The Earth's magnetic field is generated by fluid motions in the molten
iron outer core, but the details of this magnetohydrodynamic (MHD) dynamo
are poorly understood. However, rotational and magnetic forces are certainly
important. Using a rotating table we built a laboratory model for the solidification of the Earth's core (picture).
Solidification of the inner core from the outer core is thought to represent a primary
energy source for the fluid motions that drive the geodynamo, so that understanding
the convection that results from solidification may give insight into how
the Earth's magnetic field is generated. One particularly interesting style
of convection that occurs during solidification is channel convection (picture),
which results from a non-linear focusing mechanism.
In addition to possibly occurring at the Earth's inner-outer core boundary,
channel convection occurs in solidifying sea ice sheets, where it plays
an important role in the heat transfer between the atmosphere and the ocean,
and in single crystal nickel-base alloys used in gas turbines, where it
can be deleterious.
When a melt solidifies, the crystallographic axes of the crystals may become
aligned, resulting in what is known as a solidification texture. This texture
affects a material's mechanical and electrical properties. Using a variety
of materials such as sea ice and zinc-tin alloys we have been studying
the effects of fluid flow (picture), annealing, and deformation on solidification textures.
We have also been studying the ultrasonic
properties of castings. The work is in part motivated by seismic inferences
that the Earth's solid inner core exhibits a texture. We are exploring
the possibility that this texture may in part be a result of fluid flow
in the outer core during solidification of the inner core, or annealing
and deformation after solidification. We are also
interested in applications of this work to solidifying sea ice sheets in
the Arctic and off Antartica (picture).
An inner core slip-sliding away
Michael I. Bergman,
Nature466, 697-698 (2010).
Grain growth and loss of texture during annealing of alloys,
and the translation of Earth's inner core
Michael I. Bergman, Daniel J. Lewis, Ingyin Hla Myint, Liam Slivka, Shun-ichiro Karato, and Andrew Abreu,
Geophys. Res. Lett.37, L22313 (2010).
Inner core dynamics
Ikuro Sumita and Michael I. Bergman,
in Elsevier Treatise on Geophysics, ed. Gerald Schubert, 299-318 (2007).
A laboratory model for solidification of Earth's core Michael I. Bergman, Marget Macleod-Silberstein, Michael Haskel, Benjamin Chandler, and Nsikan Akpan,
Phys. Earth Planet. Int.153, 150-164 (2005).
Transverse solidification textures in hexagonal close-packed alloys Michael I. Bergman, Sameer Agrawal, Michael Carter, Marget Macleod-Silberstein,
J. Crystal Growth255, 204-211 (2003).
Preferred crystal orientations due to melt convection during directional
solidification Michael I. Bergman, David M. Cole, & Jackson R. Jones, J. Geophys.
Res. 107, ECV 6-1 - 6-8 (2002).
Suppression of channel convection in solidifying Pb-Sn alloys via an
applied magnetic field Michael I. Bergman, David R. Fearn, & Jeremy Bloxham, Met. Trans.
A30, 1809-1815 (1999).
Estimates of the Earth's inner core grain size Michael I. Bergman,Geophys. Res. Lett.25, 1593-1596 (1998).
Measurements of elastic anisotropy due to solidification texturing and
the implications for the earth's inner core Michael I. Bergman,
Nature389, 60-63 (1997).