Role of dynamic topography in sustaining the Nile River over 30 million years
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Ocean surface topography
Nile delta
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Summary In this paper we attempt to find a solution of the following problem. It appears reasonable to expect that if thermal convection occurs in the Earth’s mantle, it may also occur within the Moon and Mars. The dimensions of these latter two bodies are comparable to the thickness of the Earth’s mantle. Presumably the amount of radioactive heat generated per unit mass is similar in all three bodies. Yet the surface morphology of the Earth, which many scientists believe arises ultimately from mantle convection, differs markedly from that of the Moon or Mars. The explanation advanced here for this difference is based on the effect produced on convection in the mantle by the presence of a low ‘viscosity ’ or low creep strength layer. It is assumed that the low velocity layer of the mantle is such a low creep strength layer. A low viscosity layer changes the amount of ‘coupling’ between the outer crust and mantle convection. (The crust is ‘coupled’ if mantle convection produces stresses in the crust which are large enough to deform it plastically. The crust is ‘decoupled’ from the mantle if these stresses are insufficient to produce plastic deformation.) The theory assumes that the viscosity or creep strength is essentially zero in the low viscosity layer. The analysis is similar to that developed earlier for the calculation of stresses within the mantle. We find that the deeper within the mantle lies the low viscosity layer the greater is the coupling of the outer crust to the mantle convection currents. If the low creep strength layer lies close to the surface the outer crust is decoupled from the interior. According to the literature the depth of the low velocity layer is determined by the temperature and pressure profiles within the mantle. If the temperature profile were kept constant but the pressure reduced by changing g, the gravitational acceleration, the low velocity zone would be moved to a level closer to the surface. It is proposed that because of this interdependence between temperature, pressure and depth of the low velocity layer, the low velocity layers in Mars and the Moon (if such layers exist in these bodies) lie much closer to the surface than does the corresponding layer in the Earth. Hence the outer crusts of Mars and the Moon are more nearly decoupled from the interior than is the Earth’s crust.
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Abstract Two new quantitative terms, the box replacement ratio and the pervasion ratio, are employed to investigate convective mantle mixing in this paper. The former is the ratio of the mantle boxes that have been replaced at least once by other mantle boxes to the total number of mantle boxes. The latter is the ratio of initial to final volumetric density of tracers which were initially confined in a small space. In this study we assume that mantle convection is steady, it is driven by plate motion or density anomalies in the mantle and the viscosities in the upper and lower mantle are different. In our numerical experiment the mantle is divided into 20736 boxes (5° × 5° × 300 km) and there are 2376 tracers placed on 5° × 5° grids at 100 km depth and at 100 km above CMB. The results show that the box replacement ratio of convective mantle mixing is over 80% for the whole mantle and is over 90% for upper mantle after 4 billion years. It means that most mantle boxes have been replaced. For two groups of 1681 tracers in a small space of 10° × 10° (at intervals of 0.25°) placed at the top and bottom of the mantle their pervasion ratios approach a constant and the initial tracers become widely distributed in the mantle after 3 billion years. These results indicate that the Earth's mantle, except for the lithosphere and “D” layer, has been mixed very well in the past 4 billion years. It is not likely that initial heterogeneity boxes greater than 5° × 5° × 300 km remain in the present mantle.
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Core–mantle boundary
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Abstract. Much effort is being made to extract the dynamic components of the Earth's topography driven by density heterogeneities in the mantle. Seismically mapped density anomalies have been used as an input into mantle convection models to predict the present-day mantle flow and stresses applied on the Earth's surface, resulting in dynamic topography. However, mantle convection models give dynamic topography amplitudes generally larger by a factor of ∼2, depending on the flow wavelength, compared to dynamic topography amplitudes obtained by removing the isostatically compensated topography from the Earth's topography. In this paper, we use 3-D numerical experiments to evaluate the extent to which the dynamic topography depends on mantle rheology. We calculate the amplitude of instantaneous dynamic topography induced by the motion of a small spherical density anomaly (∼100 km radius) embedded into the mantle. Our experiments show that, at relatively short wavelengths (<1000 km), the amplitude of dynamic topography, in the case of non-Newtonian mantle rheology, is reduced by a factor of ∼2 compared to isoviscous rheology. This is explained by the formation of a low-viscosity channel beneath the lithosphere and a decrease in thickness of the mechanical lithosphere due to induced local reduction in viscosity. The latter is often neglected in global mantle convection models. Although our results are strictly valid for flow wavelengths less than 1000 km, we note that in non-Newtonian rheology all wavelengths are coupled, and the dynamic topography at long wavelengths will be influenced.
