Category: Tectonics

The ‘boring billion’ years of the Mesoproterozoic: plate tectonics and the eukaryotes

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The emergence of the eukaryotes – of which we are a late-entry member – has been debated for quite a while. In 2023 Earth-logs reportedthat a study of ‘biomarker’ organic chemicals in Proterozoic sediments suggests that eukaryotes cannot be traced back further than about 900 Ma ago using such an approach. At about the same time another biomarker study showed signs of a eukaryote presence at around 1050 Ma. Both outcomes seriously contradicted a ‘molecular-clock’ approach based on the DNA of modern members of the Eukarya and estimates of the rate of genetic mutation. That method sought to deduce the time in the past when the last eukaryotic common ancestor (LECA) appeared. It pointed to about 2 Ga ago, i.e. a few hundred million years after the Great Oxygenation Event got underway. Since eukaryote metabolism depends on oxygen, the molecular-clock result seems reasonable. The biomarker evidence does not. But were the Palaeo- and Mesoproterozoic Eras truly ‘boring’? A recent paper by Dietmar Müller and colleagues from the Universities of Sydney and Adelaide, Australia definitely shows that geologically they were far from that (Müller, R.D. et al. 2025. Mid-Proterozoic expansion of passive margins and reduction in volcanic outgassing supported marine oxygenation and eukaryogenesis. Earth and Planetary Science Letters, v. 672; DOI: 10.1016/j.epsl.2025.119683).

Carbon influx (million tons per year) into tectonic plates and into the ocean-atmosphere system from 1800 Ma to present. The colour bands represent: total carbon influx into the atmosphere (mauve); sequestered in tectonic plates (green); net atmospheric influx i.e. total minus carbon sequestered into plates (orange). The widths of the bands show the uncertainties of the calculated masses shown as darker coloured lines.

From 1800 to 800 Ma two supercontinents– Nuna-Columbia and Rodinia – aggregated nearly all existing continental masses, and then broke apart. Continents had collided and then split asunder to drift. So plate tectonics was very active and encompassed the entire planet, as Müller et al’s palaeogeographic animation reveals dramatically. Tectonics behaved in much the same fashion through the succeeding Neoproterozoic and Phanerozoic to build-up then fragment the more familiar supercontinent of Pangaea. Such dynamic events emit magma to form new oceanic lithosphere at oceanic rift systems and arc volcanoes above subduction zones, interspersed with plume-related large igneous provinces and they wax and wane. Inevitably, such partial melting delivered carbon dioxide to the atmosphere. Reaction on land and in the rubbly flanks of spreading ridges between new lithosphere and dissolved CO2 drew down and sequestered some of that gas in the form of solid carbonate minerals. Continental collisions raised the land surface and the pace of weathering, which also acted as a carbon sink. But they also involved metamorphism that released carbon dioxide from limestones involved in the crustal transformation. This protracted and changing tectonic evolution is completely bound up through the rock cycle with geochemical change in the carbon cycle.

From the latest knowledge of the tectonic and other factors behind the accretion and break-up of Nuna and Rodinia, Müller et al. were able to model the changes in the carbon cycle during the ‘boring billion’ and their effects on climate and the chemistry of the oceans. For instance, about 1.46 Ga ago, the total length of continental margins doubled while Nuna broke apart. That would have hugely increased the area of shallow shelf seas where living processes would have been concentrated, including the photosynthetic emission of oxygen. In an evolutionary sense this increased, diversified and separated the ecological niches in which evolution could prosper. It also increased the sequestration of greenhouse gas through reactions on the flanks of a multiplicity of oceanic rift systems, thereby cooling the planet. Translating this into a geochemical model of the changing carbon cycle (see figure) suggests that the rate of carbon addition to the atmosphere (outgassing) halved during the Mesoproterozoic. The carbon cycle and probable global cooling bound up with Nuna’s breakup ended with the start of Rodinia’s aggregation about 1000 Ma ago and the time that biomarkers first indicate the presence of eukaryotes.

Simplified structures of (a) a prokaryote cell; (b) a simple eukaryote animal cell. Plants also contain organelles called chloroplasts

So, did tectonics play a major role in the rise of the Eukarya? Well, of course it did, as much as it was subsequently the changing background to the appearance of the Ediacaran animals and the evolutionary carnival of the Phanerozoic. But did it affect the billion-year delay of ‘eukaryogenesis’ during prolonged availability of the oxygen that such a biological revolution demanded? Possibly not. Lyn Margulis’s hypothesis of the origin of the basic eukaryote cell by a process of ‘endosymbiosis’ is still the best candidate 50 years on. She suggested that such cells were built from various forms of bacteria and archaea successively being engulfed within a cell wall to function together through symbiosis. Compared with prokaryote cells those of the eukaryotes are enormously complex. At each stage the symbionts had to be or become compatible to survive. It is highly unlikely that all components entered the relationship together. Each possible kind of cell assembly was also subject to evolutionary pressures. This clearly was a slow evolutionary process, probably only surviving from stage to stage because of the global presence of a little oxygen. But the eukaryote cell may also have been forced to restart again and again until a stable form emerged.

See also: New Clues Show Earth’s “Boring Billion” Sparked the Rise of Life. SciTechDaily, 3  November 2025

The final closure of the Iapetus Ocean

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A symposium hosted by the Royal Society in 1965 aimed at resurrecting Alfred Wegener’s hypothesis of continental drift. During the half century since Wegener made his proposal in 1915, it had been studiously ignored by most geologists. The majority had bumbled along with the fixist ideology of their Victorian predecessors. The symposium launched what can only be regarded as a revolution in the Earth Sciences. In the three years following the symposium, the basic elements of plate tectonics had emerged from a flurry of papers, mainly centred on geophysical evidence. Geology itself became part of this cause célèbre through young scientists eager to make a name for themselves. The geological history of Britain, together with that of the eastern North America, became beneficiaries only four years after the Royal Society meeting (Dewey, J. 1969. Evolution of the Appalachian/Caledonian Orogen. Nature 222, 124–129; DOI: 10.1038/222124a0).

