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Nature of Science

In some instances mantle clearly drives changes in the crust, as in the Hawaiian Islands. However, there is ongoing debate whether oceanic crust subduction and mid ocean ridge spreading is driven by a push or pull mechanism. In very broad terms, oceanic crust is made up of basalt and continental crust is made up of rocks similar to granite.

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The lithosphere is physically distinct from the below-lying layers due to its cool temperatures and typically extends km in depth. The asthenosphere begins at the bottom of the lithosphere and extends approximately km into the Earth. The asthenosphere acts as the lubricating layer below the lithosphere that allows the lithosphere to move over the Earth's surface. The mantle starts at the Mohorovicic Discontinuity , also known as the Moho. The Moho is defined as the density contrast from less dense crust to denser mantle and where seismic wave velocities increase.

The mantle acts similar to plastic and at very high temperatures and pressures the rock is deformable at geologic timescales. This deformation causes a convection like process in the mantle where you have larige-scale upwelling and downwelling zones. The mantle extends down to 2, km into the Earth's surface Temperatures that range from to degrees Celsius in the upper portion to over 4, degrees Celsius near the core boundary. Earth's mantle is believed to be composed of bulk mineralogy similar to peridotite. The video above provides a glimpse into the global circulation of mantle magma around the Earth.

The outer core is the liquid largely iron layer of the earth that lies below the mantle. Geologists have confirmed that the outer core is liquid due to seismic surveys of Earth's interior. The outer core is 2, km thick and goes down to approximately 3, km into the earth. When Earth was just beginning to cool billions of years ago, heavier elements sunk down into the center of the Earth, while less dense elements rose to the surface.

Therefore, we see a general increase in density, as you get closer to the center of the Earth. The outer core is hot enough to be melted but not under quite enough pressure to make the iron solid again, as seen in the inner core. However, two seismic studies found no evidence for a stratification Alexandrakis and Eaton, ; Irving et al. Second, the limited set of simulations presented in this study does not allow for the derivation of scaling laws, which prevents any direct extrapolation to the Earth's core.

Third, several physical ingredients are not considered in the present study such as the magnetic field, thermal convection, and turbulence. Table 2. List of the main studies that proposed some constraints on the stratified layer at the top of the Earth's core based on either magnetic or seismic observations. The presence of a magnetic field can influence the dynamics of buoyant parcels of fluid in a non-trivial way Moffatt and Loper, ; Sreenivasan and Narasimhan, and should be incorporated in future studies. One may also argue that adding the destabilizing effect of thermal convection omitted in this study could destroy the chemically stratified layer.

Although some preliminary simulations of thermochemical convection not shown in this paper suggest this is not the case—at least when both fields contribute equally to the buoyancy flux—the dynamics of thermochemical convection remains to be explored. Note that thermal effects may not necessarily be destabilizing. The recently revised value of the thermal conductivity de Koker et al. The presence of a thermally stratified layer at the top of the core may ease the formation of chemical stratification as well.

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The present study also ignores any possible double-diffusive phenomena that may affect the dynamics of a stably stratified layer in various ways. On the other hand, the method of light elements injection in our simulations may not adequately reflect the complex chemical processes occurring in the inner core boundary region. Indeed, injecting light elements via a homogeneous source term above the bottom boundary tends to generate plumes with a sheet-like structure instead of the cylindrical shape that would be expected if plumes erupt from isolated chimneys.

Furthermore, the scaling law 22 may not be relevant to predict the size of chemical plumes in core conditions if these plumes erupt from localized chimneys formed in the mushy layer. The initial width of the plumes would in that case likely be controlled by the dynamics of the mushy layer rather than by the Rayleigh-Taylor destabilization of a homogeneous light layer.

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Unfortunately, the size of the chimneys formed in the mush is presently unknown in core conditions. An alternative scenario is the presence of a slurry layer above the inner core boundary Loper and Roberts, ; Gubbins et al. It is not clear whether the nature of the first instability affects what we observe in the shell away from the boundary. Another crucial question is whether chemical plumes would experience a complete turbulent mixing in core conditions. Laboratory experiments by Jellinek et al.

Assuming the scaling law 22 can be used as a good proxy for the size of chemical plumes in core conditions, we can estimate. However, as rotation is known to partially inhibit lateral entrainment of fluid and subsequent mixing Helfrich and Battisti, ; Fernando et al. The local Rossby number based on the plumes length scale called for by Equation 22 writes.

Future investigations will be necessary to quantify the mixing of chemical plumes and the role played by rotation in more turbulent flow regimes. One may also question the relevance of the infinite Schmidt number approximation used in the simulations. The dynamics of a rising chemical plume is susceptible to be affected by chemical diffusion if the latter acts at a length scale comparable with the size of the plume or larger during its typical rising time.

