has become compelling, heightening a controversy that dates back half a century.
The sharpened resolution of geophysical and geochemical observations, in
combination with high-level computational simulations of the complex timedependent flow of the mantle, is leading to new insights about the existence and
long-term (billion-year) survival of distinct geological regions within the deep
interior.
The Core-Mantle Boundary (CMB)
The contrasts in physical properties at the boundary between the rocky
mantle and the liquid metal of the outer core are extraordinary: the mass density
jumps by an amount greater than at the free surface, and the viscosity drops by
more than 20 orders of magnitude—from that of “solid” rock to a value not much
greater than that of water. The heat flowing across the CMB comes from energy
sources within the core large enough to power a geodynamo that produces the
Earth’s magnetic field.
A great deal has been learned about this boundary in the last decade.
Evidence of patches with highly anomalous seismic velocities (“ultralow velocity
zones”) and strong anisotropy (direction-dependent wave speeds), as well as
indications of unexpected electrical conductivity that may influence the wobble
of the Earth’s rotation, are but a few of the unexpected results of recent studies. In
combination with geophysical observations, laboratory investigations suggest
that this is a region of intense chemical reactions, perhaps because the nature of
chemical bonding is radically different at deep-Earth conditions than at the
surface (e.g., oxygen, the primary constituent of rock, is a metal at the millionatmosphere pressures of the deep mantle).
The Core Dynamo and Magnetic Field
The core dynamo generates the geomagnetic field through complex
electromagnetic and hydrodynamic interactions among convective motions within
the rotating, highly conductive liquid outer core. Although “toy models”—
simple dynamos amenable to standard theoretical analysis—have shed light on
the fundamental physics of magnetic field generation, realistic simulations of the
core dynamo require intense numerical calculations, and the first such numerical
models were developed only about five years ago. With continuing advances in
computational capabilities, these system-level models offer new possibilities for
explaining observations of the geomagnetic field and core properties.
An abundance of data recently available from direct observations, satellites,
and permanent observatories now reveals that the behavior of the
SCIENCE OPPORTUNITIES
73
About this PDF file: This new digital representation of the
original work has been recomposed from XML files created
from the original paper book, not from the original
typesetting files. Page breaks are true to the original; line lengths,
word breaks, heading styles, and other typesetting-specific
formatting, however, cannot be retained,
and some typographic errors may have been accidentally inserted.
Please use the print version of this publication as the authoritative
version for attribution.
The sharpened resolution of geophysical and geochemical observations, in
combination with high-level computational simulations of the complex timedependent flow of the mantle, is leading to new insights about the existence and
long-term (billion-year) survival of distinct geological regions within the deep
interior.
The Core-Mantle Boundary (CMB)
The contrasts in physical properties at the boundary between the rocky
mantle and the liquid metal of the outer core are extraordinary: the mass density
jumps by an amount greater than at the free surface, and the viscosity drops by
more than 20 orders of magnitude—from that of “solid” rock to a value not much
greater than that of water. The heat flowing across the CMB comes from energy
sources within the core large enough to power a geodynamo that produces the
Earth’s magnetic field.
A great deal has been learned about this boundary in the last decade.
Evidence of patches with highly anomalous seismic velocities (“ultralow velocity
zones”) and strong anisotropy (direction-dependent wave speeds), as well as
indications of unexpected electrical conductivity that may influence the wobble
of the Earth’s rotation, are but a few of the unexpected results of recent studies. In
combination with geophysical observations, laboratory investigations suggest
that this is a region of intense chemical reactions, perhaps because the nature of
chemical bonding is radically different at deep-Earth conditions than at the
surface (e.g., oxygen, the primary constituent of rock, is a metal at the millionatmosphere pressures of the deep mantle).
The Core Dynamo and Magnetic Field
The core dynamo generates the geomagnetic field through complex
electromagnetic and hydrodynamic interactions among convective motions within
the rotating, highly conductive liquid outer core. Although “toy models”—
simple dynamos amenable to standard theoretical analysis—have shed light on
the fundamental physics of magnetic field generation, realistic simulations of the
core dynamo require intense numerical calculations, and the first such numerical
models were developed only about five years ago. With continuing advances in
computational capabilities, these system-level models offer new possibilities for
explaining observations of the geomagnetic field and core properties.
An abundance of data recently available from direct observations, satellites,
and permanent observatories now reveals that the behavior of the
SCIENCE OPPORTUNITIES
73
About this PDF file: This new digital representation of the
original work has been recomposed from XML files created
from the original paper book, not from the original
typesetting files. Page breaks are true to the original; line lengths,
word breaks, heading styles, and other typesetting-specific
formatting, however, cannot be retained,
and some typographic errors may have been accidentally inserted.
Please use the print version of this publication as the authoritative
version for attribution.