In Britain John Dewey, like a few other geologists, saw plate theory as key to understanding the many peculiarities revealed by geological structure, igneous activity and stratigraphy of the early Palaeozoic. These included very different Cambrian and Ordovician fossil assemblages in Scotland and Wales, now only a few hundred kilometres apart. The Cambro-Ordovician of NW Scotland was bounded to the SE by a belt of highly deformed and metamorphosed Proterozoic to Ordovician sediments and volcanics forming the Scottish Highlands. That was terminated to the SE by a gigantic fault zone containing slivers of possible oceanic lithosphere. The contorted and ‘shuffled’ Ordovician and Silurian sediments of the Southern Uplands of Scotland. The oldest strata seemed to have ocean-floor affinities, being deposited on another sliver of ophiolites.  A few tens of km south of that there was a very different Lower Palaeozoic stratigraphy in the Lake District of northern England. It included volcanic rocks with affinities to those of modern island arcs. A gap covered by only mildly deformed later Palaeozoic shelf and terrestrial sediments, dotted by inliers of Proterozoic sediments and volcanics separated the Lake District from yet another Lower Palaeozoic assembly of arc volcanics and marine sediments in Wales. Intervening in Anglesey was another Proterozoic block of deformed sediments that also included ophiolites.

Dewey’s tectonic assessment from this geological hodge-podge, which had made Britain irresistible to geologists through the 19th and early 20th centuries, was that it had resulted from blocks of crust (terranes), once separated by thousands of kilometres, being driven into each other. Britain was thus formed by the evolution and eventual destruction of an early Palaeozoic ocean, Iapetus: a product of plate tectonics. Scotland had a fundamentally different history from England and Wales; the unification of several terranes having taken over 150 Ma of diverse tectonic processes. Dewey concluded that the line of final convergence lay at a now dead, major subduction zone – the Iapetus Suture – roughly beneath the Solway Firth. During the 56 years since Dewey’s seminal paper on the Caledonian-Appalachian Orogeny details and modifications have been added at a rate of around one to two publications per year. The latest seeks to date when and where the accretion of 6 or 7 terranes was finally completed (Waldron, J.W.F. et al. 2025. Is Britain divided by an Acadian suture?  Geology, v. 53, p. 847–852; DOI: 10.1130/G53431.1).

Kernel density plots – smoothed versions of histograms – of detrital zircon ages in Silurian and Devonian sandstones from Wales. The bracketed words are stratigraphic epochs. Credit: Waldron et al. 2025, Fig 3A

John Waldron and colleagues from the University of Alberta and Acadia University in Canada and the British Geological Survey addressed this issue by extracting zircons from four late Silurian and early Devonian sandstones in North and South Wales. These sediments had been deposited between 433 and 393 Ma ago at the southernmost edge of the British Caledonide terrane assemblage towards the end of terrane assembly. The team dated roughly 250 zircons from each sandstone using the 207Pb/206Pb and 206Pb/238U methods. Each produced a range of ages, presumed to be those of igneous rocks from whose magma the zircon grains had crystallised. These data are expressed as plots of probable frequency against age.  Each pattern of ages is assumed to be a ‘fingerprint’ for the continental crust from which the zircons were eroded and transported to their resting place in their host sediment. In this case, the researchers were hoping to see signs of continental crust from the other side of the Caledonian orogen; i.e. from the Precambrian basement of the Laurentia continent.

The three late-Silurian sediments showed distinct zircon-age peaks around 600 Ma and a spread of smaller peaks extending to 2.2 Ga. This tallied with a sediment source in Africa, from which the southernmost Caledonian terrane was said to have split and moved northwards.  The Devonian sediment lacked signs of such an African ‘heritage’ but had a prominent age peak at about 1.0 Ga, absent from the Welsh Silurian sediments.  Not only is this a sign of different sediment provenance but closely follows the known age of a widespread magmatic pulse in the Laurentian continent. So, sediment transport from the opposite side of the Iapetus Ocean across the entire Caledonian orogenic belt was only possible after the end of the Silurian Period at around 410 Ma. There must have been an intervening barrier to sediment movement from Laurentia before that, such as deep ocean water further north. Previous studies from more northern Caledonian terranes show that Laurentian zircons arrived in the Southern Uplands of Scotland and the English Lake District around 432 Ma in the mid-Silurian. Waldron et al. suggest, on these grounds that the suture marking the final closure of the Iapetus Ocean lies between the English Lake District and Anglesey, rather than beneath the Solway. They hint that the late-Silurian to early Devonian granite magmatism that permeated the northern parts of the Caledonian-Appalachian orogen formed above northward subduction of the last relics of Iapetus, which presaged widespread crustal thickening known as the Acadian orogeny in North America.

Readers interested in this episode of Earth history should download Waldron et al.’s paper for its excellent graphics, which cannot be reproduced adequately here.

Gravity survey reveals signs of Archaean tectonics in Canadian Shield

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Much of the Archaean Eon is represented by cratons, which occur at the core of continental parts of tectonic plates. Having low geothermal heat flow they are the most rigid parts of the continental crust.  The Superior Craton is an area that makes up much of the eastern part of the Canadian Shield, and formed during the Late Archaean from ~4.3 to 2.6 billion years (Ga) ago. Covering an area in excess of 1.5 million km2, it is the world’s largest craton. One of its most intensely studied components is the Abitibi Terrane, which hosts many mines. A granite-greenstone terrain, it consists of volcano-sedimentary supracrustal rocks in several typically linear greenstone belts separated by areas of mainly intrusive granitic bodies. Many Archaean terrains show much the same ‘stripey’ aspect on the grand scale. Greenstone belts are dominated by metamorphosed basaltic volcanic rock, together with lesser proportions of ultramafic lavas and intrusions, and overlying metasedimentary rocks, also of Archaean age. Various hypotheses have been suggested for the formation of granite-greenstone terrains, the latest turning to a process of ‘sagduction’. However the relative flat nature of cratonic areas tells geologists little about their deeper parts. They tend to have resisted large-scale later deformation by their very nature, so none have been tilted or wholly obducted onto other such stable crustal masses during later collisional tectonic processes. Geophysics does offer insights however, using seismic profiling, geomagnetic and gravity surveys.

The Geological Survey of Canada has produced masses of geophysical data as a means of coping with the vast size and logistical challenges of the Canadian Shield. Recently five Canadian geoscientists have used gravity data from the Canadian Geodetic Survey to model the deep crust beneath the huge Abitibi granite-greenstone terrain, specifically addressing variations in its density in three dimensions. They also used cross sections produced by seismic reflection and refraction data along 2-D survey lines (Galley, C. et al. 2025. Archean rifts and triple-junctions revealed by gravity modeling of the southern Superior Craton. Nature Communications, v. 16, article 8872; DOI: 10.1038/s41467-025-63931-z). The group found that entirely new insights emerge from the variation in crustal density down to its base at the Moho (Mohorovičić discontinuity). These data show large linear bulges in the Moho separated by broad zones of thicker crust.

