Fast (" cold ") and slow (" hot ") anomalies in the mantle, as imaged by seismic tomographic model SB4L18 (G. Masters, G. Laske, H. Bolton, A. Dziewonski, in Mineral Physics and Seismic Tomography, S. Karato et al. eds, AGU, Washington DC, 2000). (a) Looking from the south, with the CMB at the bottom of the image and the surface at the top of the image, downwelling slabs (blue) appear to sink to the CMB in most areas. Two large megaplumes (red) are observed underneath Africa (left of image) and the Pacific (center of image). (b) Looking down from above, the pattern of continents and oceans is clearly visible. Contoured velocity values are 0.6% fast (blue) and 1% slow (red).
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