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Earth's crustal evolution

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Surface map of oceanic crust showing the generation of younger (red) crust and eventual destruction of older (blue) crust. This demonstrates the crustal spatial evolution at the Earth's surface dictated by plate tectonics.

Crustal evolution involves the formation, destruction and renewal of the rocky outer shell at the Earth's surface.

The unique composition of the Earth's crust in comparison to other terrestrial planets accounts for the complex series of crustal processes that have taken place throughout the planet's history. Mars, Venus, Mercury and other planetary bodies have compositionally uniform crusts unlike that of the Earth which contains both oceanic and continental plates.[1] This separation provides the constant regeneration of crustal material to take place through plate tectonics to occur.

Determining the formation and overall life cycle of this unique crust is most accurately plotted through direct sampling. However, this source of evidence for the early crust is rarely preserved and so hypothetical models based on present day samples are used to extrapolate likely predictions. There is therefore much dispute over the true mechanisms of early crustal evolution.

Early crust

Types of crust

Primordial crust

The initial crystallisation of minerals from the magma ocean forms the primordial crust. This solidification of the mantle edge at approximately 4.43Ga produced continents composed of komatiite, an ultramafic rock rich in magnesium with a high melting point and low dynamic viscosity.[2] One line of research proposes that differences in densities caused the separation of crustal rocks; upper crust largely composed of fractionated gabbros and lower crust composed of anorthosites. The result of initial crystallisation formed a primordial crust roughly 60km in depth. [3]

The primordial crust was regularly broken and re-formed by impacts involving other planetesimals which continued for several hundred million years after accretion, approximately 4.4Ga.[4] Further to this, none of the original crust remains on the surface at the present day. This is due to Earth's high erosional rates and the subduction and subsequent destruction of tectonic plates throughout it's 4.5 Ga history. It is therefore hard to accurately predict the composition and formation of the earliest crust.

Secondary crust

Recycling of existing primordial crust contributes to the production of secondary crust. Partial melting of the existing crust increases the mafic content of the melt producing basaltic secondary crust.[5] A further method of formation due to the decay of radioactive elements within the Earth releasing heat energy and eventually causing the partial melting of upper mantle, also producing basaltic lavas.[6] As a result, most secondary crust on Earth is formed at mid ocean ridges forming the oceanic crust.

Tertiary crust

Tertiary crust is the most differentiated type of crust and so has a composition vastly different to that of the bulk Earth.[7] The present day continental crust is an example of a tertiary crust. Although contributing 0.57% of the mantle mass, the tertiary crust contains over 20% of the abundance of incompatible elements.[7] This a result of its generation from the subduction and partial melting of secondary crust where it undergoes further fractional crystallisation, evolving its chemical composition through the selective removal of certain compatible elements.[7]

A phase diagram showing the order of crystallisation within the early mantle to form the early crust. The early mantle adiabats show crystallisation took place from from the base; above approximately 25GPa (deep mantle) perovskites would begin to crystallise, below 25GPa (upper mantle) olivine would crystallise.

Mechanisms of formation

The early Earth was entirely molten. This was due to high temperatures created and maintained by the following processes:

  • Compression of the early atmosphere
  • Rapid axial rotation
  • Regular impacts with neighbouring planetesimals.[8]

The mantle remained hotter than modern day temperatures throughout the Archean.[9] Over time the Earth began to cool as planetary accretion slowed and heat stored within the magma ocean was lost to space through radiation.

A theory for the initiation of magma solidification states that once cool enough, the cooler base of the magma ocean would begin to crystallise first. This is because pressure of 25GPa at the surface cause the solidus to lower.[10] The formation of a thin 'chill-crust' at the extreme surface would provide thermal insulation to the shallow sub surface, keeping it warm enough to maintain the mechanism of crystallisation from the deep magma ocean.[10]

The composition of the crystals produced during the crystallisation of the magma ocean varied with depth. Experiments involving the melting of peridotite magma show that deep in the ocean (>≈700m), the main mineral present would be Mg-perovskite. Whereas olivine would dominate in the shallower areas along with it's high pressure polymorphs e.g. garnet and majorite.[11]


A contributing theory to the formation of the first continental crust is through intrusive plutonic volcanism. The product of these eruptions formed a hot, thick lithosphere which underwent regular cycling with the mantle.[12] The heat released by this form of volcanism, as well as assisting mantle convection, increased the geothermal gradient of the early crust.[13]

Crustal dichotomy

Crustal dichotomy represents the distinct contrast in composition and nature of the oceanic and continental plates, which together form the overall crust.

Timing

Development of a the base of an impact crater on the Earth's surface showing infill of basaltic partial melts from mantle. This solidified to form the early differentiated oceanic crust.

Oceanic and continental crusts are produced and maintained through plate tectonic processes. However the same mechanisms are unlikely to have produced the crustal dichotomy of the early lithosphere. Sections of the thin, low density continental lithosphere could not have been sub-ducted under each other. Taking these mechanisms to be true, a proposed timing for crustal dichotomy is put forward stating that dichotomy must have taken place before the commencement of global plate tectonics. This is so a difference in crustal density could be established to facilitate plate subduction.[14]

The first instance of early subduction is known to be dated at approximately 4Ga[15], therefore crustal dichotomy occurred during early stages of crustal evolution.

