Category: Tectonics

The oldest and most stable parts of the continents are known as cratons, after the Greek word for strength κράτο (kratos). All the present continents have at least one craton (Africa and South America have 4 each, and Eurasia 6 or 7). Each has remained unaffected by major deformation for a billion years or more, even during continent-to-continent collisions in which they participated. Almost all cratons began to form during the Archaean Eon before 2500 Ma, but most became rigid long after. Several theories have been suggested to account for their durability, one commonly accepted being that somehow the crust ‘ripened’ so that most of the heat-producing radioactive isotopes of U, Th and K were moved by igneous and metamorphic processes to the uppermost crust, along with water; most cratons expose fragments of anhydrous granulites of tonalitic composition. These bear evidence of having formed at the base of the continental crust and have been heavily depleted in “granitophile” trace elements. As a result they cannot undergo partial melting under normal geothermal conditions and where they remain at great depth are much cooler than younger, deep crust. The other dominant feature of cratonic lithosphere is a mantle portion that is anomalously thick (sometimes down to 250 km); in some cases there is little if any sign of asthenosphere beneath such ‘keels’. Research on rocks brought up from the ‘roots’ of cratons by the kimberlite magmas famous for their diamonds points to that deep mantle itself having conferred great rigidity and thus longevity (Peslier, A.H. et al. 2010. Olivine water contents in the continental lithosphere and the longevity of cratons. Nature, v. 467, p. 78-81).

The presence of water in minerals that make up igneous and metamorphic rocks enables them to begin melting at lower temperatures than their dry equivalents, and also to behave in a more plastic fashion under stress. Anne Peslier of NASA in Houston and her US and German colleagues analysed the minerals in ultramafic mantle rocks dragged upwards by kimberlites that punched through the Kaapvaal craton in southern Africa long after it formed. The dominant mantle mineral is olivine (50-80%), generally thought of as anhydrous but typically containing a few hundred parts per million by weight. Olivines in the Kaapvaal mantle xenoliths become drier with increasing depth of their formation (determined from their mineralogy in which garnet is stable at the deepest levels). At depths around 150-250 km low water content in olivine makes it and the mantle itself 20 to 3000 times stronger than the asthenosphere, which protects it from the underlying flow associated with tectonic motions.

How such root zone of continents may have formed has been addressed by two papers on seismic structure beneath the best studied craton; that of the Canadian Shield (Yuan, H. & Romanowicz, B. 2010. Lithospheric layering in the North American craton. Nature, v. 466, p. 1063-1068; Miller, M.S. & Eaton, D.W. 2010. Formation of cratonic mantle keels by arc accretion: Evidence from S receiver functions. Geophysical Research Letters, v. 37, doi:10.1029/2010GL044366). In the first, Yuan and Romanowicz of the Berkeley Seismological Laboratory, California use a form of seismic tomography to map anisotropy in the mantle along transects that cross the ancient core of the North American continent. Their results chart the depth of the base of the lithosphere and also define two layers in the lithospheric mantle. The upper layer (down to 150 km) only occurs beneath the Archaean craton, and the top of the asthenosphere ranges from 100-240 km down: at its deepest beneath the craton. The sub-craton mantle they ascribe to chemical depletion of its upper part during early lithospheric evolution, and later addition of the less chemically evolved deeper layer. Miller and Eaton of the Universities of California USA and Calgary Canada used S-wave data from eight seismic stations extending from WSW to ENE over the western cordillera and the Canadian Shield to the Arctic islands of Canada. Their results show a similar variation in dept of the base of the lithosphere and resolve several roughly eastward-dipping boundaries in the sub-craton lithospheric mantle, which they link to Precambrian volcanic arcs preserved in the upper crust above them; i.e. suggesting that the upper layer in the first paper stems from a major episode of arc accretion that built the Canadian Shield.

