The Nature Of Convection In The Mantle

The evidence for convective flow in the mantle, from seismic tomography and studies of the regional elevation and subsidence of the Earth's surface, strongly suggests that there are two main driving forces for this convection. The negative buoyancy of cold subducting lithosphere would appear to determine the main sites of downwelling, and the positive buoyancy of hot, low viscosity material originating in the lowermost, D", layer of the mantle determines the upwellings. These two complementary modes of convection in the mantle have been termed the plate and plume modes, respectively (Davies, 1999). Both have their origins in thermal boundary layers: the plate mode in the lithosphere immediately beneath the Earth's surface, and the plume mode in the D" layer of the mantle, immediately above the core-mantle boundary. As Davies (1993) has aptly put it, the plate mode is crucial in cooling the mantle, by the creation of oceanic lithosphere, and the plume mode releases heat from the core. The heat released by the plate mode is thought to be much greater than that released from the core as the mantle is heated internally by radioactivity. One might expect therefore that the plate mode is dominant. These two very different modes of convection need not necessarily be strongly coupled. However it is noteworthy that the two major upwellings at the present day, beneath southern Africa and the south central Pacific, are at the centers of the expanding ring of subduction zones around what was Gondwana and the contracting ring of subduction zones around the Pacific respectively, and hence distant from the cooling effect of the subducting slabs that appear to extend to the core-mantle boundary ((Plate 12.2 (between pp. 244 and 245), Fig. 12.12). It is also striking that these two active upwellings do not correspond directly to mid-ocean ridges. This is consistent with the interpretation of the upwelling beneath ridges being entirely passive. Meguin & Romanowicz (2000) and Montelli et al. (2004b) note that there is evidence in their mantle tomographic models for lateral flow in the upper mantle from the African upwelling to the Atlantic and Indian Ocean ridges, and from the Pacific upwelling to the East Pacific Rise. If so this would complete the elusive route of the return flow from subduction zones to mid-ocean ridges, or at least provide one such route.

The scale, or wavelength, of this gross pattern of convection in the mantle is greater than that predicted by analogue experiments and early numerical models assuming a Rayleigh number greater than 106. It transpires that this is because these models assumed uniform viscosity throughout the convecting layer. In the Earth's mantle the viscosity varies with both temperature and pressure. For the relevant temperature gradient in the mantle the effect of increasing pressure with depth almost certainly means that the viscosity of the lower mantle is significantly greater than that of the upper mantle. Bunge et al. (1997) investigated three-

Mantle Convection

Figure 12.12 Cartoon showing an approximately equatorial section through the Earth and illustrating the possible relationship of subduction zones, superswells, plumes, and mid-ocean ridges (MOR) to the gross pattern of circulation in the mantle. Note that deep-seated or primary plumes, such as Afar, Reunion, Tristan, Hawaii, Easter, and Louisville, are peripheral to the superswells, and that secondary plumes are common above the Pacific superswell. The mid-ocean ridges are a passive response to the plate separation and not systematically related to the main convective pattern.

Figure 12.12 Cartoon showing an approximately equatorial section through the Earth and illustrating the possible relationship of subduction zones, superswells, plumes, and mid-ocean ridges (MOR) to the gross pattern of circulation in the mantle. Note that deep-seated or primary plumes, such as Afar, Reunion, Tristan, Hawaii, Easter, and Louisville, are peripheral to the superswells, and that secondary plumes are common above the Pacific superswell. The mid-ocean ridges are a passive response to the plate separation and not systematically related to the main convective pattern.

dimensional spherical convection models of the mantle in which the viscosity of the lower mantle was 30 times that of the upper mantle. They found that not only was the wavelength of the resulting convection greater but that long linear downwellings formed from the upper boundary layer; both effects making the pattern of convection very comparable to that deduced for the mantle. The convective pattern also had greater temporal stability.

Researchers also have investigated the effect on mantle convection of the endothermic phase change at a depth of 660 km, the base of the transition zone. For plausible physical characteristics of this phase change the results suggest that it might inhibit but not prevent the passage of upwellings and downwellings through it. This is consistent with the results of seismic tomography that indicate that the transition zone has some effect but that it is not sufficient to impede whole mantle convection (Montelli et al., 2004b).

The chemical heterogeneity of layer D" (Section 12.8.4) means that it acts as a thermochemical, rather than a thermal boundary layer. Indeed where it is hottest it is essentially a thermal boundary layer over a chemical boundary layer, the ultra-low velocity zone (ULVZ). Upwellings of the low viscosity, low density thermal boundary layer at these points entrain the low viscosity but higher density chemical boundary layer to a height of 50-100 km depending on the strength of the upwelling (Fig. 12.13). Analogue experiments (Davaille, 1999) indicate that the nature of the upwelling depends on the ratio of the stabilizing chemical density anomaly to the destabilizing thermal density anomaly. If this is greater than 1, a plume-like upwelling forms; if it is approximately 0.5, thermals (broad upwellings or domes) are produced. In either case the entrainment of the dense chemical boundary layer is thought to stabilize the location of the plume or thermal upwelling (Jellinek & Manga, 2004). However, as a result of the greater stability ratio, plumes will tend to be very long-lived.

If this general picture of convection in the mantle is correct the roles of subduction zones and a chemical boundary layer at the base of the mantle are crucial in determining the pattern and nature of the convection. Indeed it could be argued that the location of subduction zones is most fundamental in that they not only determine downwellings occur but also where the boundary layer at the core-mantle boundary is hottest, and hence where upwellings occur. However, subduction zones are transient features within the context of

Figure 12.13 Cartoon of the D" layer where it is hotter than its average temperature. These regions include an ultra-low-velocity zone (ULVZ), thought to be characterized by partial melt and chemical heterogeneity, chemical and melt scatterers throughout and, possibly, the points of origin of plumes (redrawn, with permission, from Garnero, 2000. Annual Review of Earth and Planetary Sciences, 28. Copyright © 2000, Annual Reviews).

Figure 12.13 Cartoon of the D" layer where it is hotter than its average temperature. These regions include an ultra-low-velocity zone (ULVZ), thought to be characterized by partial melt and chemical heterogeneity, chemical and melt scatterers throughout and, possibly, the points of origin of plumes (redrawn, with permission, from Garnero, 2000. Annual Review of Earth and Planetary Sciences, 28. Copyright © 2000, Annual Reviews).

geologic time. Within the supercontinent cycle (Section 11.5) there are times when subduction zones are initiated, as a result of continental break-up, and terminated by continent-continent collision. Such events could initiate changes in the gross pattern of convection in the mantle and even change the distribution of mass within the Earth causing a change in the location of the rotational axis, that is, the axis about which the moment of inertia is a maximum. This would be particularly true if the initial development of subduction zones includes a build-up of subducted material in the transition zone that ultimately avalanches down into the lower mantle. Such True Polar Wander (Section 5.6) is thought to have occurred between 130 and 50 Ma ago (Besse & Cour-tillot, 2002), a time period bracketed by the break-up of Pangea, with the initiation of subduction zones, and the collision of India with Eurasia and a major change in the rate of subduction in this zone. The change in direction of the Hawaiian-Emperor seamount chain, and the change in the relative motion between the Pacific and Indo-Atlantic hotspot reference frames (Section 5.5) also occurs at the time of the Indian collision, 40-50 Ma ago. Thus, these too might reflect the consequent changes in the thermal regime and pattern of convection in the mantle, and, hence, the relative positions of the two major convection cells within the African and Pacific hemispheres.

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