Geology of the Abitibi Terrane (upper),; Depth to the Moho beneath the Abitibi Terrane with rifts and VMS deposits superimposed (lower). Credit: After Galley et al. Figs 1 and 5.

Galley et al. suggest that the zones are former sites of lithospheric extensional tectonics and crustal thinning: rifts from which ultramafic to mafic magmas emerged. They consider them to be akin to modern mid-ocean and continental rifts. Most of the rifts roughly parallel the trend of the greenstone belts and the large, long-lived faults that run west to east across the Abitibi Terrain. This suggests that rifts formed under the more ductile lithospheric condition of the Neoarchaean set the gross fabric of the granites and greenstones. Moreover, there are signs of two triple junctions where three rifts converge: fundamental features of modern plate tectonics. However, both rifts and junctions are on a smaller scale than those active at present. The rift patterns suggest plate tectonics in miniature, perhaps indicative of more vigorous mantle convection during the Archaean Eon.

There is an interesting spin-off. The Abitibi Terrane is rich in a variety of mineral resources, especially volcanic massive-sulfide deposits (VMS). Most of them are associated with the suggested rift zones. Such deposits form through sea-floor hydrothermal processes, which Archaean rifting and triple junctions would have focused to generate clusters of ‘black smokers’ precipitating large amounts of metal sulfides. Galley et al’s work is set to be applied to other large cratons, including those that formed earlier in the Archaean: the Pilbara and Kaapvaal cratons of Australia and South Africa. That could yield better insights into earlier tectonic processes and test some of the hypotheses proposed for them

See also: Archaean Rifts, Triple Junctions Mapped via Gravity Modeling. Scienmag, 6 October 2025

Sagduction of greenstone belts and formation of Archaean continental crust

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Simplified geological map of the Archaean Yilgarn Craton in Western Australia. Credit: Geological Survey of Western Australia

Every ancient craton seen from space shows patterns that are unique to Archaean continental crust: elongated, ‘canoe-shaped’ greenstone belts enveloped by granitic gneisses, both of which are punctured by domes of younger, less deformed granites. The Yilgarn Craton of Western Australia is a typical granite-greenstone terrain. Greenstone belts contain lavas of ultramafic, basaltic and andesitic compositions, which in undeformed settings show the typical pillow structures formed by submarine volcanic extrusion. There are also layered mafic to ultramafic complexes, formed by fractional crystallisation, minor sedimentary sequences and occasionally more felsic lavas and ashes. The enveloping grey gneisses are dominantly highly deformed tonalite-trondhjemite-granodiorite (TTG) composition that suggest that they formed from large volumes of sodium-rich, silicic magmas, probably generated at depth by partial melting of hydrated basaltic rocks.

The heat producing radioactive isotopes of potassium, uranium and thorium in both the Archaean mantle and crust would have been more abundant before 2.5 Ga ago, because they decay over time. Consequently the Earth’s interior would have then generated more heat than now, gradually to escape by thermal conduction towards the cooler surface. The presence of pillow lavas and detrital sediments in greenstone belts indicate that surface temperatures during the Archaean Eon were below the boiling point of water; in fact probably much the same as in the tropics at present. Indeed there is evidence that Earth was then a water world. It may even have been so during the Hadean, as revealed by the oxygen-isotope data in 4.4 Ga zircon grains. The broad conclusion from such findings is that the Archaean geothermal gradient was much steeper; there would have been a greater temperature increase with depth than now and new crust would have cooled more slowly. Subduction of cool lithosphere would have been less likely than in later times, especially as higher mantle heat production would have generated new crust more quickly. Another likely possibility is that far more heat would have been moved by convection: there would have been more mantle-penetrating plumes and they would have been larger. Large mantle plumes of the Phanerozoic have generated vast ocean floor plateaus, such as the Kerguelen and Ontong Java Plateau.

A group of geoscience researchers at The University of Hong Kong and international colleagues recently completed a geological and geochemical study of the North China Craton, analysing their data in the light of recently emerging views on Archaean processes (Dingyi Zhao et al, A two-stage mantle plume-sagduction origin of Archean continental crust revealed by water and oxygen isotopes of TTGs, Science Advances, v. 11, article eadr9513  ; DOI: 10.1126/sciadv.adr9513).They found compelling evidence that ~2.5 Ga-old Neoarchaean TTG gneisses in the North China granite-greenstone terrain formed by partial melting of an earlier mafic-ultramafic greenstone crust with high water content. They consider this to support a two-stage model for the generation of the North China Craton’s crust above a vast mantle plume. The first stage at around 2.7 Ga was the arrival of the plume at the base of the lithosphere, which partially melted as a result of the decompression of the rising ultramafic plume. The resulting mafic magma created an oceanic plateau partly by underplating the older lithosphere, intruding it and erupting onto the older ocean floor. This created the precursors of the craton’s greenstones, the upper part of which interacted directly with seawater to become hydrothermally altered. They underwent minor partial melting to produce small TTG intrusions. A second plume arriving at ~2.5 Ga resulted in sinking of the greenstones under their own weight to mix or ‘hybridise’ with the re-heated lower crust. This caused the greenstones substantially to partially melt and so generate voluminous TTG magmas that rose as the greenstones subsided. . It seems likely that this dynamic, hot environment deformed the TTGs as they rose to create the grey gneisses so typical of Archaean granite-greenstone terranes. [Note: The key evidence for Dingyi Zhao et al.’s conclusions is that the two TTG pulses yielded the 2.7 and 2.5 Ga ages, and show significantly different oxygen isotope data (δ18O)].

Two stages of TTG gneiss formation in the North China Craton and the sinking (sagduction) of greenstone belts in the second phase. Credit: Dingyi Zhao et al., Fig 4)

Such a petrogenetic scenario, termed sagduction by Dingyi Zhao and colleagues, also helps explain the unique keel-like nature of greenstone belts, and abundant evidence of vertical tectonics in many Archaean terrains (see: Vertical tectonics and formation of Archaean crust; January 2002), Their model is not entirely new, but is better supported by data than earlier, more speculative ideas. That such processes have been recognised in the Neoarchaean – the North China Craton is one of the youngest granite-greenstone terrains – may well apply to far older Archaean continental crust generation. It is perhaps the last of a series of such events that began in the Hadean, as summarised in the previous Earth-logs post.

