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Earth's crustal evolution

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

Evolution on this scale can take place because the unique composition of the Earth's crust in comparison to other terrestrial planets. Mars, Venus, Mercury and other planetary bodies have compositionally uniform crusts unlike that of the Earth which contains both oceanic and continental plates. This separation provides the platform for plate tectonics to occur.

Determining the life cycle of the crust is most accurately plotted through direct sampling. This source evidence for the early crust is rarely preserved and so hypothetical models based on present day samples are used to extrapolate this uncertainty. This leads to much dispute and controversy regarding mechanisms of crustal evolution.

Early crust

Mechanisms of formation

Modelled view of mantle thermodynamics showing how hotter melt rises and causes the advection of cooler material downwards. This process reinforced a thermal separation in the magma ocean by decreasing surface temperatures. This contributed to the formation of an initial crust.

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.[1]

The mantle remained hotter than modern day temperatures throughout the Archean.[2] Over time the Earth began to cool as planetary accretion slowed and heat stored within the magma ocean was lost to space through radiation. Once cool enough, the cooler base of the magma ocean would begin to crystallise. This is because pressure of 25GPa at the surface cause the solidus to lower.[3] The formation of a thin 'chill-crust' at the extreme surface would provide thermal insulation to the shallow sub surface, reinforcing the mechanism of deep magma ocean crystallisation.

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.[4]

Heat pipe volcanism is a model for early Earth thermodynamics and in turn contributes to the process by which a cooler, solid lithosphere formed from a molten ocean. Convection is initiated when melt, extracted from the mantle, rises towards the surface due to it's higher temperature and lower density compared to that of the surrounding mantle . The section of the mantle removed where the melt was produced is advected downwards, meanwhile the rising melt loses heat and is deposited or erupted near the surface. A cold lithosphere is able to be created from this process as the rising and erupting melt overlays cooler surface mantle which causes the continual transfer of cooler temperatures downwards.[5]

A further contributing theory to the formation of the first continental crust is also through intrusive plutonic volcanism.[6] The product was a hot, thick lithosphere which underwent regular cycling with the mantle.[7] The heat released by this form of volcanism, as well as assisting mantle convection, increased the geothermal gradient of the early crust.[6] Where heat pipe volcanism would produce a crust too cold to sustain, a combination of these two mechanisms produces a early lithosphere with a geothermal gradient, as well as a composition, that is fitting with that of Earth-like proto-crust.[8]

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 4.53Ga produced continents composed of komatiite, an ultramafic rock rich in magnesium with a high melting point and low dynamic viscosity.[9] Initial internal differentiation dictated that the upper crust was largely composed of fractionated gabbros, the middle of anorthosite with the base of the crust reaching ≈ 60km in depth. [10]

The primordial crust was regularly broken up and re-formed as impacts involving other planetesimals continued for several hundred million years after accretion.[11] Further to this, none of the original crust remains on the surface at the present day due to the Earth's high erosional rates and subduction of tectonic plates throughout it's 4.5 Ga history. This makes it hard to accurately predict the composition and formation of the earliest crust. The destructive processes on Earth do not exist on others planetary bodies such as the Moon, therefore much of the present crust is the same composition as that of its primordial crust which from a magma ocean, like Earth.

The major rock type on the Moon are plagioclase rich anorthosites, through a process of partial melting where less dense plagioclase crystals would accumulate at the surface.[12] Although comparable, the Earth would not have undergone the same process of primordial crust formation. The hydrous phases received by the Earth from the outer solar system reacted with the terrestrial magma ocean keeping it molten for longer than the Moon where the weak gravitational field and anhydrous magma allowed for the formation of a buoyant lithosphere.[13]

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 [14]. Another method of formation is a result of radioactive elements within the Earth decaying, releasing heat energy and eventually causing partial melting of the mantle also producing basaltic lavas.[15] As a result most secondary crust on Earth is formed at mid ocean ridges forming the oceanic crust, whereas on the Moon much of the maria basalts are a product of such processes.

Tertiary crust

Tertiary crust is the most differentiated type of crust and so has a composition vastly different to that of the bulk Earth. Although contributing 0.57% of the mantle mass, the tertiary crust contains over 20% of the abundance of incompatible elements, a product of subducting and partial melting of secondary crust.[16]

Crustal dichotomy

Timing

Development of an impact crater on the Earth's surface showing infill and partial meting at the base, forming the earliest 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 subducted under each other. Therefore crustal dichotomy must have taken place before the commencement of global plate tectonics, in order to create a difference in crustal density for subduction.[17]

The first instance of early subduction is known to be dated at approximately 4Ga[18], therefore the crustal dichotomy occurred during early stages of crustal evolution.

Impact cratering

Large and numerous impact craters can be recognised on planetary bodies across the Solar System, most noticeably the Moon, dating to the period of the Late Heavy Bombardment which terminated approximately 4Ga[19]. Earth could not have escaped the nature of these impacts meaning it is due to the planets 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.[17] This estimate based on the lunar surface is a minimum so predictions are given as lower limits.