The oldest and most stable parts of the continents are known as cratons, after the Greek word for strength κράτο (kratos). All the present continents have at least one craton (Africa and South America have 4 each, and Eurasia 6 or 7). Each has remained unaffected by major deformation for a billion years or more, even during continent-to-continent collisions in which they participated. Almost all cratons began to form during the Archaean Eon before 2500 Ma, but most became rigid long after. Several theories have been suggested to account for their durability, one commonly accepted being that somehow the crust ‘ripened’ so that most of the heat-producing radioactive isotopes of U, Th and K were moved by igneous and metamorphic processes to the uppermost crust, along with water; most cratons expose fragments of anhydrous granulites of tonalitic composition. These bear evidence of having formed at the base of the continental crust and have been heavily depleted in “granitophile” trace elements. As a result they cannot undergo partial melting under normal geothermal conditions and where they remain at great depth are much cooler than younger, deep crust. The other dominant feature of cratonic lithosphere is a mantle portion that is anomalously thick (sometimes down to 250 km); in some cases there is little if any sign of asthenosphere beneath such ‘keels’. Research on rocks brought up from the ‘roots’ of cratons by the kimberlite magmas famous for their diamonds points to that deep mantle itself having conferred great rigidity and thus longevity (Peslier, A.H. et al. 2010. Olivine water contents in the continental lithosphere and the longevity of cratons. Nature, v. 467, p. 78-81).

The presence of water in minerals that make up igneous and metamorphic rocks enables them to begin melting at lower temperatures than their dry equivalents, and also to behave in a more plastic fashion under stress. Anne Peslier of NASA in Houston and her US and German colleagues analysed the minerals in ultramafic mantle rocks dragged upwards by kimberlites that punched through the Kaapvaal craton in southern Africa long after it formed. The dominant mantle mineral is olivine (50-80%), generally thought of as anhydrous but typically containing a few hundred parts per million by weight. Olivines in the Kaapvaal mantle xenoliths become drier with increasing depth of their formation (determined from their mineralogy in which garnet is stable at the deepest levels). At depths around 150-250 km low water content in olivine makes it and the mantle itself 20 to 3000 times stronger than the asthenosphere, which protects it from the underlying flow associated with tectonic motions.

How such root zone of continents may have formed has been addressed by two papers on seismic structure beneath the best studied craton; that of the Canadian Shield (Yuan, H. & Romanowicz, B. 2010. Lithospheric layering in the North American craton. Nature, v. 466, p. 1063-1068; Miller, M.S. & Eaton, D.W. 2010. Formation of cratonic mantle keels by arc accretion: Evidence from S receiver functions. Geophysical Research Letters, v. 37, doi:10.1029/2010GL044366).In the first, Yuan and Romanowicz of the Berkeley Seismological Laboratory, California use a form of seismic tomography to map anisotropy in the mantle along transects that cross the ancient core of the North American continent. Their results chart the depth of the base of the lithosphere and also define two layers in the lithospheric mantle. The upper layer (down to 150 km) only occurs beneath the Archaean craton, and the top of the asthenosphere ranges from 100-240 km down: at its deepest beneath the craton. The sub-craton mantle they ascribe to chemical depletion of its upper part during early lithospheric evolution, and later addition of the less chemically evolved deeper layer. Miller and Eaton of the Universities of California USA and Calgary Canada used S-wave data from eight seismic stations extending from WSW to ENE over the western cordillera and the Canadian Shield to the Arctic islands of Canada. Their results show a similar variation in dept of the base of the lithosphere and resolve several roughly eastward-dipping boundaries in the sub-craton lithospheric mantle, which they link to Precambrian volcanic arcs preserved in the upper crust above them; i.e. suggesting that the upper layer in the first paper stems from a major episode of arc accretion that built the Canadian Shield.

