2023-07-13 12:11:39
The shape of the Earth’s surface determined by the Earth’s gravitational field is called a gravitational geoid, without taking into account the orography of the continents and oceans. The geoid is not an ellipsoid, since it presents a “gravitational orography” with elevations and depressions, which depends on the density of the mantle and its temperature. The most striking feature of the geoid is its global minimum, a depression of almost 110 equivalent meters in the Indian Ocean. Many hypotheses have been proposed to explain its origin. A new one is posted on Geophysical Research Letters, based on computational models of the evolution in the last 140 million years of the mantle under the crust; a fascinating explanation based on 7 of the 29 models studied, the ones that best fit current seismological estimates. Said explanation, however reasonable it may seem to us, should be taken as one more hypothesis; Further studies are needed to settle this question. By the way, it has caught my attention that some media speak of a “giant hole” in the Indian Ocean as if it were orographic.
Computer models offer us a surprising explanation of the gravity anomaly under the Indian Ocean. The Tethys Ocean (which lay between the continents of Gondwana and Laurasia 140 million years ago) closed and its floor subducted (subducted) as present-day India moved from south to north of Tethys (where it collided with present-day India). Asia to form the Himalayas) and the Indian Ocean was formed. Because the Earth’s lithosphere is less dense and cooler than the mantle, these remnants of low-density oceanic lithosphere would still persist in the Earth’s mantle below the Indian Ocean; They would already be in the deepest part of the mantle (the core-mantle boundary, CMB) where they have reached after more than 100 million years. The surprise is that, as a reaction to the cooling due to these remnants, hot plumes would appear from the CMB to the upper mantle, which would now lie under Central Africa. The combination of these hot plumes and the cold regions due to subduction would be responsible for the reduction in density under the Indian Ocean that is observed as a gravity anomaly today. Furthermore, such an anomaly would be very recent, around 20 million years old, and modeled as having a very short duration, perhaps only a few million years.
A good example of using computational geophysics to predict the past by fitting models to the present. But we must be cautious with the conclusions of this type of mantle convective models; they are three-dimensional models, but very simplified, depending on initial and boundary conditions, and on many physical parameters that we ignore (for which a reasonable estimate is used). The article is Debanjan Pal, Attreyee Ghosh, “How the Indian Ocean Geoid Low Was Formed,” Geophysical Research Letters 50: e2022GL102694 (16 May 2023), doi: https://doi.org/10.1029/2022GL102694. More informative information in Tom Metcalfe, “Giant ‘Gravity Hole’ in the Ocean May Be the Ghost of an Ancient Sea,” Scientific American 26 Jun 2023. The figure that opens this piece is from Aleš Bezděk, Josef Sebera, “Matlab script for 3D visualizing geodata on a rotating globe,”Computers & Geosciences 56: 127-130 (05 May 2023), doi: https://doi.org/10.1016/j.cageo.2013.03.007.
The CitcomS code has been used for the simulation of the mantle convection, which solves the equations of conservation of energy, momentum and mass in a three-dimensional spherical shell. Of course, the code involves a number of simplifications, for example, it assumes that the mantle is an incompressible fluid with infinite Prandtl number. Also, the spatiotemporal resolution of the code is coarse, even though it has been run on the Indian supercomputer Param Pravega (which is not in the June 2023 Top 500but according to some sources would be more powerful than Param Siddhi, who is ranked 131: for comparison, MareNostrum is ranked 98). But what I would like to highlight is that the depends a lot on the initial and boundary conditions, which in turn depend on the estimated evolution of the tectonic plates. It also depends on the constitutive equations used for the mantle; These include six parameters that affect the formation of feathers (key in the new hypothesis), among which buoyancy stands out (buyoancy ratio) and the dependence of viscosity on temperature and radius (the mantle is described with four distinct layers). Different values have been taken for these parameters in the 19 different models considered; but, obviously, a comprehensive exploration of this large parameter space would have required thousands of models. Only 7 of the 19 models reproduce the minimum IOGL observed (with a correlation greater than 0.75); Among them, the one that offers the highest correlation (0.80) has been chosen to draw conclusions (which according to the authors are quite robust when compared with the other six models).
Mantle temperature maps show plumes that are hot (red and brown regions) and cool regions (blue and light blue regions). I found it curious that the units are not indicated in the temperature scales (the dimensionless interval between 0.4 and 0.6 is used) and that the article does not explicitly clarify said dimensionality. However, as it is indicated that an average mantle temperature of 1000 °C is assumed, of 0 °C in the crust and of 2000 °C at the core-mantle boundary, I think we can lower that the dimensionless temperature must be multiplied by 2000 ° C; thus the hot plumes that reach 0.6 would have a temperature of about 1200 °C. However, I have some doubts about it, since the article mentions that hot plumes reach temperatures between 1400 °C and 1500 °C, which would correspond to dimensionless values between 0.7 and 0.75. This kind of thing happens when a computational physicist reads articles on computational geophysics; certain details are omitted because it is assumed that everyone knows them, which is not always true.
In summary, it seems very curious to me that the explanation of what happens in the mantle under the Indian Ocean requires taking into account phenomena that are observed in the mantle under Central Africa. And even more curious that pieces (or plates, of slabs in English) of the lithosphere of the extinct Tethys Ocean sinking into the mantle to the core-mantle boundary. As an explanation I find it fascinating. What I don’t know is what geophysicists experts in this field will think about this new hypothesis.
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