When was the first continental crust formed

This is consistent with an increase in the SiO 2 content of the upper crust through time from 3. Large et al. However they also noted increased contributions from more mafic source rocks in the period 2. Kamber argued that chemical weathering of predominantly ultra mafic upper continental crust is needed to explain the typical trace element signatures of Archean marine chemical sediments.

Condie presented map normalized compositions of upper crust estimated from the proportions of common rock types, and compared those with values estimated from cratonic shales. Yet, it remains difficult to infer the composition of the bulk crust from estimates of the upper crust. Figure 7. Most upper crustal rocks are sedimentary and granitic rocks derived from pre-existing crustal rocks Hutton, ; Figure 8.

Thus, it is difficult to estimate the composition of new continental crust, and hence the tectonic settings in which the continental crust was generated. While Nd model ages are prone to significant errors Arndt and Goldstein, , they provide a broad estimate of when segments of new crust were generated.

Figure 8. Schematic sketch section through the continental crust, initially drawn by Tony Kemp Hawkesworth and Kemp, b. Dhuime et al. In terms of silica contents, the new crust is similar to that of many greenstone belt volcanic rocks. New continental crust formed after 3 Ga gradually became more intermediate in composition, and hence more like the calc-alkaline andesitic crust that dominates the continental record today.

Figure 9. The curve for Sr isotopes in seawater is from Shields and Veizer , and the dashed portion is from Satkoski et al. A is granular barites Satkoski et al. The gray shaded histogram illustrates the secular variation in the proportion of subaerial LIPs Kump and Barley, The curve for the proportion of emerged land is from Flament et al.

One consequence of increases in the thickness of the continental crust, is that more of it will emerge above sea level, and the models of Flament et al. Crustal emergence is reflected in the proportion of large igneous provinces LIPs erupted on land Kump and Barley, , and there will be an increased contribution of continental crust to the chemistry of the oceans, and in particular an increase in the Sr isotope ratio of seawater.

Relatively few carbonate samples are available from the late Archean Figure 9 , and the dashed curve, at higher Sr isotope ratios than the Shields and Veizer curve, is from Satkoski et al. At this stage there is broad agreement in the inferences drawn from the different approaches, and the inferred crustal thicknesses at the site of crust generation of Dhuime et al.

This was in part because the depletion of the upper mantle was linked to the generation of the continental crust, and so it was important to establish the volumes of continental crust that had been generated at different times in Earth history. This was tied to the volumes of depleted mantle that resulted, whether those volumes might be restricted to the upper mantle, and the extent to which that was a constraint on layered rather than whole mantle convection.

More recently there has been renewed interest because the rates of crustal growth constrain the settings in which new continental crust was generated. The continental crust is defined differently in different studies and using different approaches. It is the layer of granitic, sedimentary and metamorphic rocks that form the continents, including the areas of shallow seabed close to their shores, known as continental shelves.

It is buoyant and high standing compared to the rocks of the oceanic crust. The constituent rocks of the continents span 4 Ga of Earth history, and have an andesitic bulk composition. In this discussion we are concerned with models for the evolution of the continental crust. Whereas the continental crust includes igneous rocks that might also be generated in the oceans, they are regarded as continental if their record is retained in the continents, typically because they have been involved in subsequent geological processes, such as partial melting, metamorphism, deformation, erosion and sedimentation.

Figure 10A contrasts the present day record of the relative volumes of rocks of different geological ages Goodwin, with a number of models for how the volume of continental crust has changed with time. The curves are presented in three groups, and they are all anchored to the present day volume of the continental crust. We infer that significant volumes of crust have been destroyed, and the crust that is preserved today is a biased record, not least because the Archean crust is under-represented Hawkesworth et al.

These curves are all based on the presently preserved proportions of rocks of either different geological or crust formation ages Condie and Aster, Yet, as these proportions are unlikely to reflect the volumes of rock that were present at different times in Earth history, the second group of curves is of crustal growth models independent of the relative volumes of crust of different ages preserved today. Figure A Crustal growth curves grouped into those that rely on age distributions preserved in present day geology dashed and red curves , those that constrain the volumes of crust in the past independent of present day age distributions dark purple and green curves , and those that invoke relative crust volumes for a theoretical reason, such as there might be more crust generated on a hotter Earth light purple.

