Earth's Deep Mantle

Published by the American Geophysical Union as part of the
Geophysical Monograph Series, Volume 160.

Understanding the inner workings of our planet and its relationship
to processes closer to the surface remains a frontier in the
geosciences. Manmade probes barely reach ~10 km depth and
volcanism rarely brings up samples from deeper than ~150 km.
These distances are dwarfed by Earth's dimensions, and our
knowledge of the deeper realms is pieced together from a range of
surface observables, meteorite and solar atmosphere analyses,
experimental and theoretical mineral physics and rock mechanics,
and computer simulations. A major unresolved issue concerns the
nature of mantle convection, the slow (1-5 cm/year) solid-state
stirring that helps cool the planet by transporting radiogenic and
primordial heat from Earth's interior to its surface.

Expanding our knowledge here requires input from a range of
geoscience disciplines, including seismology, geodynamics, mineral
physics, and mantle petrology and chemistry. At the same time, with
better data sets and faster computers, seismologists are producing
more detailed models of 3-D variations in the propagation speed of
different types of seismic waves; new instrumentation and access to
state-of-the-art community facilities such as synchrotrons have
enabled mineral physicists to measure rock and mineral properties
at ever larger pressures and temperatures; new generations of mass
spectrometers are allowing geo-chemists to quantify minute
concentrations of diagnostic isotopes; and with supercomputers
geodynamicists are making increasingly realistic simulations of
dynamic processes at conditions not attainable in analogue
experiments. But many questions persist. What causes the lateral
variations in seismic wavespeed that we can image with mounting
accuracy? How reliable are extrapolations of laboratory
measurements on simple materials over many orders of magnitude of
pressure and temperature? What are the effects of volatiles and
minor elements on rock and mineral properties under extreme
physical conditions? Can ab initio calculations help us understand
material behavior in conditions that are still out of reach of
laboratory measurement? What was the early evolution of our planet
and to what extent does it still influence present-day dynamics?
And how well do we know such first-order issues as the average bulk
composition of Earth?