How the earliest continental crust may have formed

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Detrital zircon grains extracted from sandstones deposited ~3 billion year (Ga) ago in Western Australia yield the ages at which these grains crystallised. The oldest formed at about 4.4 Ga; only 150 Ma after the origin of the Earth (4.55 Ga). Various lines of evidence suggest that they originally crystallized from magmas with roughly andesitic compositions, which some geochemists suggest to have formed the first continental crust (see: Zircons and early continents no longer to be sneezed at; February 2006). So far, no actual rocks of that age and composition have come to light. The oldest of these zircon grains also contain anomalously high levels of 18O, a sign that water played a role in the formation of these silicic magmas. Modern andesitic magmas – ultimately the source of most continental crust – typically form above steeply-dipping subduction zones where fluids expelled from descending oceanic crust encourage partial melting of the overriding lithospheric mantle. Higher radiogenic heat production in the Hadean and the early Archaean would probably have ensured that the increased density of later oceanic lithosphere needed for steep subduction could not have been achieved. If subduction occurred at all, it would have been at a shallow angle and unable to exert the slab-pull force that perpetuated plate tectonics in later times (see: Formation of continents without subduction, March, 2017).

Landsat image mosaic of the Palaeoarchaean granite-greenstone terrain of the Pilbara Craton, Western Australia. Granite bodies show as pale blobs, the volcanic and sedimentary greenstone belts in shades of grey.

Geoscientists have been trying to resolve this paradox for quite a while. Now a group from Australia, Germany and Austria have made what seems to be an important advance (Hartnady, M. I. H and 8 others 2025. Incipient continent formation by shallow melting of an altered mafic protocrust. Nature Communications, v. 16, article 4557; DOI: 10.1038/s41467-025-59075-9). It emerged from their geochemical studies of rocks in the Pilbara Craton of Western Australia that are about a billion years younger than the aforementioned ancient zircon grains. These are high-grade Palaeoarchaean metamorphic rocks known as migmatites that lie beneath lower-grade ‘granite-greenstone’ terrains that dominate the Craton, which Proterozoic deformation has forced to the surface. Their bulk composition is that of basalt which has been converted to amphibolite by high temperature, low pressure metamorphism (680 to 730°C at a depth of about 30 km). These metabasic rocks are laced with irregular streaks and patches of pale coloured rock made up mainly of sodium-rich feldspar and quartz, some of which cut across the foliation of the amphibolites. The authors interpret these as products of partial melting during metamorphism, and they show signs of having crystallised from a water-rich magma; i.e. their parental basaltic crust had been hydrothermally altered, probably by seawater soon after it formed. The composition of the melt rocks is that of trondhjemite, one of the most common types of granite found in Archaean continental crust. Interestingly, small amounts of trondhjemite are found in modern oceanic crust and ophiolites.

A typical migmatite from Antarctica showing dark amphibolites laced with quartzofeldspathic products of partial melting. Credit: Lunar and Planetary Laboratory, University of Arizona

The authors radiometrically dated zircon and titanite (CaTiSiO₅) – otherwise known as sphene – in the trondhjemites, to give an age of 3565 Ma. The metamorphism and partial melting took place around 30 Ma before the overlying granite-greenstone assemblages formed. They regard the amphibolites as the Palaeoarchaean equivalent of basaltic oceanic crust. Under the higher heat production of the time such primary crust would probably have approached the thickness of that at modern oceanic plateaux, such as Iceland and Ontong-Java, that formed above large mantle plumes. Michael Hartnady and colleagues surmise that this intracrustal partial melting formed a nucleus on which the Pilbara granite-greenstone terrain formed as the oldest substantial component of the Australian continent. The same nucleation may have occurred during the formation of similar early Archaean terrains that form the cores of most cratons that occur in all modern continents.

Modelling climate change since the Devonian

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A consortium of geoscientists from Australia, Britain and France, led by Andrew Merdith of the University of Adelaide examines the likely climate cooling mechanisms that may have set off the two great ‘icehouse’ intervals in the last 541 Ma (Merdith, A.S. et al. 2025. Phanerozoic icehouse climates as the result of multiple solid-Earth cooling mechanisms. Science Advances, v. 11, article eadm9798: DOI: 10.1126/sciadv.adm9798). They consider the first to be the global cooling that began in the latter part of the Devonian culminating in the Carboniferous-Permian icehouse. The second is the Cenozoic global cooling to form the permanent Antarctic ice cap around 34 Ma and culminated in cyclical ice ages on the northern continents after 2.4 Ma during the Pleistocene. They dismiss the 40 Ma long, late Ordovician to early Silurian glaciation that left its imprint on North Africa and South America –  then combined in the Gondwana supercontinent. The data about two of the parameters used in their model – the degree of early colonisation of the continents by plants and their influence on terrestrial weathering are uncertain in that protracted event.  Yet the Hirnantian glaciation reached 20°S at its maximum extent in the Late Ordovician around 444 Ma to cover about a third of Gondwana: it was larger than the present Antarctic ice cap. For that reason, their study spans only Devonian and later times.

Fluctuation in evidence for the extent of glacial conditions since the Devonian: the ‘ice line’ is grey. The count of glacial proxy occurrences in each 10° of latitude through time is shown in the colour key. Credit: Merdith et al., Fig 2A.

Merdith et al. rely on four climatic proxies. The first of these comprises indicators of cold climates, such as glacial dropstones, tillites and evidence in sedimentary rocks of crystals of hydrated calcium carbonate (ikaite – CaCO3.6H2O) that bizarrely forms only at around 0°C . From such occurrences it is possible to define an ‘ice line’ linking different latitudes through geological time. Then there are estimates of global average surface temperature; low-latitude sea surface temperature; and estimates of atmospheric CO2. The ‘ice-line’ data records an additional, long period of glaciation in the Jurassic and early Cretaceous, but evidence does not extend to latitudes lower than 60°. It is regarded by Merdith et al. as an episode of ‘cooling’ rather than an ‘icehouse’. Their model assesses sources and sinks of CO2­ since the Devonian Period.

