The D layer

It has long been recognized that the greatest contrasts in physical properties and chemical composition within the Earth occur at the core-mantle boundary and that this is almost certainly the location of a thermo-chemi-cal boundary layer (Section 2.8.6). Initially, seismologists were unable to detect any layering in the lower mantle and referred to it as Layer D (Bullen, 1949). Subsequently it was realized that a layer at the base of the mantle, perhaps 2-300 km thick, has distinctive, if variable, characteristics; typically lower seismic velocities or a lower velocity gradient than in the lower mantle above. Hence the lower mantle is now divided into two seismologic layers D' and D". With further refinements in seismologic techniques, studies of seismic waves reflected, refracted and diffracted at the core-mantle boundary have revealed remarkable details of the complexity and lateral variability of layer D". The geographic distribution of earthquakes and seis-mologic observatories is such that not all parts of the layer can be studied in the same degree of detail. Clearly for such a remote layer, that is now thought to have vertical and horizontal variability analogous to that of the lithosphere, this poses quite a challenge for future seismologic studies.

Figure 12.11 illustrates the picture that is emerging of the nature of layer D" for three very different regions for which detailed studies have been possible: beneath central America, Hawaii, and southern Africa. The upper boundary of the layer is characterized by a velocity discontinity. Below this there may be an increase or decrease in the seismic velocities, particularly the shear wave velocity, or a decrease in the velocity gradient with depth. A velocity increase is most marked beneath regions where there are subducting slabs such as Central America (Fig. 12.11a). In a 5- to 50-km-thick layer immediately above the core-mantle boundary there is often a zone of ultra-low seismic velocities, with decreases in the shear wave velocity of 10-50%. This implies partial melting with more than 15% melt (Thybo et al., 2003). These ultra-low velocity zones (ULVZ) are most extensively developed beneath major hotspots such as Hawaii (Fig. 12.11b) and beneath the superswells, and inferred upwellings, of the central Pacific and southern Africa (Fig. 12.11c). Unlike the variations in seismic velocity in the main part of the lower mantle, that are thought to be largely due to temperature differences, the marked

South Africa

South Africa

N. America

CMB depression, chemical reaction repository

Fluid outer core

Figure 12.11 Sections through the Earth's interior beneath regions centered on (a) central America, (b) Hawaii, and (c) South Africa, illustrating variations in the nature of the D" layer (reproduced from Garnero, 2004, Science 304,834-6, with permission from the AAAS).

410 discontinuity 660 discontinuity

D" discontinuity D" anisotropy-

Low-velocity scatterer-

Ultralow-velocity patches, plume genesis

Core-mantle boundary (CMB)

CMB depression, chemical reaction repository

Fluid outer core

Figure 12.11 Sections through the Earth's interior beneath regions centered on (a) central America, (b) Hawaii, and (c) South Africa, illustrating variations in the nature of the D" layer (reproduced from Garnero, 2004, Science 304,834-6, with permission from the AAAS).

N. America lateral and vertical variations within layer D" may be caused by variations in chemical composition, mineral-ogic phase changes and/or varying degrees of partial melting, in addition to temperature differences. Compositional variations may be due to the mixing of molten iron from the core with the perovskite of the mantle to form new high-pressure minerals (Section 2.8.6). It is thought that this is most likely to occur in the ULVZs where it is facilitated by higher temperatures, partial melting, and low viscosity. The result would be a chemically distinct, high-density layer but with a low viscosity. A phase change in perovskite to a denser and strongly anisotropic form is an interesting possibility as some parts of the D" layer exhibit a marked anisotropy. It is thought that this anisotropy may be induced by subducted slabs beneath down-wellings and by shear flow beneath upwellings.

It seems likely that the slabs of subducted lithosphere that sink into the lower mantle affect the nature of the D" layer beneath them, most notably its temperature. This in turn will modulate the flow of heat from the core which will influence convection in the core and the nature of the Earth's magnetic field, and determine where flow may occur within and above layer D".

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How To Have A Perfect Boating Experience

How To Have A Perfect Boating Experience

Lets start by identifying what exactly certain boats are. Sometimes the terminology can get lost on beginners, so well look at some of the most common boats and what theyre called. These boats are exactly what the name implies. They are meant to be used for fishing. Most fishing boats are powered by outboard motors, and many also have a trolling motor mounted on the bow. Bass boats can be made of aluminium or fibreglass.

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