Calculated current distribution of heat flow at the core-mantle boundary. The heat flow beneath subduction zones of the last 300 million years exceeds the heat flux below areas where no subduction occurred many times over.

On
a time scale of tens to hundreds of millions of years, the geomagnetic
field may be influenced by currents in the mantle. The frequent polarity
reversals of Earth’s magnetic field can also be connected with
processes in the mantle. These are the research results presented by a
group of geoscientists in the new advance edition of Nature Geoscience
on Sunday, July 29th. The results show how the rapid processes in the
outer core, which flows at rates of up to about one millimeter per
second, are coupled with the processes in the mantle, which occur more
in the velocity range of centimeters per year.

The
international group of scientists led by A. Biggin of the University of
Liverpool included members of the GFZ German Research Centre for
Geosciences, the IPGP Paris, the universities of Oslo and Utrecht, and
other partners.

   

It
is known that the Earth’s magnetic field is produced by convection
currents of an electrically conducting iron-nickel alloyin the liquid
core, about 3,000 kilometers below the earth’s surface. The geomagnetic
field is highly variable, there are changes in Earth’s magnetic field on
a multitude of spatial and temporal scales. Above the liquid outer core
is the mantle, the rock in which behaves plastically deformable due to
the intense heat and high pressure. At the boundary between Earth’s core
and mantle at 2900 km depth there is an intense heat exchange, which is
on the one hand directed from the Earth’s core into the mantle. On the
other hand, processes within Earth’s mantle in turn also affect the heat
flow. The interesting question is how the much slower flow in the solid
mantle influences the heat flow and its spatial distribution at the
core-mantle boundary, and how this will affect the Earth’s magnetic
field which is produced due to the much faster currents in the Earth’s
core.

   

Key variable heat transfer

“The
key variable is the heat flow. A cooler mantle accelerates the flow of
heat from the hot core of the Earth, and in this way alters the also
heat-driven convection in the Earth’s core”, said Bernhard Steinberger
of the GFZ German Research Centre for Geosciences. “Ocean floor sinking
into the mantle due to tectonic processes can lead to cooling in the
mantle. They cause at these sites an increased heat flow into the cooler
parts, namely until they have been heated to the ambient temperature.”
That might take several hundred million years, however.

   

Conversely,
the hot core of the Earth leads to the ascent of heated rocks in form
of large bubbles, so-called mantle plumes that separate from the
core-mantle boundary and make their way up to the surface of the earth.
This is how Hawaii came into existence. This increases the local heat
flux out of the earth’s core and in turn modifies the generator of the
geomagnetic field.

   

Reversals of the magnetic field

Effect of the subduction processes on the distribution of heat flow from the core into the mantle. Spatial distribution of the subduction of oceanic plates over the last 300 million years. The darker the color, the higher the rate of subduction.

In
the Earth’s history, polarity reversals of the geomagnetic field are
nothing extraordinary. The most recent took place only 780 000 years
ago, geologically speaking a very short period of time. The research
team was able to determine that in the period of 200 to 80 million years
before present, reversals initially happened more often, namely up to
ten times in hundred million years.

“Surprisingly,
these reversals stopped about 120 million years ago and were absent for
nearly 40 million years,” explains GFZ scientist Sachs. Scientists
presume that the reason for this is a concurrent reorientation of the
whole mantle and crust with a shift in the geographic and magnetic poles
of about 30°. Known as “true polar wander”, thisprocess is caused by a
change in density distribution in the mantle. If it increases the heat
flux in equatorial regions, it would presumably lead to more frequent
field reversals, if it decreases it, the field reversal might not occur.

   

Looking to the future

According
to current knowledge, therefore, an influence of plate tectonics and
mantle convection on the Earth’s magnetic field seems quite possible.
The article also shows, however, that further research is still needed
for a better understanding of these relationships. In particular, more
episodes of “true polar wander” should be derived from paleomagnetic
data, and it should be determined whether these are usually associated
with an altered behavior of the magnetic field (e.g. frequency of field
reversal). Also, future models for the generation of the geomagnetic
field should investigate the influence of the spatial and temporal
variation of the heat flux at the core-mantle boundary in more detail.

   

Possible links between long-term geomagnetic variations and whole-mantle convection processes

Source: Helmholtz Center, Potsdam