These are the starting conditions for the box model illustrated in Figure 11 Dhuime et al. One of the areas of debate is whether crustal growth curves are more realistic if they are based on archives in the crust, or from the mantle, and the role of the underlying assumptions in the application of such archives Payne et al. Widely used crustal archives include Nd isotopes in shales, which sample the presently preserved upper crust, and zircon which tends to sample more felsic lithologies.

Both are modeled in terms of the proportions of new and reworked crustal material in each time slice, and they effectively provide minimum estimates of the proportions of crust because they tend to sample more differentiated reservoirs within the continental crust Dhuime et al.

Mantle based archives do not sample the crust, and they rely instead on a framework in which depletion of the upper mantle is due to the generation of the continental crust Campbell, If the generation of the continental crust was responsible for depletion of the upper mantle, the presence of persistent chondritic mantle might indicate that relatively small volumes of felsic crust had been extracted.

The second group of curves in Figure 10 seeks to estimate the volumes of continental crust at different times in Earth history, independent of the relative volumes preserved today.

They are based on the proportions of reworked and juvenile crust in the zircon record Belousova et al. A limitation with these approaches is that the curves are based on cumulative growth of the crust and sum to unity at the present day, and hence no curve can have a past volume greater than the current volume. It is also likely that such curves represent minimum estimates for the volumes of continental crust at different times see also Guo and Korenaga, , in part because the crustal material modeled has to have been in the crust for long enough to develop distinctive isotope ratios, and because zircons predominantly crystallize from relatively felsic magmas, and the record may be biased toward such differentiated lithologies.

The curves of Fyfe , Armstrong represent a third group Figure 10 in which the curves are more schematic in character, but they were amongst the first to highlight that crustal growth involved both the generation of continental crust through the extraction and crystallization of magma from the mantle, and its destruction through recycling back into the mantle.

The paucity of preserved old continental crust, as represented in the present day age distribution, reflects this recycling process. Korenaga a sought to argue that the difference between crustal growth curves based on crustal and mantle records would indicate the volumes of continental crust that had been destroyed, but that may be to confuse the messenger with the message. Using a trial-and-error approach, the rates of crust generation and destruction for each crustal segment were adjusted at the end of a series of Myr time steps and crust generation rates were assumed to vary smoothly through time.

The estimated rates of crust generation broadly follow the evolution of mantle temperatures Figure 11 , but the rates of crust destruction have a marked peak in the period 3. A model for the changes in the volume and rates of generation and destruction of the continental crust through time Dhuime et al.

The inset shows the smooth evolution of the mantle temperature through time, after Herzberg et al. The high peak of destruction rate at 3. The peak in destruction rates also appears to be a feature of the transition from a stagnant lid to a plate tectonic world Bedard, ; Dhuime et al.

The box model for the growth of the continental crust Dhuime et al. The latter figure is significantly less than the figure of 2. However, as Tang et al. Thus, the different estimates can be reconciled if the Tang et al. This is consistent with the model of Hawkesworth and Kemp a , who presented mass balance arguments that the residence times of material in the lower crust may have been as much as 6 times less than those in the upper crust.

Sketch of the early Earth evolution, based on the models of Capitanio et al. A Transition from the Hadean, with relatively high rates of crust generation and recycling, to a lid-and-plate tectonics as a model for the Archean.

C Schematic section of continental crust illustrating likely different residence times in the upper and lower crust. The relative ease at which more mafic denser crust may be recycled has been touched on in the previous section, and there are a number of geodynamic models in which residual lower crustal material is recycled through drips back into the mantle Moore and Webb, ; Johnson et al. Such material is typically regarded as the residual mafic material from which TTG magmas, for example, have been extracted during crustal differentiation.

However, it might also include relatively dense mafic juvenile continental crust generated before 3 Ga Figure 7. Capitanio et al. The Capitanio et al. These are very preliminary estimates but the destruction of significant volumes of crust through the development of crustal drips, might be taken to undermine the significance of the change in crustal growth rates around 3 Ga Figure 10B.

