The uppermost part of the mantle constitutes a high velocity lid typically 80-160 km thick in which seismic velocities remain constant at a figure in excess of 7.9 km s-1 or increase slightly with depth. This part of the mantle makes up the lower portion of the lithosphere (Section 2.12). Beneath the lithosphere lies a low velocity zone extending to a depth of approximately 300 km. This appears to be present beneath most regions of the Earth with the exception of the mantle beneath cratonic areas. From the base of this zone seismic velocities increase slowly until a major discontinuity is reached at a depth of 410 km, marking the upper region of the transition zone. There is a further velocity discontinuity at a depth of 660 km, the base of the transition zone.
Within the lower mantle velocities increase slowly with depth until the basal 200-300 km where gradients decrease and low velocities are present. This lowermost layer, at the core-mantle boundary, is known as Layer D" (Section 12.8.4) (Knittle & Jeanloz, 1991). Seismic studies have detected strong lateral heterogeneities and the presence of thin (5-50 km thick) ultra-low velocity zones at the base of Layer D" (Garnero et al., 1998).
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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.