The discovery in the 1970s that some low-angled faults have an extensional or normal sense of displacement stemmed from extensional systems in the continental crust, exemplified by the Basin and Range Province of western North America. Yet the largest extensional systems on Earth are those associated with mid-ocean ridges, and in the 1980s some of those were shown to involve low-angled detachments too. Michael Cheadle and Craig Grimes (University of Wyoming and Mississippi State University, USA) review the latest word on oceanic extensional complexes revealed at the AGO Chapman Conference in May 2010 (Cheadle, M. & Grimes, C. 2010. To fault or not to fault. Nature Geoscience, v. 3, p.454-456). As in continental extension, this kind of deformation at divergent margins may produce core complexes uplifted as a result of tectonic unroofing by low-angled detachments, thereby revealing oceanic mantle lithosphere on the ocean floor. Such peculiarities seem to be absent from fast spreading ridges such as the East Pacific Rise and occur where spreading is slow. They are best developed where spreading is starved of magma injection to produce the classic sheeted-dyke complexes of the middle oceanic crust, and with unusually thick oceanic lithosphere. Yet the ocean floor must spread at these localities, and that is achieved by extensional tectonics that accommodates up to 125 km of spreading with next to no magmatism: 4 Ma-worth of spreading.

For extensional faults to develop into low-angled detachments rocks must be weak, otherwise simple steep, domino-style faults would form. Penetration of seawater down faults weakens oceanic lithosphere through hydration reactions that produce clays and serpentines, which encourage the formation of ductile shear zones. Interestingly, some of the largest hydrothermal systems on the mid-Atlantic Ridge coincide with core complexes, and exude hydrogen – a product of serpentinisation – as well as methane and metal-rich brines.

The last 40 to 50 years have seen the theory of plate tectonics supported by more and more empirical evidence from sea-floor magnetism, seismicity, bathymetry and a growing number of other features that relate to Earth’s dynamism. Yet the original concepts of rigid plates and their dislocation from one another and the underlying mantle have been undermined to a degree by the wealth of data now available. Increasing resolution of seismic tomography is revealing what is happening in the depths of the mantle on which growing confidence can be placed. Matching these increasingly revealing sources of data has been the computing power to try to blend them all with rheological theory and thereby model the way the world works. The latest of these modelling ventures does seem to move plate theory onto a significantly higher plane (Stadler, G. et al. 2010. The dynamics of plate tectonics and mantle flow: from local to global scales. Science, v. 329, p. 1033-1038). The keys to this step are: increasingly sophisticated software that encompasses the contributory factors, akin to models used by mechanical and hydraulic engineers; faster computing that allows a decrease in the size of the 3-D cells used in assessing all the interactions as realistically as possible, and a great deal of graphic creativity so that we can visualise the results. At its centre is varying rock strength, the principal ‘engineering’ input derived from seismic tomography, blended with the gravitational and thermal forces that drive Earth’s ‘engine’.

Stadler et al.’s development divides up the planet into a 3-D mesh whose resolution varies according to the likely complexity of motions within and upon the Earth. For instance there is not much call for detail for what lies below abyssal plains of the ocean floor, so available computing power can be focused on the more intricate parts of the tectonic set-up, especially subduction zones that are both the most spectacular features of the Earth’s behaviour and the source of the main force that drives its surface parts – slab pull. Already the approach is producing more questions than answers. For instance, building in the data that show something of convection in the deep mantle makes the model’s output for the more shallow-seated and better known processes deviate more than expected from what is observed – less comprehensive and more coarse approaches previously seemed to be match deep and shallow processes quite well. This is a difficult topic to express merely in words, but fortunately the paper has been made freely available at http://users.ices.utexas.edu/~carsten/papers/StadlerGurnisBursteddeEtAl10.pdf

See also: Becker, T. 2010. Fine-scale modelling of global plate tectonics. Science, v. 329, p. 1020-1021.