However, the stagnant lid regime is characterized by the generation and destruction of mafic crust effectively through deeper levels of the continental crust, and as such it differs from the more differentiated continental crust that is returned to the mantle through subduction.

This change is broadly accompanied by an increase in the Sr isotope ratio of seawater Figure 9 , which reflects an increase in the volume and perhaps in the degree of differentiation of emergent continental crust, and hence of increased continental weathering Flament et al. Upper mantle xenon isotope ratios reflect contributions from atmospheric noble gases recycled into the mantle via subduction, and this progressively overprinted the initial mantle signature.

The results of Peron and Moreira included the highest excesses in —— Xe reported in present day mantle relative to the atmosphere, strongly suggesting a chondritic origin for xenon in the mantle prior to recycling of atmospheric xenon.

A number of studies have highlighted other changes that characterize the late Archean e. These are summarized in Figure 13 , and we note that the late Archean is when there is a marked increase in the volumes of crust preserved Figure 10A , and that may have had an influence on some of the distributions plotted.

The oldest supercontinent is late Archean in age, and palaeomagnetic data from the Kaapvaal and Pilbara cratons for the interval — Ma and from the Superior, Kaapvaal and Kola-Karelia cratons for — Ma suggest significant relative movements Cawood et al.

Brenner et al. A summary of some of the changes that appear to mark the end of the Archean. Sedimentary rock associations thought to have accumulated in passive continental margin settings, and hence to mark the onset of sea-floor spreading, are present in the late Archean, as are the oldest foreland basin deposits associated with lithospheric convergence Catuneanu, ; Hofmann et al.

There is an increase in the amounts of crustal reworking, as recorded in Hf isotope ratios in zircon, and the periods of increased crustal reworking match up with the peaks of zircon crystallization ages and supercontinent assembly Figure 4 , which we link to collisional tectonics and crustal thickening Hawkesworth et al. High-grade metamorphic terrains Brown, , , and significant volumes of S-type granites Laurent et al.

The composition of new continental crust changed from mafic to more intermediate compositions around 3 Ga Figure 7 and there was an increase in the thickness of continental crust at the sites of crust generation Figure 9. The composition of the upper crust became more felsic from the late Archean Taylor and McLennan, ; Condie, ; Kamber, ; Millet et al.

A number of strands of evidence indicate that the continental lithosphere was not strong enough to support crustal thickening Rey and Coltice, , and hence facilitate crustal reworking and high grade metamorphism, until the late Archean. The progression from TTG to more potassic granites marks the stabilization of Archean cratonic lithosphere, and this happened between 3.

The late Archean appears to be when the lithosphere became strong enough to support high-relief crust, and this was when significant volumes of mantle lithosphere stabilized, as reflected in Re depletion model ages Pearson and Wittig, Furthermore, as the continental crust started to emerge at this time, it became available for erosion and weathering, thus impacting on the composition of the atmosphere Kramers, ; Campbell and Allen, ; Kump, ; Lee et al. The late Archean was therefore the time with evidence for lateral movement of different fragments of continental lithosphere, and for sufficient strength to facilitate the development of plates and crustal thickening.

There is no evidence that this was a time when much less crust was generated, and so it is taken to mark a marked increase in the volumes of crust destroyed Dhuime et al. The crust was likely also relatively differentiated as some of the crustal growth curves are based on variations in Hf isotope ratios in zircon, which tends to crystallize from relatively felsic magmas, and the increased destruction of such crust is linked to the development of subduction-driven plate tectonics as the dominant mechanism by which new continental crust was generated.

This is a time of considerable interest in the early history of the Earth; it has been the subject of high profile international meetings e. Big data sets encourage the matching of patterns, and there is increasing focus on the underlying processes, and how those may best be tested. At this stage we would identify:. The need for better approaches to link information from different scales;. The exploration of past landscapes and topographic relief from the sedimentary record;.