The main natural source of the principal greenhouse gas CO2 is degassing through volcanism expelled from the mantle and breakdown of carbonate rock in subducted lithosphere. Natural sequestration of carbon involves weathering of exposed rock that releases dissolved CO2 and ions of calcium and magnesium.   A recently compiled set of plate reconstructions that chart the waxing and waning of tectonics since the Devonian Period allows them to model the tectonically driven release of carbon over time, with time scales on the order of tens to hundreds of Ma. The familiar Milanković forcing cycles on the order of tens to hundreds of ka are thus of no significance in Merdith et al.’s  broader conception of icehouse episodes  Their modelling shows high degassing during the Cretaceous, modern levels during the late Palaeozoic and early Mesozoic, and low emissions during the Devonian. The model also suggests that cooling stemmed from variations in the positions and configuration of continents over time.  Another crucial factor is the tempo of exposure of rocks that are most prone to weathering. The most important are rocks of the ocean lithosphere incorporated into the continents to form ophiolite masses. The release of soluble products of weathering into ocean basins through time acts as a fluctuating means of ‘fertilising’ so that more carbon can be sequestered in deep sediments in the form of organisms’ unoxidised tissue and hard parts made of calcium carbonates and phosphates. Less silicate weathering results in a boost to atmospheric CO2.

Only two long, true icehouse episodes emerge from the empirical proxy data, expressed by the ‘ice-line’ plots. Restricting the modelling to single global processes that might be expected to influence degassing or carbon sequestration produces no good fits to the climatic proxy data. Running the model with all the drivers “off” produces more or less continuous icehouse conditions since the Devonian. The model’s climate-related outputs thus imply that many complex processes working together in syncopation may have driven the gross climate vagaries over the last 400 Ma or so. A planet of Earth’s size without such complexity would throughout that period have had a high-CO2 warm climate. According to Andrew Merdith its fluctuation from greenhouse to icehouse conditions in the late Palaeozoic and the Cenozoic were probably due to “coincidental combination of very low rates of global volcanism, and highly dispersed continents with big mountains, which allow for lots of global rainfall and therefore amplify reactions that remove carbon from the atmosphere”.

Geological history is, almost by definition, somewhat rambling. So, despite despite the large investment in seeking a computed explanation of data drawn from the record, the outcome reflects that in a less than coherent account. To state that many complex processes working at once may have driven climate vagaries over the last 400 Ma or so, is hardly a major advance: palaeoclimatologists have said more or less the same for a couple of decades or more, but have mainly proposed single driving mechanisms. One aspect of Merdith et al.’s  results seems to be of particular interest. ‘Icehouse’ conditions seem to be rare events interspersed with broader ice-free periods. We evolved within the mammal-dominated ecosystems on the continents during the latest of these anomalous climatic episodes. And we and those ecosystems now rely on a cool world. As the supervisor of the project commented, ‘Over its long history, the Earth likes it hot, but our human society does not’.

Readers may like to venture into how some philosophers of science deal with a far bigger question; ‘Is intelligent life a rare, chance event throughout the universe?’ That is, might we be alone in the cosmos? In the same issue of Science Advances is a paper centred on just such questions (Mills, D.B. et al. 2025. A reassessment of the “hard-steps” model for the evolution of intelligent life. Science Advances, v. 11, article eads5698; DOI: 10.1126/sciadv.ads5698). It stems from cosmologist Brandon Carter’s ‘Anthropic Principle’ first developed at Nicolas Copernicus’s 500th birthday celebrations in 1973. This has since been much debated by scientists and philosophers – a gross understatement as it knocks the spots off the Drake Equation. To take the edge off what seems to be a daunting task, Mills et al. consider a corollary of the Anthropic Principle, the ‘hard steps model’. That, in a nutshell, postulates that the origin of humanity and its ability to ponder on observations of the universe required a successful evolutionary passage through a number of hard steps. It predicts that such intelligence is ‘exceedingly rare’ in the universe. Icehouse conditions are respectable candidates for evolutionary ‘hard steps’, and in the history of Earth there have been five of them.

A fully revised edition of Steve Drury’s book Stepping Stones: The Making of Our Home World can now be downloaded as a free eBook

How India accelerated towards Eurasia at the end of the Cretaceous

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About 70 Ma ago the magnetic striping of the Indian Ocean floor suggests that the Indian subcontinent was then moving towards the huge, almost stationary Eurasian continent at about 8 cm per year. Over the next 5 Ma this convergence rate underwent a tectonically startling acceleration to reach 18 cm yr-1 by around the time of the Cretaceous-Palaeogene boundary (65 Ma): more than doubling the approach rate. Thereafter it slowed, eventually to a few centimetres per year once collision and building of the Himalayan mountain belt were more or less complete about 30 Ma ago. This cannot easily be explained by a speeding up of the sea-floor spreading rate at an Indian Ocean ridge to the south, 18 cm yr-1 being as fast as tectonic forces can manage at present. At that time ocean floor to the north of India was being subducted beneath Eurasia, and basaltic volcanism was flooding what is now the Deccan Plateau on western India. A couple of suggestions have been made: two northward subduction zones may have developed or the mantle plume feeding the Deccan flood basalts may have driven the tectonic acceleration. A third possibility is that the subduction was somehow lubricated. That approach has recently been considered by geoscientists from China and Singapore  (Zhou, H. et al. 2024. India–Eurasia convergence speed-up by passive-margin sediment subduction. Nature, v. 635, p. 114-120; DOI: 10.1038/s41586-024-08069-6).

Hao Zhou and colleagues studied the isotopic and trace-element geochemistry of volcanic and plutonic igneous complexes to the north of the Himalaya. They were emplaced in arc environments in three stages: from 98 to 89; 65 to 60; and 57 to 50 Ma. In this tectonic setting fluids rise from the subducted slab to induce the mantle part of the overriding lithosphere to partially melt. That yields magmas which penetrate the crust above. The first and last magmatic events produced similar isotopic and trace-element ‘signatures’, which suggest fluids rose from subducted ocean lithosphere.  But those in the latest Cretaceous to earliest Palaeocene are markedly different. Instead of showing signs of their magmas being entirely mantle derived like the earlier and later groups, the 65 to 60 Ma rocks exhibit clear evidence of partial melting having incorporated materials that had originated in older continental crust. The authors suggest that this crustal contamination stemmed from sediments that had been deposited at the northern margin of the Indian subcontinent during the Mesozoic. These sediments had formed by weathering of the ancient rocks that underpin India, transport of the debris by rivers and deposition on the seafloor as water-saturated sands, silts and clays. Once those sediments were subducted beneath what is now Tibet they would yield fluids with a geochemical ‘fingerprint’ inherited from old continental crust. Moreover, far more fluids than subducted oceanic crust could ever release would rise into the overriding lithosphere than.