Formation

Impact cratering

Large and numerous impact craters can be recognised on planetary bodies across the Solar System. These craters date back to a period where there was an increased frequency and intensity of asteroid impacts with terrestrial planets, known as the Late Heavy Bombardment, which terminated approximately 4Ga[16]. The Earth would have sustained the same relative intensity of cratering as other planetesimals in the Solar System. It is therefore only due to Earth's high erosional rates and constant plate tectonics that the craters are not visible today. Scaling up the number and size of impact craters seen on the Moon to fit the size of Earth provides an estimate as the extent of the effect the Late Heavy Bombardment. It is predicted that at least 50% of the Earth's initial crust was covered in impact basins.[14] This estimate, based on the lunar surface, is a minimum and so predictions are interpreted as lower limits.

Effects

The main effects of impact cratering on the early lithosphere were:

  • Formation of large craters. Isostatic rebound would adjust the depth of the craters making them relatively shallow in comparison to their diameter[17]; some reaching 4km deep and 1000km in diameter.[14]
  • Topographic division between the low-lying impact basins and the now elevated surface.
  • Release in pressure at the surface from the removal of overburden. This produced a greater increase in temperature with depth below the surface. Increased surface temperatures caused the partial melting of mantle which erupted and deposited within the surface basins. The pyrolite mantle would have produced basaltic partial melts, compositionally contrasting to the existing sialic crust.[14]

On the global scale impact cratering converted roughly half of the 'continental' crust into terrestrial maria, creating the basis of the dichotomy seen today.[17]

Initiation of plate tectonics

A schematic evolutionary diagram showing the impact of a mantle plume on the early lithosphere (dark blue) and surface proto-crust (brown). This initiated subduction and subsequent global plate tectonics within a previously unseparated lithosphere that had no lateral surface movement.

Plume induced subduction

The formation and development of plumes in the early mantle contributed to triggering the lateral movement of crust across the Earth's surface.[13] The effect of upwelling mantle plumes on the lithosphere can be seen today through local depressions around hotspots such as Hawaii. The scale of this impact is much less than that exhibited in the Archean eon where mantle temperatures were much greater. Localised areas of hot mantle rose to the surface through a central plume wedge, weakening the damaged and already thin lithosphere.[13] Once the plume head breaks the surface, crust either side of the head is forced downwards through the conservation of mass, initiating subduction.[18] Numerical modelling shows only strongly energetic plumes are capable of weakening the lithosphere enough to rupture it, such plumes would have been present in the hot Archean mantle.[19]

Pre-tectonic subduction can also be inferred from the internal volcanism on Venus. Artemis Corona is a large plume formed by the upwelling of mantle derived magma and is on a scale potentially comparable to that in the Archean mantle.[1] Models using its known characteristics showed that continued magmatism from conductive heat through the plume caused gravitational collapse. The weight of collapse caused the spreading of the surrounding crust outwards and subsequent subduction around the margins.[20] The anhydrous nature of the crust on Venus prevents it from sliding past each other, whereas through the study of oxygen isotopes, the presence of water on Earth can be confirmed from 4.3Ga[21]. Thus, this model helps provide a mechanism for how plate tectonics could have been triggered on Earth, although it does not demonstrate that subduction was initiated at the earliest confirmed presence of water on Earth. Based on these models, the onset of subduction and plate tectonics is dated at 3.6Ga.[20]

Late Heavy Bombardment

Impact cratering also had consequences for both the development of plume-induced subduction and the establishment of global plate tectonics.[16] The steepening of geothermal gradients could have directly enhanced convective mantle transport which now beneath an increasingly fractured lithosphere could have created stresses great enough to cause rifting and the separation of crust into plates.[16]

Growth rates

Plots showing the rate of continental crustal growth over time as a percentage of the total mass, along with associated thickness of the newly generated crust. The plot of crustal reworking represents the amount of post formational alteration undergone by the crust. The dramatic increase in crustal reworking and reduction in the rate of crustal growth at approximately 3Ga represents the onset of subduction and plate tectonics

Lithological dating

Crustal growth rates can be used to calculate estimates for the age of the continental crust. This can be done through analysis of igneous rocks with the same isotopic composition as initial mantle rock. These igneous rocks are dated and assumed to be direct evidence of new continental crust formation.[21] The resulting ages of isotopically juvenile igneous rocks give distinct peaks, representing an increased proportion of igneous rock and therefore increased crust growth, at 2.7, 1.9 and 1.2 Ga. The validity of these results is questioned as the peaks could represent periods of preservation rather than increased continental crust generation. This is reinforced by the fact that such peaks are not observed in recent geologic time where it is given that magmatism resulting from the plate subduction has strongly contributed to producing new crust.[22]