Ways to evaluate crustal strength from geological evidence, and to link it to the conditions required for the development of plates and the onset of plate tectonics;. Obtaining better constraints on the range of metamorphic conditions in the Archean, and the onset of relatively high pressure metamorphism;. Developing closer links between metamorphic conditions, the geochemistry of magmatic rocks, and tectonic style in the Archean;. The global links between lithospheric extension and compression i.

Yet stories become more finely tuned in the telling, and there are areas of seeming confusion that it may be helpful to have considered again. More than in most subjects, scale is a complex issue in the Earth Sciences, and it remains difficult to deal with.

The laws of physics operate over a wide range in scales, and yet the journey from mineral structures and conditions of crystallization to global tectonics can be a long and tortuous road, often with many cul-de-sacs on the way. The reading of this road map is hindered because the evidence preserved in the geological record is both incomplete, and in some aspects, biased. The narrative is a history, with all the concerns of who writes it, supported by an exceptional range of analytical techniques applied to the rocks and minerals preserved.

Yet it is the context of what is analyzed that is critical, and that can be hard to establish. We have tried to emphasize the consequences of plate tectonics, and when it may have become the dominant tectonic regime, rather than focus on the hunt for the oldest example of rocks that may have been generated by possible plate tectonic processes. It is inferred that the effects were themselves global in scale, and thus there needs to be a way to sample them.

This is most readily done through global databases, but traditionally it has also been approached through analysis of material from well-mixed reservoirs, such as the mantle. Aspects of thermal records can reflect global conditions e. The strength of the lithosphere is another global signature that should reflect when it becomes strong enough to be thickened by tectonic processes, and to be subducted. This may be marked by the stabilization of Archean crust and lithosphere, and the formation of cratons, and it is striking that such stabilization took place at different times in different parts of the world over Myr Cawood et al.

The development of new tectonic regimes takes s of millions of years. There is a marked increase in the volumes of rock preserved from the end of the Archean, and yet it also appears to mark a time of considerable changes, globally Figure 13 , which we conclude marks the period in which plate tectonics became the dominant tectonic regime on Earth.

The ideas have been developed in discussions between all three authors, and CH took the lead in writing this review. All authors contributed to the article and approved the submitted version.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. The growth of continental crust. Tectonophysics , 1— Chemical geodynamics. Tectonophysics 81, — Isotope geodynamics. Earth Planet. Chemical structure and evolution of the mantle and continents determined by inversion of Nd and Sr isotopic data. Theoretical methods. The growth of the continent through geological time studied by Nd isotope analysis of shales.

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The late Archean record: a puzzle in ca. Lithos 71, 99— Bradley, D. Passive margins through earth history. Earth Sci. Secular trends in the geologic record and the supercontinent cycle. Brenner, A. Paleomagnetic evidence for modern-like plate motion velocities at 3. Brown, M. Duality of thermal regimes is the distinctive characteristic of plate tectonics since the Neoarchean. Geology 34, — Metamorphic conditions in orogenic belts: a record of secular change.

The contribution of metamorphic petrology to understanding lithosphere evolution and geodynamics. Cameron, W. Boninites, komatiites and ophiolitic basalts. Nature , — Campbell, I. Formation of supercontinents linked to increases in atmospheric oxygen. Capitanio, F. Reconciling thermal regimes and tectonics of the early Earth. Geology 47, — Lithosphere differentiation in the early Earth controls Archean tectonics.

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Jenkin, P. New images from Mars point to ancient river delta that formed billions of years ago October 7, Earth-monitoring Landsat 9 satellite launches in California September 27, Similarly, oceanic crust is also layered, and each layer varies slightly in its composition [ 21 ].

In general, oceanic crust is basaltic and is rich in minerals and substances like silicon, oxygen and magnesium. To determine the chemical composition, it is important to look into mid-ocean ridge basalt MORB. All the MORB reflects the mean composition of no or the zero-age ocean crust apart from back-arc basins [ 22 ]. The evolution of the crust would refer to the gradual development of the crust over time.