more empirical evidence from sea-floor magnetism, seismicity, bathymetry and a growing number of other features that relate to Earth’s dynamism. Yet the original concepts of rigid plates and their dislocation from one another and the underlying mantle have been undermined to a degree by the wealth of data now available. Increasing resolution of seismic tomography is revealing what is happening in the depths of the mantle on which growing confidence can be placed. Matching these increasingly revealing sources of data has been the computing power to try to blend them all with rheological theory and thereby model the way the world works. The latest of these modelling ventures does seem to move plate theory onto a significantly higher plane (Stadler, G. et al. 2010. The dynamics of plate tectonics and mantle flow: from local to global scales. Science, v. 329, p. 1033-1038). The keys to this step are: increasingly sophisticated software that encompasses the contributory factors, akin to models used by mechanical and hydraulic engineers; faster computing that allows a decrease in the size of the 3-D cells used in assessing all the interactions as realistically as possible, and a great deal of graphic creativity so that we can visualise the results. At its centre is varying rock strength, the principal ‘engineering’ input derived from seismic tomography, blended with the gravitational and thermal forces that drive Earth’s ‘engine’

Stadler et al.’s development divides up the planet into a 3-D mesh whose resolution varies according to the likely complexity of motions within and upon the Earth. For instance there is not much call for detail for what lies below abyssal plains of the ocean floor, so available computing power can be focused on the more intricate parts of the tectonic set-up, especially subduction zones that are both the most spectacular features of the Earth’s behaviour and the source of the main force that drives its surface parts – slab pull. Already the approach is producing more questions than answers. For instance, building in the data that show something of convection in the deep mantle makes the model’s output for the more shallow-seated and better known processes deviate more than expected from what is observed – less comprehensive and more coarse approaches previously seemed to be match deep and shallow processes quite well. This is a difficult topic to express merely in words, but fortunately the paper has been made freely available at http://users.ices.utexas.edu/~carsten/papers/StadlerGurnisBursteddeEtAl10.pdf

See also: Becker, T. 2010. Fine-scale modelling of global plate tectonics. Science, v. 329, p. 1020-1021.

Low-angle extensional detachments at ocean ridges

The discovery in the 1970s that some low-angled faults have an extensional or normal sense of displacement stemmed from extensional systems in the continental crust, exemplified by the Basin and Range Province of western North America. Yet the largest extensional systems on Earth are those associated with mid-ocean ridges, and in the 1980s some of those were shown to involve low-angled detachments too. Michael Cheadle and Craig Grimes (University of Wyoming and Mississippi State University, USA) review the latest word on oceanic extensional complexes revealed at the AGO Chapman Conference in May 2010 (Cheadle, M. & Grimes, C. 2010. To fault or not to fault. Nature Geoscience, v. 3, p.454-456). As in continental extension, this kind of deformation at divergent margins may produce core complexes uplifted as a result of tectonic unroofing by low-angled detachments, thereby revealing oceanic mantle lithosphere on the ocean floor. Such peculiarities seem to be absent from fast spreading ridges such as the East Pacific Rise and occur where spreading is slow. They are best developed where spreading is starved of magma injection to produce the classic sheeted-dyke complexes of the middle oceanic crust, and with unusually thick oceanic lithosphere. Yet the ocean floor must spread at these localities, and that is achieved by extensional tectonics that accommodates up to 125 km of spreading with next to no magmatism: 4 Ma-worth of spreading.

For extensional faults to develop into low-angled detachments rocks must be weak, otherwise simple steep, domino-style faults would form. Penetration of seawater down faults weakens oceanic lithosphere through hydration reactions that produce clays and serpentines, which encourage the formation of ductile shear zones. Interestingly, some of the largest hydrothermal systems on the mid-Atlantic Ridge coincide with core complexes, and exude hydrogen – a product of serpentinisation – as well as methane and metal-rich brines

