Bruce A. Buffett, Professor, Department of the Geophysical Sciences and the College

Sediments at the Top of Earth's Core

Unusual physical properties at the core-mantle boundary have been inferred from seismic and geodetic observations in recent years. Both types of observations can be explained by a layer of silicate sediment, which accumulates at the top of the core as Earth cools. Compaction of the sediments expels most of the liquid iron but leaves behind a small amount of core material, which is entrained in mantle convection and may account for the isotopic signatures of core material in some hotspot plumes. Extraction of light elements from the liquid core also enhances the vigor of convection in the core and may increase the power available to the geodynamo.

Figure 1: Earth's cross section. Sedimentation at the top of Earth's core in the small boxed area is sketched in Figure 2 below.

Figure 2: Schematic cross-section of Earth's core and lower mantle. Light elements are segregated into the outer core as the inner core grows by solidification. The increasing concentration of light elements in the outer core causes excess light elements to precipitate as a sediment. The sediment accumulates in depressions at the top of the core, which may account for the ultra-low-velocity zones (ULVZs) inferred from seismological observations. Click the figure for a larger (101 kbytes) version.