The fluids rising from a subducted wedge of sediments may have reduced friction between the overriding Eurasian lithosphere and the subducted slab derived from the Indian tectonic plate. That scenario would not only have lubricated subduction, but allowed compressive forces in the overriding lithosphere to relax. Both would have allowed convergence of the two plates to move significantly faster as the sediments were progressively consumed. Once completed, convergence would have slowed without such ‘lubrication’.Earlier continent-continent collision zones, such as those that united Pangaea and older supercontinents may well have involved such tectonic surges. And the same kind of process may eventually speed up the reassembly of the latest distribution of continents.

Watch an animation of the India-Eurasia convergence (just over 3 minutes long)compiled by Christopher Scotese of Northwestern University in Evanston, Illinois, USA, which is a component of his Paleomap Project. It starts by following India from its current position to its origin in the break-up of Gondwanaland ~100 Ma ago. The last half reverses the motions to show India’s slow collision with Eurasia.

Drip tectonics beneath Türkiye

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Tectonics and geomorphology of Turkey showing the main fault systems. The Konya basin is enclosed by the grey rectangle at centre. (Credit: Taymaz et al. Geological Society of London, Special Publication 291, p1-16, Fig 1)

The 1.5-2.0 km high Central Anatolian plateau in Türkiye has been rising since ~11 Ma ago: an uplift of about 1 km in the last 8 Ma. However, part of the southern Plateau shows signs of rapidly subsidence that has created the Konya Basin, marked by young lake sediments. Interferometric radar (InSAR) data from the European Space Agency’s Sentinel-1 satellite, which detects active movement of the Earth’s surface, reveal a crude, doughnut-shaped area of the surface that is subsiding at up to 50 mm per year. This ring of subsidence surrounds a core of active uplift that is about 50 km across (see the first figure). Expressed crudely, active subsidence suggests an excess of mass beneath the affected area, whereas uplift implies a mass deficit; in both cases within the lithosphere. So, when the InSAR data were published in 2020, it became clear that the lithosphere beneath Anatolia is doing something very strange.

Vertical velocities affecting the surface in the Konya Basin derived from InSAR data, velocities colour-coded cyan to blue show subsidence, yellow to red suggesting that the surface is rising. (Credit: Andersen et al., Fig 1c)

Canadian and Turkish geophysicists set out to find a tectonic reason for such aberrant behaviour (Andersen, A.J.  et al. 2024. Multistage lithospheric drips control active basin formation within an uplifting orogenic plateau. Nature Communications, v. 15, Article 7899; DOI: 10.1038/s41467-024-52126-7). They wondered if a process known as ‘drip tectonics’, first mooted as an explanation of anomalous features in some mountain belts in 2004 (see: Mantle dripping off mountain roots, October 2004; and A drop off the old block? May 2008) might be applicable to the Anatolian Plateau. The essence of this process is similar to the slab-pull force at the heart of subduction. Burial and cooling of basaltic material in oceanic lithosphere being driven beneath another tectonic plate converts its igneous mineralogy to the metamorphic rock eclogite, whose density exceeds that of mantle rocks. Gravity then acts to pull the changed material downwards. However, Anatolia shows little sign of subduction. But the mantle beneath shows seismic speed anomalies that hint at anomalously dense material.

Seismic tomography shows that in a large volume 100 to 200 km beneath the central part of the Plateau S-waves travel faster than in the surrounding mantle. The higher speed suggests a body that is denser and more rigid than its surroundings. This could be a sinking, detached block of ‘eclogitised’ lithosphere whose disconnection from the remaining continental lithosphere has been causing the uplift of the Plateau that began in the Late Miocene. A smaller high-speed anomaly lies directly under the Konya Basin, but at a shallower depth (50 to 80 km) just beneath the lithosphere-asthenosphere boundary. The authors suggest that this is another piece of the lower lithosphere that is beginning to sink and become a ‘drip’. Still mechanically attached to the lithosphere the sinking dense block is dragging the surface down.

Andersen et al. instead of relying on computer modelling created a laboratory analogue. This consisted of a tank full of a fluid polymer whose viscosity is a thousand times that of maple syrup that represents the Earth’s deep mantle beneath. They mimicked an overlying  plate by a layer of the same material with additional clay to render it more viscous – the model’s lithospheric mantle – with a ‘crust’ made of a sand of ceramic and silica spherules. A dense seed inserted into the model lithospheric mantle began to sink, dragging that material downwards in a ‘drip’. After that ‘drip’ had reached the bottom of the tank hours later, it became clear that another, smaller drip materialised along the track of the first and also began to sink. Monitoring of the surface of the ‘crust’ revealed that the initial drip did result in a basin. But the further down the drip fell the basin gradually became shallower: there was surface uplift. Once the initial drip had ‘bottomed-out’ the basin began to deepen again as the secondary drip formed and slowly moved downwards. The model seems to match the authors’ interpretation of the geophysics beneath the Anatolian Plateau. One drip created the potential for a lesser one, a bit like in inversion of the well-known slo-mo videos of a drop of milk falling into a glass of milk, when following the drop’s entry a smaller drop rebounds from the milky surface.

Cartoons of drip tectonics beneath the Anatolian Plateau. (a) Lower lithosphere detached from beneath Anatolia in the Late Miocene (10 to 8 Ma) descends into the mantle as it is ‘eclogitised’; (b) a smaller block beneath the Konya Basin beginning to ‘drip’, but still attached to the lithosphere. (Credit: Andersen et al., Fig 4)

In Anatolia the last 10 Ma has not been just ups and downs of the surface corresponding to drip tectonics. That was accompanied by volcanism, which can be explained by upwelling of mantle material displaced by lithospheric drips. When mantle rises and the pressure drops partial melting can occur, provided the mantle material rises faster than it can lose heat: adiabatic melting.