One of the elements comprising the canon of plate tectonics is that as plates spread away from constructive margins the depth to the ocean floor increases in direct proportion to the square root of the underling lithosphere’s age. This is generally considered to reflect steady passive cooling and increasing density of initially hot lithosphere produced at ridge systems. The resulting slope of the ocean floor is said to result in one of the gravitational forces that sustain plate tectonics – ‘ridge slide’. The Pacific Ocean floor is a good test for the hypothesis, but unfortunately does not show a linear depth vs Öage relationship (Adam, C. & Vidal, V. 2010. Mantle flow drives the subsidence of oceanic plates. Science, v. 328, p. 83-85). Instead, the ocean floor flattens out beyond a threshold distance, which has been a source of puzzlement for decades. However, a plot of depth against the square root of distance from the ridge along estimated lines of mantle convective flow is consistently linear. The depth curve seems therefore to reflect past changes in the direction of sea-floor spreading and changes in the deeper mantle convection, thereby linking reality to the original model for continental drift that had mantle convection at its heart. That view was discarded by geophysicists on account of a widespread belief that the asthenosphere was too weak to transmit forces from below to the rigid lithospheric plates.

Riven by the effects of at least two Wilson cycles of rifting drifting and collision, and then covered by a variety of later sediments, late-Precambrian rocks at high latitudes around today’s North Atlantic are nowhere near as coherent as their counterparts in, for instance, Africa. Also they have a long history of field investigation that began long before the unifying theory of plate tectonics, using a parochial rather than a ‘joined-up’ approach. Consequently there is a vast literature, as witness that of say the Moines or the Dalradian in Scotland, which has strangely acted as a hindrance rather than a boon to synthesisers: not that attempts haven’t been made in recent decades. Interestingly, a multi-hemisphere approach to unification, combining Australian and British geologists, seems to have made a great deal of ground (Cawood, P.A. et al. 2010. Neoproterozoic orogeny along the margin of Rodinia: Valhalla orogen, North Atlantic. Geology, v. 38, p. 99-102).

The Rodinia (‘Motherland’) supercontinent united all continental lithosphere at the end of the Mesoproterozoic era, existed between 1100 and 750 Ma, then broke into eight drifting continents during the Neoproterozoic. Like the later Pangaea (‘all of mother Earth’) formed when all these wandering masses finally clanged together again, conditions deep in the interior of Rodinia were probably tectonically and geomorphologically almost static. All the action would have been around its rim, towards which much of global sea-floor spreading ultimately was directed. Far older continental material now juxtaposed across the high-latitude North Atlantic was in just such an exposed position at the edge of the supercontinent; Greenland abutting the present Baltic crystalline mass. Local sea-floor spreading twisted Baltica from this part of Rodinia in a clockwise manner, to leave a large triangular sea in its wake. This Asgard Sea (why not Toblerone?) received debris from uplifted masses of older crust, to fill a deep sedimentary basin ready for deformation should tectonics warrant that. Two such episodes (980-910, 830-710 Ma) created the older Neoproterozoic metamorphic belts which have long drawn geologists to study Greenland, Scotland and Scandinavia in great detail: for British geologists the attraction was the complexity of the Moine Schists in which John Ramsay famously laid the foundations of modern polyphase structural analysis in the late 1950s and 1960s. A noteworthy point is that by comparison with most mountain belts, the Valhalla orogen took an awfully long time to form: around 300 Ma.

An old theory resurrected

Before the wide acceptance of sea-floor spreading and continental drift geoscientists had to seek explanations for the common occurrence of very similar fossils on now widely separated land masses. On the other hand, Alfred Wegener used observations such as the presence of fossilised tongue-like Glossopteris leaves in the Permian sediments of all the southern continents, and similar distributions of reptiles to support his theory. His detractors tried to explain away the fossil evidence by suggesting now-vanished land bridges, ‘island hopping’, floating seeds, and natural Noah’s Arks carrying animals and so on. With the discovery of irrefutable evidence for sea-floor spreading Wegener was vindicated, albeit long after his death, and the views of his detractors became ridiculed and neglected in their turn. But one puzzle remained: the fauna of Madagascar. Beginning about 170 Ma ago, Madagascar along with India parted company with Africa, to the extent that Madagascar is now more than 430 km off the East African coast (India moved much further independently).