Effects

The main impacts 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[20]; some reaching 4km deep and 1000km in diameter.[17]
  • Topographic division between the low-lying impact basins and the now elevated surface.
  • Release in pressure at the surface from the removal of overburden caused a shift in the geothermal gradient allowing for the partial melting of mantle and subsequent flooding of the basins. The pyrolite mantle would have produced basaltic partial melts, compositionally contrasting to the existing sialic crust.[17]

On the global scale impact cratering converted roughly half of the 'continental' crust into terrestrial maria, creating the basis of the dichotomy seen today.[20]

Initiation of plate tectonics

Plume induced subduction

A schematic evolutionary diagram showing the impact of a mantle plume on the early lithosphere (dark blue) and surface proto-crust (brown), initiating subduction and subsequent global plate tectonics

The formation and development of mantle plumes in the early mantle contributed in triggering the lateral movement of crust across the Earth's surface. The effect of upwelling mantle plumes on the lithosphere can be seen today through local depressions around hotspots such as Hawaii. The scale of the impact is however not comparable to the Archean eon, where localised areas of hot mantle rose to the surface where they flattened before a central plume wedge rose further, weakening the damaged and already thin lithosphere. Once the plume head breaks the surface, crust either side of the head is forced downwards through the conservation of mass, initiating subduction.[21] 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.[22]

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. Models using its known characteristics showed that continued magmatism from conductive heat through the plume caused gravitational collapse, the weight of which caused the spreading of the surrounding crust outwards and subsequent subduction around the margins.[23] The anhydrous nature of the crust on Venus prevents the 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[24]. Thus, this model helps provide a mechanism for how plate tectonics could have been triggered on Earth.

Late Heavy Bombardment

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

Growth rates

Surface map of oceanic crust showing young crust (red) and old crust (blue), demonstrating its spatial evolution at the Earth's surface.

Crustal growth rates can be used calculate estimates for the ages of continental crust. This can be done through isotopic comparison, where igneous rocks with the same isotopic composition as initial mantle rock are dated and are assumed to be evidence of the formation of new continental crust. The resulting view of the ages of isotopically juvenile igneous rocks give distinct peaks, representing an increased proportion of igneous rock, at 2.7, 1.9 and 1.2 Ga. The validity of these results is questioned as the peaks could well represent periods of preservation rather than increased continental crust generation. This is backed up 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.[25]

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. Although representative of large periods of time, limitations are found where samples do not solely represent magmatic production events instead mixing of sediments produces a mix of original and altered isotope ratios.[25]

Zircon minerals can be both detrital from sedimentary rocks and crystalline in igneous, and therefore can provide a more accurate estimate of growth rates. Further to this, zircon minerals can be subject to Hf and O isotope ratio analysis. This is important as as Hf isotopes indicate whether a rock originates from the mantle or existing rock and high δ18O values of zircons represent rock recycled at the Earth's surface and thus potentially producing mixed samples.[26] The outcome of this 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. [25]

References

  1. ^ Erickson, Jon (2014-05-14). Historical Geology: Understanding Our Planet's Past. Infobase Publishing. ISBN 9781438109640.
  2. ^ "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.
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  9. ^ 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.
  10. ^ "Hadean Earth and primordial continents: The cradle of prebiotic life". Geoscience Frontiers. 8 (2): 309–327. 2017-03-01. doi:10.1016/j.gsf.2016.07.005. ISSN 1674-9871.
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  12. ^ Walker, David (1983). "Lunar and terrestrial crust formation". Journal of Geophysical Research. 88 (S01): B17. doi:10.1029/jb088is01p00b17. ISSN 0148-0227.
  13. ^ Albarède, Francis; Blichert-Toft, Janne (2007-12-19). "The split fate of the early Earth, Mars, Venus, and Moon". Geochemistry (Cosmochemistry). 339. doi:10.1016/j.crte.2007.09.006.
  14. ^ Condie, Kent C. (2011-08-25). Earth as an Evolving Planetary System. Academic Press. ISBN 9780123852274.
  15. ^ Taylor, Stuart Ross (1985). The Continental Crust: Its Composition and Evolution. Blackwell Scientific Publications.
  16. ^ "The continental crust: Its composition and evolution". 1985-01-01. {{cite journal}}: Cite journal requires |journal= (help)
  17. ^ a b c d "Crustal evolution of the early earth: The role of major impacts". Precambrian Research. 10 (3–4): 195–216. 1980-02-01. doi:10.1016/0301-9268(80)90012-1. ISSN 0301-9268.
  18. ^ Bercovici, David; Ricard, Yanick (2014-04-24). "Plate tectonics, damage and inheritance". Nature. 508 (7497): 513–516. doi:10.1038/nature13072. ISSN 1476-4687. PMID 24717430.
  19. ^ "Bombardment of the early Solar System : Nature Geoscience". www.nature.com. Retrieved 2018-10-01.
  20. ^ 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.
  21. ^ "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.
  22. ^ 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)
  23. ^ 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.
  24. ^ "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.
  25. ^ a b c 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.
  26. ^ 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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