The onset of weathering in the late Archaean and stabilisation of the continents

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Distribution of exposed Archaean cratons. The blue Proterozoic areas may, in part be underlain by cratons. (Credit: Groves, D.I. & Santosh, M. DOI:10.1016/j.gr.2020.06.008)

About 50% of continental crust is of Archaean age (2.5 to 4.0 Ga) in huge blocks above lithosphere more than 150 km thick. Younger continental lithosphere is significantly thinner – as low as 40 km. Since the end of the Archaean Eon these blocks have remained tectonically stable and only show signs of extensional, brittle fracture that have been exploited by basaltic dyke swarms. Such crystalline monstrosities have remained rigid for 2.5 billion years. They are termed cratons from the Greek word κράτο (kratos) for ‘might’ or ‘strength’. Numbers of cratons have been pushed together by later tectonics to form continental ‘cores’, separated from one another by highly deformed ‘mobile belts’ formed by younger collisional orogenies. Africa and South America have 4 cratons each, Eurasia 6 or 7, the other continents all have one

Considering how much cratons have been stressed by later tectonic forces, their implacable rigidity might seem surprising. This rigidity is thought to be due to cratons’ unusually low amounts of the main heat-producing elements (HPE) potassium, uranium and thorium, the decay of whose radioactive isotopes produces surface heat flow. Cratons have the lowest surface heat flow on the planet, so in bulk they must have low HPE content. This stems from the nature of cratons’ deepest parts: almost anhydrous, once igneous rocks of intermediate average composition known as granulites. They formed by metamorphism of earlier crustal rocks at depths of up to 70km, which drove out most of their original HPEs and water. The upper cratonic crust has much the same complement of HPEs as that of more recent continental crust. This bulk depletion of cratons has maintained unusually low temperatures in their deep continental crust. That has been immune from partial melting and thus ductile deformation since it formed.

Three billion year-old TTG gneiss in the Outer Hebrides, Scotland. (Credit: British Geological Survey)

Jesse Reimink and Andrew Smye of Pennsylvania State University, USA have considered the geochemistry and history of the world’s cratons to address the long-standing issue of their stability and longevity (Reimink, J.R. & Smye, A.J. 2024. Subaerial weathering drove stabilization of continents. Nature, v. 629, online article; DOI: 10.1038/s41586-024-07307-1). Their main focus is on how the Archaean lower crust lost most of it HPEs, and where they went. During much of the Archaean continental crust formed by partial melting of hydrated basaltic rocks at shallow depths. That generated sodium-rich silicic magmas from which the dominant grey tonalite-trondhjemite-granodiorite (TTG) gneisses of Archaean crust formed by extreme ductile deformation. Though TTGs originally contained sufficient heat-producing capacity to make them ductile during the early Archaean there is little evidence that they underwent extensive partial melting themselves. But they did after 3.0 Ga to produce swarms of granite plutons in the upper Archaean crust.

Complementing the late-Archaean granite ‘swarm’ are deep-crustal granulites with low HPE contents, which mainly formed around the same time. The granulites contain highly metamorphosed sedimentary rocks, which seem to have been sliced into the Archaean crust during its ductile deformation phase. Some of them have compositions that suggest that they are derived from clay-rich shales, their proportion reaching about 30% of all granulite-facies metasediments. Clay minerals are the products of chemical weathering of silicon- and aluminium-rich igneous rocks exposed to the atmosphere. When they form, they host K, U and Th. Also, their composition and high initial water contents are conducive to partial melting under high-temperature conditions, to become a source of granitic magmas. Crustal weathering is key to Reimink and Smye’s hypothesis for the development of cratons in the late Archaean.

There is growing evidence that high Archaean heat flow through oceanic lithosphere – the mantle contained more undecayed HPE isotopes than now – reduced its density. As a result Archaean oceanic basins were considerably shallower than they became in later times. Because of the lower volume of the basins during the Archaean, seawater extended across much of the continental surface. For most of the Archaean Eon Earth was a ‘waterworld’, with little subaerial weathering of its TTG upper crust. As the volume of exposed continental crust increased so did surface weathering to form clay minerals that selectively absorbed HPEs. Over time shales became tectonically incorporated deep into the thickening Archaean continental crust to form a zone with increased heat producing capacity and a higher water content. Once deep enough and heated by their own content of HPE they began partially melting to yield voluminous granitic magmas to which they contributed their load of HPEs. Being lower in density than the bulk of TTG crust the granite melts would have risen to reach the upper crust. They also took in HPEs from the deep TTG crust itself. According to Reimink and Smye this would have concentrated continental heat production in the upper crust, leaving the deeper crust drier, less able to melt and assume ductile properties, and thus to create the cratons.

The authors believe that such a redistribution of heat production in the ancient continental crust did not need any major change in global tectonics. All it required was decreasing oceanic heat flow to create deeper and more voluminous ocean basins, allowing more continental surface to emerge above sea level and dynamic burial of sedimentary products of subaerial weathering. They conclude: “The geological record can then be cast in terms of a pre-emergence (TTG-dominated) and post-emergence (granite-dominated) planet.” That seems very neat … but it seems unlikely that samples can be drilled from the depths where the ‘action’ took place. Geologists depend on exposures of Archaean middle to deep crust brought to the surface by fortuitous later tectonics.

A new explanation for the Neoproterozoic Snowball Earth episodes

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The Cryogenian Period that lasted from 860 to 635 million years ago is aptly named, for it encompassed two maybe three episodes of glaciation. Each left a mark on every modern continent and extended from the poles to the Equator. In some way, this series of long, frigid catastrophes seems to have been instrumental in a decisive change in Earth’s biology that emerged as fossils during the following Ediacaran Period (635 to 541 Ma). That saw the sudden appearance of multicelled organisms whose macrofossil remains – enigmatic bag-like, quilted and ribbed animals – are found in sedimentary rocks in Australia, eastern Canada and NW Europe. Their type locality is in the Ediacara Hills of South Australia, and there can be little doubt that they were the ultimate ancestors of all succeeding animal phyla. Indeed one of them Helminthoidichnites, a stubby worm-like animal, is a candidate for the first bilaterian animal and thus our own ultimate ancestor. Using the index for Palaeobiology or the Search Earth-logs pane you can discover more about them in 12 posts from 2006 to 2023. The issue here concerns the question: Why did Snowball Earth conditions develop? Again, refresh your knowledge of them, if you wish, using the index for Palaeoclimatology or Search Earth-logs. From 2000 onwards you will find 18 posts: the most for any specific topic covered by Earth-logs. The most recent are Kicking-off planetary Snowball conditions (August 2020) and Signs of Milankovich Effect during Snowball Earth episodes (July 2021): see also: Chapter 17 in Stepping Stones.