Madagascar, of course, is famous for its lemurs but its fauna includes other animals found nowhere else. Another oddity is that late-Mesozoic Malagasy sediments have yielded no evidence for ancestors to these animals, so the fauna could not have evolved from African stock set adrift with the microcontinent. The only explanation then seems to be that the little animal ancestors drifted on vegetation rafts from Africa – note this would be more unlikely for large animals. Yet today’s current patterns make any drift toward Madagascar highly unlikely. The puzzle may have been resolved, if one believes computer modelling, by the different surface flow patterns of the Indian Ocean during the Palaeocene (Ali, J.R. & Huber, M. 2010. Mammalian biodiversity on Madagascar controlled by ocean currents. Nature, v. 463, p. 653-656). At that time the drifting island was further south than it is now, and currents would intermittently have flowed from East Africa towards it. As it was driven northwards, so it entered the influence of the westward flowing, South Equatorial Current that now isolates it from its parent continent. The idea of rafting, first developed in 1940 by George Gaylord Simpson, an opponent of anything smacking of continental drift, also seems the only possibility if the arrival of New World monkeys in South America and other oddities are to be explained.

See also: Krause, D.W. 2010. Washed up in Madagascar. Nature, v. 463, p. 613-614.

The most distinctive products of the high-pressure, low-temperature metamorphism along subduction zones are stunningly coloured blueschists formed from ocean-floor basalts, their colour deriving from the sodium-rich amphibole glaucophane. Yet the defining mineral for subduction-zone metamorphism is lawsonite, which takes up the calcium from plagioclase feldspar that becomes unstable. Having formed at depths of up to 100 km, blueschists found at the surface had to rise slowly from mantle depths after metamorphism. Consequently, it is nearly impossible to unravel the date of their formation from those of later events. Being basaltic, blueschists also lack the usual elements whose unstable isotopes are commonly used for radiometric dating: potassium, rubidium, uranium and thorium. However, they do contain rare-earth elements, an isotope of one (¹⁷⁶Lu) being unstable. Applying the Lu-Hf dating method to lawsonite ties down precisely when basalts achieved the narrow P-T range at which lawsonite forms (Mulcahy, S.R. et al. 2009. Lawsonite Lu-Hf geochronology: A new geochronometer for subduction zone processes. Geology, v. 37, p. 987-990). Sean Mulcahy of the Unigversity of Nevada and colleagues from Washington State chose a sample from the type locality for lawsonite discovered in the late 19^th century by Andrew Lawson: the Franciscan blueschists of the Tiburon Peninsula in California. The Franciscan Complex formed during subduction at 145.5 Ma.

Phew, there is a mantle plume under Hawaii after all

Along with constructive and destructive plate boundaries volcanic hotspots within plates and sometimes at plate boundaries epitomise modern Earth science. Assuming that they are fixed points of reference allows the absolute motions of tectonic plates to be worked out, although it seems that some do move around. The evidence for hotspots being fixed or at least moving much more slowly than do plates are the chains of extinct volcanic islands or seamounts that extend away from active volcanic centres in the direction of plate motion. The most debated aspect of hotspots is whether they stem from processes in the upper mantle just beneath the asthenosphere or are the heads of cylindrical plumes of hot mantle that rise from the region next to the outer core. Seismic tomography has been claimed capable of resolving between the two possibilities, but its spatial resolution depends very much on the spacing of seismometers that provide the data that tomography subjects to highly complex processing. Some have claimed that the resolution of early tomography lends itself to producing artefacts that look like sought-after mantle structures (see Geoscience consensus challenged in EPN of December 2003).