Continental crust transforms into oceanic crust in a cyclic and dynamic process [ 23 ]. Where the old crust is being destroyed at convergent boundaries, new crust is being created at divergent boundaries. When rifting first occurs at divergent boundaries, the crust-mantle system transforms due to the temperature, and a rift forms.

Subduction of the low-velocity zone in the upper part of the crust is the main mechanism overlooking the beginning of crustal attenuation. Intruding magma, originating from the mantle under the rift, modifies the intermediate and lower crustal layers. Before the new oceanic crust is created, the intermediate crust disappears completely, and the underneath crustal layer is critically modified by bouts of magma from the mantle sources.

New oceanic crust is then produced from the ridge and spreads out from the spreading centre towards the subduction zone where the crust is eventually destroyed. Components of the crust will return to the upper crust in different forms such as igneous intrusions and contribute to the formation of new continental crust [ 21 ]. Depending on the type of plate boundary and the types of plates involved, the resultant processes and landforms formed differ.

The different phenomena that occur contribute to the evolution of the crust. Another example of the evolution of the crust due to endogenous processes is volcanism, where material from the mantle or the deep crust is deposited onto the surface where it contributes in renewing the crust surface with new igneous rock and landforms. In some places the crust is weaker such as along plate boundaries, the magma forces its way through the rock, extruding rock and releasing pressure, which is why volcanic activity tends to occur near the borders of tectonic plates, for example, the Pacific Ring of Fire [ 22 ].

The composition and origin of the lava determine the type of volcanic landform created, with more fluid mafic lava forming structures such as shield volcanoes and more viscous felsic lava forming structures such as stratovolcanoes from the accumulation of ejecta.

However, in cases where magma does not breach the surface, the magma in horns or magma chambers may solidify to form intrusive or plutonic rocks. Over time, the surrounding softer rock erodes away, revealing the harder plutonic rock beneath, which creates landforms such as plutons, batholiths, dykes, sills, laccoliths and volcanic necks.

The evolutionary processes mentioned above were all a result of forces originating from within the Earth. However, the crust is also shaped by a multitude of processes from external forces such as climate and extraterrestrial material.

An overt example of an extraterrestrial force on the crust would be an impact crater, in which materials from space such as asteroids, meteoroids or comets collide with the Earth, leaving scars on the surface.

The size of the impactor and extension diameter of the resultant impact crater is a decisive factor on the type of crater formed, with crater diameters above 2 km for sedimentary rocks and 4 km for crystalline rocks having a more complex impact structure as opposed to a simple bowl shape [ 25 ]. Climate and weathering are also significant drivers in the continued evolution of the crust. And while the parameters that control climate are complex and not fully understood, its effects can be seen widely.

These processes can be observed in many forms, such as the exposure of batholiths by the erosion of soft rock, the carving of the Grand Canyon or the deposition of sediment by fluvial processes to create river deltas [ 26 ].

Additionally, biological processes also play a role in weathering and erosion. For example, plant roots hold the soil together, providing resistance to erosion [ 25 ]. Plants and burrowing animals also contribute to the mechanical breakdown of rock through wedging caused by growth and burrowing, respectively.

And while we may be unable to observe all geological evolutionary phenomena in the span of a human lifetime, we have more than enough examples and evidence to show that truly drastic changes occur in geological time. While the crust may only comprise the superficial layer of the Earth, it is truly a dynamic and fascinating thing to learn about.

Superficially appearing to be a solid immutable covering of rock on our world, it is actually a collection of gargantuan rock plates of heterogeneous composition floating upon an equally colossal ocean of magma that is the outer mantle. A large-scale exploration program that could reach deeply enough into the crust for a meaningful sample would press the limits of modern drilling technology and would, in any event, be prohibitively expensive.

Fortunately, a simpler solution is at hand. Nature has already accomplished a widespread sampling through the erosion and deposition of sediments. Lowly muds, now turned into solid sedimentary rock, give a surprisingly good average composition for the exposed continental crust.

These samples are, however, missing those elements that are soluble in water, such as sodium and calcium. Among the insoluble materials that are transferred from the crust into sediments without distortion in their relative abundances are the 14 rare-earth elements, known to geochemists as REEs. These elemental tags are uniquely useful in deciphering crustal composition because their atoms do not fit neatly into the crystal structure of most common minerals.

