more mafic basalts rather than directly from the mantle. That early Archaean mantle melting probably did generate vast amounts such primary magma is generally acknowledged and confirmed by the common occurrence of komatiitic lavas with much higher magnesium content than common basalts of modern constructive margins. In essence, Johnson et al. favour thermal reworking of primitive Archaean crust, rather than reworking in a plate tectonic cycle.

More on continental growth and plate tectonics

See also

When did Plate Tectonics begin on Earth, and what came before?

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.