One hotspot that has all the characteristics of a plume head, but which seismic tomography has been unable to detect is the volcanically active Big Island of the Hawaiian chain. The response to that somewhat embarrassing failure has been to deploy 30-odd seismometers on the seabed immediately around Hawaii and then to shift them to a wider spacing further from the island between 2005 to 2007. Together with 10 stations on the islands themselves, the array recorded 2146 S-wave arrivals from 97 earthquakes (Wolfe, C.J. et al. 2009. Mantle shear-wave velocity structure beneath the Hawaiian hot spot. Science, v. 326, p. 1388-1390). The results are reassuring, for the show in detail that indeed there is a vertical zone of low S-wave speeds indicating hotter, less rigid mantle that extends down to at least 1200 km. It is several hundred kilometres across, and is indeed a plume surrounded by a ‘tube’ of colder more rigid mantle.

See also: Kerr, R.A. 2009. Sea-floor study gives plumes from the deep mantle a boost. Science, v. 326, p. 1330.

Hot tectonics in the Archaean

The first thing that strikes you when looking at a small-scale geological maps of many deformed Archaean terrains – most of them are deformed – is how different they seem compared with those of later aeons. Bulbous granitic plutons separate slim and irregular, sometimes cusp-adorned areas of volcanic and sedimentary rocks. This is classic granite-greenstone terrain. Many geologists who have worked on Archaean rocks find it hard to pin down signs of ‘modern’ plate tectonics and the typical orogens of continent-continent collision zones, yet non-uniformitarian ideas on Archaean tectonics have become passé in the last 25-30 years. That seems odd, considering that the Earth’s internal heat production by radioactive decay must have been higher as less radioactive U, Th and K isotopes would have decayed in the very distant past. Convective mantle flow would have been faster, lithosphere would not have been so thick as now, and plates would have moved more rapidly in order that radioactive heat and that left over from early accretion and the Moon-forming event could escape. Whichever way one looks at such a scenario – plates as big as modern ones or more small plates – there is no escaping that younger, warmer lithosphere would have re-entered the mantle. Geochemistry of Archaean granitic rocks is so different from those of later aeons that their formative processes must have differed too. Quite probably descending basaltic crust would not have dehydrated to produce eclogite under low-T, high-P conditions, and that would prevent steep subduction, so that slab-pull may not have been the driving force for Archaean tectonics.

Two recent papers refresh the idea that the present is not entirely a key to the Earth’s Archaean past. One suggests an entirely alien kind of orogenic activity: that of very hot deformation of weak lithosphere (Chardon, D. et al. 2009. Flow of ultra-hot orogens: A view from the Precambrian, clues for the Phanerozoic. Tectonophysics, v. 477, p. 105-108). Dominique Chardon of the Université de Toulouse and colleagues from the Université de Rennes, highlight the dominance in orogens of the Archaean and early Proterozoic of ductile deformation imposed on massive accretion of magma produced by mantle processes, compared with the dominantly brittle style that dominates modern, cold orogens. Accumulated radiometric dating of the main building material of the continents – diorites and grandiorites – indicates that the 1.5 Ga of the Archaean witnessed the formation of not only the earliest continental crust but most (65%) of the rest of it. A summary of an emerging explanation for explosive continent production appeared in the first 2010 issue of Scientific American (Simpson, S. 2009. Violent origins of continents. Scientific American v. 302(1), p. 46-53). This rests on rapidly growing evidence, much unearthed by Andrew Glikson of the Australian National University, for the influence of major impacts that flung debris far and wide and perturbed the mantle’s thermal structure on a massive scale (Glikson, A. 2008. Field evidence for Eros-scale asteroids and impact forcing of Precambrian geodynamic episodes, Kaapvaal (south Africa) and Pilbara (Western Australia) cratons. Earth and Planetary Science Letters, v. 267, p. 558-570). Beds of impact-related spherules are turning up throughout Archaean greenstone-belt sequences. There are also megabreccias that could be debris lifted by tsunamis vcaused by impacts in the Archaean oceans. Glikson has demonstrated that the timing of such evidence closely matches that of magmatic outbursts that created continental crust. He has proposed that the thermal effects of the large impacts set in motion or deflected a large number of convective mantle plumes that drove the necessary magmatism.