The boundary between Earth's core and mantle is a fundamental discontinuity where silicate minerals come into contact with molten iron alloys. Because it is so remote, nearly 2900 km below the surface, it can only be studied from afar. New studies areallowing a glimpse of the lowermost part of the mantle and raising new questions about this region. A very thin layer in which seismic velocities drop by 10% or more (1,2) may indicate melting at the base of the mantle (3). The lower mantle may be anisotropic, perhaps as a consequence of fine-scale compositional variations (see figure). Moreover, seismic studies have also revealed several hundred meters of small-scale topography on the core-mantle boundary (4).
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