Ocean surface topography
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Hotspot (geology)
Core–mantle boundary
Planetary differentiation
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Mantle convection patterns of the past are not well known, yet an understanding of changing mantle convection characteristics is fundamental to understanding the evolution of plate tectonics. There are very few ways to examine mantle characteristics of the past. Changes in spreading rate and volcanic activity with time have been used to draw conclusions about historic changes in mantle activity. Mantle temperature has been found to be related to crustal thickness. With this relationship, crustal thicknesses may now yield new conclusions about historic changes in mantle characteristics. We have inferred changes in mantle convection patterns throughout the last 180 m.y. by examining variations in assumed crustal thickness within the Pacific basin. Crustal thicknesses were calculated from residual depth anomalies by assuming that residual depth anomalies are the result of isostatic compensation of variations in crustal thickness. Crustal thickness is determined at the time of crustal formation and is dependent upon the temperature of the mantle source material. Intraplate hot spot volcanism effects on crustal thickness were not ignored. Examination of variations in crustal thickness of crust of different ages can reveal information about changing temperatures of the mantle at the ridge through time. We have learned that mantle temperatures at the ridge during the mid‐Cretaceous were more variable than those temperatures at the ridge after the mid‐Cretaceous. Furthermore, we have inferred from the data that mantle temperatures at hot spots were higher during the mid‐Cretaceous than those at hot spots existing after the mid‐Cretaceous. We suggest that mantle convection at the ridge was more rapid during the mid‐Cretaceous causing a higher variability of temperatures at the ridge. We also note that this period of increased mantle convection is concurrent with the increased mantle temperatures at hot spots within the Pacific basin.
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Crustal recycling
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Core–mantle boundary
Planetary differentiation
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Advancements in the study of mantle convection and the material movements in the deep Earth interior
Now geo-science realized that the mantle convection is not the imagination of few geodynamical studies. It is the main frame of the mantle thermodynamic system. The study of the mantle convection and plate tectonics has been transformed from the simple discussions of the driving mechanics either passive or active into the discussions of en united mantle thermodynamic system. The mantle convection system (including plumes) not only the main subjects in the study of the mantle evolution, but also in the study of the continent formation and evolution. At the same time, the geophysical (in particular seismic tomography) and geo-chemical observations turn into the main way to investigate the mantle convection system. However, these two groups of data set are inconsistent in the inferring the structure of mantle convection. Some new mantle thermodynamic models based on the geo- chemical data became the strong challenge to the study of the mantle convection.
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Mantle plume
Core–mantle boundary
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In this paper we attempt to find a solution of the following problem. It appears reasonable to expect that if thermal convection occurs in the Earth's mantle, it may also occur within the Moon and Mars. The dimensions of these latter two bodies are comparable to the thickness of the Earth's mantle. Presumably the amount of radioactive heat generated per unit mass is similar in all three bodies. Yet the surface morphology of the Earth, which many scientists believe arises ultimately from mantle convection, differs markedly from that of the Moon or Mars. The explanation advanced here for this difference is based on the effect produced on convection in the mantle by the presence of a low ‘viscosity’ or low creep strength layer. It is assumed that the low velocity layer of the mantle is such a low creep strength layer. A low viscosity layer changes the amount of ‘coupling’ between the outer crust and mantle convection. (The crust is ‘coupled’ if mantle convection produces stresses in the crust which are large enough to deform it plastically. The crust is ‘decoupled’ from the mantle if these stresses are insufficient to produce plastic deformation.) The theory assumes that the viscosity or creep strength is essentially zero in the low viscosity layer. The analysis is similar to that developed earlier for the calculation of stresses within the mantle. We find that the deeper within the mantle lies the low viscosity layer the greater is the coupling of the outer crust to the mantle convection currents. If the low creep strength layer lies close to the surface the outer crust is decoupled from the interior. According to the literature the depth of the low velocity layer is determined by the temperature and pressure profiles within the mantle. If the temperature profile were kept constant but the pressure reduced by changing g, the gravitational acceleration, the low velocity zone would be moved to a level closer to the surface. It is proposed that because of this interdependence between temperature, pressure and depth of the low velocity layer, the low velocity layers in Mars and the Moon (if such layers exist in these bodies) lie much closer to the surface than does the corresponding layer in the Earth. Hence the outer crusts of Mars and the Moon are more nearly decoupled from the interior than is the Earth's crust.
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Radiogenic nuclide
Planetary differentiation
Hotspot (geology)
Mantle plume
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