They tend instead to be concentrated in the late-forming granitic products of a cooling magma that make up most of the continental crust. Because the REE patterns found in a variety of sediments are so similar, geochemists surmise that weathering, erosion and sedimentation must mix different igneous source rocks efficiently enough to create an overall sample of the continental crust. All the members of the REE group establish a signature of upper crustal composition and preserve, in the shapes of the elemental abundance patterns, a record of the igneous events that may have influenced the makeup of the crust.

Using these geochemical tracers, geologists have, for example, determined that the composition of the upper part of the continental crust approximates that of granodiorite, an ordinary igneous rock that consists largely of light-colored quartz and feldspar, along with a peppering of various dark minerals.

Deep within the continental crust, below about 10 to 15 kilometers, rock of a more basaltic composition is probably common. The exact nature of this material remains controversial, and geologists are currently testing their ideas using measurements of the heat produced within the crust by the important radioactive elements uranium, thorium and 40 K, the radioactive isotope of potassium.

But it seems reasonable that at least parts of this inaccessible and enigmatic region may consist of basalt trapped and underplated beneath the lower-density continents. It is this physical property of granitic rock--low density--that explains why most of the continents are not submerged.

Continental crust rises on average meters above sea level, and some 15 percent of the continental area extends over two kilometers in elevation. These great heights contrast markedly with the depths of ocean floors, which average about four kilometers below sea level--a direct consequence of their being lined by dense oceanic crust composed mostly of basalt and a thin veneer of sediment. At the base of the crust lies the so-called Mohorovicic discontinuity a tongue-twisting name geologists invariably shorten to "Moho".

This deep surface marks a radical change in composition to an extremely dense rock rich in the mineral olivine that everywhere underlies both oceans and continents. Geophysical studies using seismic waves have traced the Moho worldwide.

Such research has also indicated that the mantle below the continents may be permanently attached at the top.

These relatively cool subcrustal "keels" can be as much as kilometers thick and appear to ride with the continents during their plate-tectonic wanderings. Support for this notion comes from the analysis of tiny mineral inclusions found within diamonds, which are thought to originate deep in this subcrustal region. Measurements show that diamonds can be up to three billion years old and thus demonstrate the antiquity of the deep continental roots.

It is curious to reflect that less than 50 years ago, there was no evidence that the rocks lining ocean basins differed in any fundamental way from those found on land. The oceans were simply thought to be floored with foundered or sunken continents. This perception grew naturally enough from the concept that the continental crust was a world-encircling feature that had arisen as a kind of scum on an initially molten planet.

Although it now appears certain that Earth did in fact melt very early, it seems that a primary granitic crust, of the type presumed decades ago, never actually existed. To answer this question, one needs to consider the earliest history of the solar system. In the region of the primordial solar nebula occupied by Earths orbit, gas was mostly swept away, and only rocky debris large enough to survive intense early solar activity accumulated. These objects themselves must have grown by accretion, before finally falling together to form our planet, a process that required about 50 million to million years.

Late in this stage of formation, a massive planetesimal, perhaps one the size of Mars, crashed into the nearly fully formed Earth. The rocky mantle of the impactor was ejected into orbit and became the moon while the metallic core of the body fell into Earth. As might be expected, this event proved catastrophic: it totally melted the newly formed planet. As Earth later cooled and solidified, an early basaltic crust probably formed. It is likely that at this stage the surface of Earth resembled the current appearance of Venus; however, none of this primary crust has survived.

Whether it sank into the mantle in a manner similar to that taking place on Earth or piled up in localized masses until it was thick enough to transform into a denser rock and sink remains uncertain.

In any event, there is no evidence of substantial granitic crust at this early stage. Telltale evidence of such a crust should have survived in the form of scattered grains of the mineral zircon, which forms within granite and is very resistant to erosion. Although a few ancient zircons dating from near this time have been found the oldest examples are from sedimentary rocks in Australia and are about 4.

More information about the early crust comes from the most ancient rocks to have survived intact.