One reason why Snowball Earths are so enigmatic is that CO2 concentrations in the Neoproterozoic atmospheric were far higher than they are at present. In fact since the Hadean Earth has largely been prevented from being perpetually frozen over by a powerful atmospheric greenhouse effect. Four Ga ago solar heating was about 70 % less intense than today, because of the ‘Faint Young Sun’ paradox. There was a long episode of glaciation (from 2.5 to 2.2 Ga) at the start of the Palaeoproterozoic Era during which the Great Oxygenation Event (GOE) occurred once photosynthesis by oxygenic bacteria became far more common than those that produced methane. This resulted in wholesale oxidation to carbon dioxide of atmospheric methane whose loss drove down the early greenhouse effect – perhaps a narrow escape from the fate of Venus. There followed the ‘boring billion years’ of the Mesoproterozoic during which tectonic processes seem to have been less active. in that geologically tedious episode important proxies (carbon and sulfur isotopes) that relate to the surface part of the Earth System ‘flat-lined’.  The plethora of research centred on the Cryogenian glacial events seems to have stemmed from the by-then greater complexity of the Precambrian Earth System.

Since the GOE the main drivers of Earth’s climate have been the emission of CO2 and SO2 by volcanism, the sedimentary burial of carbonates and organic carbon in the deep oceans, and weathering. Volcanism in the context of climate is a two-edged sword: CO2 emission results in greenhouse warming, and SO2 that enters the stratosphere helps reflect solar radiation away leading to cooling. Silicate minerals in rocks are attacked by hydrogen ions (H+) produced by the solution of CO2 in rain water to form a weak acid (H2CO3: carbonic acid). A very simple example of such chemical weathering is the breakdown of calcium silicate:

CaSiO3  +  2CO2  + 3H2O  =  Ca2+  +  2HCO3–  +  H4SiO4  

The reaction results in calcium and bicarbonate ions being dissolved in water, eventually to enter the oceans where they are recombined in the shells of planktonic organisms as calcium carbonate. On death, their shells sink and end up in ocean-floor sediments along with unoxidised organic carbon compounds. The net result of this part of the carbon cycle is reduction in atmospheric CO2 and a decreased greenhouse effect: increased silicate weathering cools down the climate. Overall, internal processes – particularly volcanism – and surface processes – weathering and carbonate burial – interact. During the ‘boring billion’ they seem to have been in balance. The two processes lie at the core of attempts to model global climate behaviour in the past, along with what is known about developments in plate tectonics – continental break-up, seafloor spreading and orogenies – and large igneous events resulting from mantle plumes. A group of geoscientists from the Universities of Sydney and Adelaide, Australia have evaluated the tectonic factors that may have contributed to the first and longest Snowball Earth of the Neoproterozoic: the Sturtian glaciation (717 to 661 Ma) (Dutkiewicz, A. et al. 2024. Duration of Sturtian “Snowball Earth” glaciation linked to exceptionally low mid-ocean ridge outgassing. Geology, v. 52, online early publication; DOI: 10.1130/G51669.1).

Palaeogeographic reconstructions (Robinson projection) during the early part of the Sturtian global glaciation: LEFT based on geological data from Neoproterozoic terrains on modern continents; RIGHT based on palaeomagnetic pole positions from those terrains. Acronyms refer to each terrains, e.g. Am is Amazonia, WAC is the West African Craton. Orange lines are ocean ridges, those with teeth are subduction zone. (Credit: Dutkiewicz et al., parts of Fig. 1)

Shortly before the Sturtian began there was a major flood volcanism event, forming the Franklin large igneous province, remains of which are in Arctic Canada. The Franklin LIP is a subject of interest for triggering the Sturtian, by way of a ‘volcanic winter’ effect from SO2 emissions or as a sink for CO2­ through its weathering. But both can be ruled out as no subsequent LIP is associated with global cooling and the later, equally intense Marinoan global glaciation (655 to 632 Ma) was bereft of a preceding LIP. Moreover, a world of growing frigidity probably could not sustain the degree of chemical weathering to launch a massive depletion in atmospheric CO2. In search of an alternative, Adriana Dutkiewicz and colleagues turned to the plate movements of the early Neoproterozoic. Since 2020 there have been two notable developments in modelling global tectonics of that time, which was dominated by the evolution of the Rodinia supercontinent. One is based largely on geological data from the surviving remnants of Rodinia (download animation), the other uses palaeomagnetic pole positions to fix their relative positions: the results are very different (download animation).

Variations in ocean ridge lengths, spreading rates and oceanic crust production during the Neoproterozoic estimated from the geological (orange) and palaeomagnetic (blue) models. Credit: Dutkiewicz et al., parts of Fig. 2)

The geology-based model has Rodinia beginning to break up around 800 Ma ago with a lengthening of global constructive plate margins during disassembly. The resulting continental drift involved an increase in the rate of oceanic crust formation from 3.5 to 5.0 km2 yr-1. Around 760 Ma new crust production more than halved and continued at a much slowed rate throughout the Cryogenian and the early part of the Ediacaran Period.  The palaeomagnetic model delays breakup of the Rodinia supercontinent until 750 Ma, and instead of the rate of crust production declining through the Cryogenian it more than doubles and remains higher than in the geological model until the late Ediacaran. The production of new oceanic crust is likely to govern the rate at which CO2 is out-gassed from the mantle to the atmosphere. The geology-based model suggests that from 750 to 580 Ma annual CO2 additions could have been significantly below what occurred during the Pleistocene ice ages since 2.5 Ma ago. Taking into account the lower solar heat emission, such a drop is a plausible explanation for the recurrent Snowball Earths of the Neoproterozoic. On the other hand, the model based on palaeomagnetic data suggests significant warming during the Cryogenian contrary to a mass of geological evidence for the opposite.

A prolonged decrease in tectonic activity thus seems to be a plausible trigger for global glaciation. Moreover, reconstruction of Precambrian global tectonics using available palaeomagnetic data seems to be flawed, perhaps fatally. One may ask, given the trends in tectonic data: How did the Earth repeatedly emerge from Snowball episodes? The authors suggest that the slowing or shut-down of silicate weathering during glaciations allowed atmospheric CO2 to gradually build up as a result of on-land volcanism associated with subduction zones that are a quintessential part of any tectonic scenario.

This kind of explanation for recovery of a planet and its biosphere locked in glaciation is in fact not new. From the outset of the Snowball Earth hypothesis much the same escape mechanisms were speculated and endlessly discussed. Adriana Dutkiewicz and colleagues have fleshed out such ideas quite nicely, stressing a central role for tectonics. But the glaring disparities between the two models show that geoscientists remain ‘not quite there’. For one thing, carbon isotope data from the Cryogenian and Ediacaran Periods went haywire: living processes almost certainly played a major role in the Neoproterozoic climatic dialectic.