This time must be compared with the typical time it takes for a chemical plume or blob to reach the top boundary, which likely depends on the convective regime. For intermediate Schmidt numbers, chemical diffusion would therefore not be negligible for the dynamics of chemical plumes. Chemical diffusion could also be a significant contributor to the long-term dynamics of the chemically stratified layer, since the latter builds over long time scales in the simulations.

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In the case of turbulent flows, chemical diffusion may also act at small scales and allow compositional mixing. Only stirring occurs when chemical diffusion is neglected. The infinite Schmidt number approximation may thus be debatable for the modeling of chemical convection, but this end-member case is certainly a better approximation than the codensity approach in the sense that it can capture physical aspects that are specific to the weakly-diffusive compositional field.

Despite the numerous limitations of the present study listed above that prevent any direct extrapolation of our results to the Earth's core, some physical considerations can be invoked to constrain the properties of a chemically stratified layer built by the accumulation of light chemical blobs emitted at the ICB.

Here we try to obtain an order of magnitude for this density anomaly using very simple hypotheses and calculations. Following Moffatt but see Moffatt and Loper, for a more elaborate study , we assume that the dynamics of erupting chemical blobs is governed by a force balance between the Coriolis and buoyancy forces. The range of acceptable values is however constrained by the typical velocity expected for the fluid core. Since velocities have a statistical distribution, the velocities of erupting blobs may be significantly larger than the RMS velocity in the convecting region.

Earth's outer core

One may however criticize the simplicity of this assumption by arguing that the properties of the blobs erupting from the mush chimneys would rather be controlled by the dynamics of the mushy layer. Should the accumulation of chemical blobs alone be unable to explain the presence of a thick stably stratified layer at the top of the core, it might still play an important role. Gubbins and Davies studied the possible formation of a chemically stratified layer by barodiffusion. The latter could well be produced by the accumulation of incompletely mixed chemical blobs emitted at the inner core boundary.

Though the absence of scaling laws prevents us from making a prediction on the thickness of such a layer, the stratified layer rapidly extends well beyond the Ekman layer in all our simulations. We therefore suggest that the accumulation of incompletely mixed chemical plumes may form a stratified layer of sufficient thickness to allow a thicker layer to grow according to the scenario of Gubbins and Davies Using a new version of the PARODY code including a particle-in-cell method, we ran a first series of exploratory simulations of pure chemical convection at infinite Schmidt number in a rotating spherical shell.

In these simulations, light elements are injected within a thin layer above the bottom boundary. This preliminary work remains descriptive and qualitative, but reveals that chemical convection in the Earth's outer core is a rich and vast topic that warrants several more focused and quantitative studies. We showed that, principally as a result of their lower diffusivity, chemical plumes have a complex dynamics that may be influenced by several distinct instabilities in the different regions of the shell.

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Plumes may also experience jet-instability and start widening and entraining ambient fluid. A particularly interesting result is the systematic formation of a chemically stratified layer at the top of the shell in all simulations. The layer has a latitudinal topography and slowly grows owing to the accumulation of incompletely mixed chemical plumes and blobs emitted at the bottom boundary. The existence of a stably stratified layer at the top of the core has been proposed for several decades based on seismic and magnetic observations Lay and Young, ; Tanaka, ; Helffrich and Kaneshima, ; Buffett, ; Kaneshima, as well as core evolution models Labrosse, However, to our knowledge, this is the first time that the self-consistent formation of a stratified layer is observed in numerical simulations.

As our simulations operate with control parameters that are very remote from core conditions and miss some important ingredients like thermal convection, magnetic field and turbulence, it is not clear whether the results shown in this study would hold when approaching more realistic conditions. Future studies should aim at incorporating these essential ingredients and systematically exploring the parameter space in order to understand more quantitatively the physics of compositional convection and make predictions for the Earth.

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Our preliminary study suggests that a chemically stratified layer formed by the accumulation of chemically buoyant parcels of fluid released at the bottom of the core would probably be weakly stratified and thus, likely seismically invisible. However, this relies on a force balance assumption that may be irrelevant.

Constraints on the stratification that can be produced by such a scenario may rather come from the precise dynamics of the mushy layer which is presently poorly understood and deserves future investigations. MB ran the simulations, analyzed the data, and wrote the manuscript. GC, SL, and JW significantly contributed to the physical reflection, the interpretation of the results, and to the elaboration of the paper.

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. Kodama for their valuable comments which were very helpful to improve the manuscript. We also thank Philippe Cardin and Hagay Amit for fruitful discussions on this topic. Alexandrakis, C. Precise seismic-wave velocity atop Earth's core: No evidence for outer-core stratification. Earth Planet. Aubert, J. The magnetic structure of convection-driven numerical dynamos.

Aurnou, J. Experiments on convection in Earth's core tangent cylinder. Badro, J. An early geodynamo driven by exsolution of mantle components from Earth's core.

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Nature Baines, W. Turbulent buoyant convection from a source in a confined region. Fluid Mech.

Bergman, M. Chimneys on the Earth's inner-outer core boundary? Bouffard, M.