Crustal growth rates from igneous rocks can be compared to the rates generated from radiogenic isotope ratios in sedimentary rocks. Projections of growth rates using these techniques does not produce staggered peaks, instead smooth shallow curves presenting a more constant rate of crustal growth.[22] Although representative of large periods of time, limitations are found where samples do not solely represent magmatic production events. Instead samples include the mixing of sediments which produces a mix of original and altered isotope ratios.[22]

Zircon dating

Zircon minerals can be both detrital grains from sedimentary rocks and crystals in igneous rocks. Therefore a combination of zircon forms can provide a more accurate estimate of crustal growth rates. Further to this, zircon minerals can be subject to Hf and O isotope ratio analysis.[21] This is important as as Hf isotopes indicate whether a rock originates from the mantle or an existing rock. High δ18O values of zircons represent rock recycled at the Earth's surface and thus potentially producing mixed samples.[23] The outcome of this combined analysis is valid zircons showing periods of increased crustal generation at at 1.9 and 3.3Ga, the latter of which representing the time period following the commencement of global plate tectonics. [22]

References

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  2. ^ Nna-Mvondo, Delphine; Martinez-Frias, Jesus (2007-02-15). "Review komatiites: from Earth's geological settings to planetary and astrobiological contexts". Earth, Moon, and Planets. 100 (3–4): 157–179. doi:10.1007/s11038-007-9135-9. ISSN 0167-9295.
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  6. ^ Taylor, Stuart Ross (1985). The Continental Crust: Its Composition and Evolution. Blackwell Scientific Publications.
  7. ^ a b c "The continental crust: Its composition and evolution". 1985-01-01. {{cite journal}}: Cite journal requires |journal= (help)
  8. ^ Erickson, Jon (2014-05-14). Historical Geology: Understanding Our Planet's Past. Infobase Publishing. ISBN 9781438109640.
  9. ^ "A great thermal divergence in the mantle beginning 2.5 Ga: Geochemical constraints from greenstone basalts and komatiites". Geoscience Frontiers. 7 (4): 543–553. 2016-07-01. doi:10.1016/j.gsf.2016.01.006. ISSN 1674-9871.
  10. ^ a b "Early Earth differentiation". Earth and Planetary Science Letters. 225 (3–4): 253–269. 2004-09-15. doi:10.1016/j.epsl.2004.07.008. ISSN 0012-821X.
  11. ^ "Melting experiments of mantle materials under lower mantle conditions with implications for magma ocean differentiation". Physics of the Earth and Planetary Interiors. 143–144: 397–406. 2004-06-15. doi:10.1016/j.pepi.2003.09.016. ISSN 0031-9201.
  12. ^ "Generation of felsic crust in the Archean: A geodynamic modeling perspective". Precambrian Research. 271: 198–224. 2015-12-01. doi:10.1016/j.precamres.2015.10.005. ISSN 0301-9268.
  13. ^ a b c "Early Earth plume-lid tectonics: A high-resolution 3D numerical modelling approach". Journal of Geodynamics. 100: 198–214. 2016-10-01. doi:10.1016/j.jog.2016.03.004. ISSN 0264-3707.
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  16. ^ a b c "Bombardment of the early Solar System : Nature Geoscience". www.nature.com. Retrieved 2018-10-01.
  17. ^ a b "Origin of the Earth's ocean basins". Icarus. 32 (2): 235–250. 1977-10-01. doi:10.1016/0019-1035(77)90064-1. ISSN 0019-1035.
  18. ^ "Late Cretaceous plume-induced subduction initiation along the southern margin of the Caribbean and NW South America: The first documented example with implications for the onset of plate tectonics". Gondwana Research. 27 (1): 38–63. 2015-01-01. doi:10.1016/j.gr.2014.07.011. ISSN 1342-937X.
  19. ^ Gerya, T. V.; Stern, R. J.; Baes, M.; Sobolev, S. V.; Whattam, S. A. (2015-11). "Plate tectonics on the Earth triggered by plume-induced subduction initiation". Nature. 527 (7577): 221–225. doi:10.1038/nature15752. ISSN 0028-0836. {{cite journal}}: Check date values in: |date= (help)
  20. ^ a b Van Kranendonk, Martin J (2010). "Two types of Archean continental crust: Plume and plate tectonics on early Earth". American Journal of Science. 310: 1187–1209.
  21. ^ a b c "Magmatic δ18O in 4400–3900 Ma detrital zircons: A record of the alteration and recycling of crust in the Early Archean". Earth and Planetary Science Letters. 235 (3–4): 663–681. 2005-07-15. doi:10.1016/j.epsl.2005.04.028. ISSN 0012-821X.
  22. ^ a b c d Hawkesworth, C. J.; Kemp, A. I. S. (2006-10-19). "Evolution of the continental crust". Nature. 443 (7113): 811–817. doi:10.1038/nature05191. ISSN 1476-4687. PMID 17051208.
  23. ^ Condie, K. C. (2014-06). "Growth of continental crust: a balance between preservation and recycling". Mineralogical Magazine. 78 (03): 623–637. doi:10.1180/minmag.2014.078.3.11. ISSN 0026-461X. {{cite journal}}: Check date values in: |date= (help)
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