New Research Opportunities in the Earth Sciences - New Research Opportunities in the Earth Sciences - page 27

Figure 2.8 Images of Vp/Vs seismic wave velocity ratio variations in the mantle
wedge beneath Central America. Low-velocity regions in the wedge may involve
either fluids extracted from the slab or regions of partial melting caused by fluid-
assisted reduction of melting temperature extending upward from the slab/wedge
interface. SOURCE: Syracuse et al. (2008).
Quantification of Core Evolution
Our knowledge of Earth's core has advanced greatly over the past few
decades, albeit with continued surprises time and again (e.g., Nimmo, 2007). As the
core cools, the inner core grows by solidification of iron at its surface accompanied
by a depletion of the light alloy component. It was originally assumed that this would
lead to a relatively homogenous inner core structure, possibly with some thin surface
transition zone. Seismology demonstrated that the inner core has both small-scale and
large-scale heterogeneities that appear to reflect dynamical processes. Early
characterization of the heterogeneity demonstrated the presence of anisotropic
structure closely aligned with the rotation axis, but it is now recognized that there are
hemispherical patterns in the inner core structure as well as changes in anisotropic
pattern with depth (e.g., Ishii and Dziewonski, 2002; Song, 2007). Parallel
improvements in seismological constraints on outer core structure indicate that there
is a region above the inner core that has reduced velocity gradients indicative of
transitional properties (Zou et al., 2008). Greatly expanded geodynamo simulation