Earth's crustal evolution
Crustal evolution regards the formation, existence, destruction and eventual renewal of the rocky crust found on the surface of the Earth.
Evolution on this scale can take place through the unique composition of Earth's crust in comparison to other terrestrial planets. Mars, Venus, Mercury and other planetary bodies possess compositionally uniform crusts unlike that of the Earth where both oceanic and continental plates make up the overall outermost shell. In particular, crustal evolution represents the growth and destruction rates of both types of crust.
Early crust
Mechanism of formation
The Proto-Earth was entirely molten due to high temperatures created and maintained by compression of the early atmosphere, rapid axial rotation and regular impacts with neighbouring planetesimals.[1] The mantle remained hotter than modern day temperatures throughout the Archean.[2] However, over time the Earth began to cool as the frequency of planetary accretion slowed and heat stored within the magma ocean is lost to space through radiation. Once cool enough, the magma crystallises, starting from the base of the ocean, as this would cool more rapidly due to the lowering of the solidus nearer the surface where pressures are less than 25GPa.[3] The formation of a thin 'chill-crust' at the extreme surface would provide thermal insulation to the shallow sub surface, reinforcing a mechanism of deep magma ocean crystallisation.
The composition of early magma ocean crystallisation varies 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 form the mantle, rises towards the surface due to it's higher temperature 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. A cold lithosphere is able to be created from this process as the rising and eruption of melt overlays cooler surface mantle causing the continual transfer of cooler temperatures downwards.[5]
A further contribution to the formation of the first continental crust is also through volcanism, this time intrusive. This model proposes much of the early crust was formed through plutonic volcanism,[6] forming hotter and thicker 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 result of initial crystallisation from the magma ocean forms the primordial crust. This solidification of the mantle edge at approximately 4.53Ga produced continents with 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 making it hard to accurately predict its composition and formation. Such processes do not exist on others planetary bodies such as the Moon, therefore much of the present crust is the same composition as the primordial crust 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 kept 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
As radioactive elements within the Earth decay they release heat energy, eventually causing partial melting of the mantle resulting in eruptions of basaltic lavas.[14] Recycling of existing primordial crust also contributes to the production of secondary crust, further depleting the mantle of incompatible elements and thus forming 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
The most differentiated type of crust, thus with a composition vastly different to that of the bulk Earth composition. 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
Oceanic and continental crusts are produced and maintained through plate tectonic processes, however the same mechanisms are unlikely to have produced this crustal dichotomy from 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 proto-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 noticably the Moon, dating to the period of the Late Heavy Bombardment terminating approximatly 4Ga[19].
Initiation of plate tectonics
Lifespan
Relative ages
Destruction
References
- ^ Erickson, Jon (2014-05-14). Historical Geology: Understanding Our Planet's Past. Infobase Publishing. ISBN 9781438109640.
- ^ "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.
- ^ "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.
- ^ "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.
- ^ Moore, William B.; Webb, A. Alexander G. (2013-09-26). "Heat-pipe Earth". Nature. 501 (7468): 501–505. doi:10.1038/nature12473. ISSN 1476-4687. PMID 24067709.
- ^ a b "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.
- ^ "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.
- ^ Rozel, A. B.; Golabek, G. J.; Jain, C.; Tackley, P. J.; Gerya, T. (2017-05-08). "Continental crust formation on early Earth controlled by intrusive magmatism". Nature. 545 (7654): 332–335. doi:10.1038/nature22042. ISSN 0028-0836.
- ^ 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.
- ^ "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.
- ^ "Growth of planetary crusts". Tectonophysics. 161 (3–4): 147–156. 1989-04-20. doi:10.1016/0040-1951(89)90151-0. ISSN 0040-1951.
- ^ Walker, David (1983). "Lunar and terrestrial crust formation". Journal of Geophysical Research. 88 (S01): B17. doi:10.1029/jb088is01p00b17. ISSN 0148-0227.
- ^ 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.
- ^ Taylor, Stuart Ross (1985). The Continental Crust: Its Composition and Evolution. Blackwell Scientific Publications.
- ^ Condie, Kent C. (2011-08-25). Earth as an Evolving Planetary System. Academic Press. ISBN 9780123852274.
- ^ "The continental crust: Its composition and evolution". 1985-01-01.
{{cite journal}}: Cite journal requires|journal=(help) - ^ "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.
- ^ 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.
- ^ "Bombardment of the early Solar System : Nature Geoscience". www.nature.com. Retrieved 2018-10-01.