What has twisted the Earth’s core so asymmetrically out of shape? That question has been a long-standing mystery for scientists, but two new studies are shining some light on the geodynamic processes that have shaped the core of our planet over the millennia, researchers report in the April 16 issue of Science.
Marc Monnereau and colleagues knew that the solid core at the center of the Earth swells from the slow crystallization of iron in the outer core, so they began testing millions of different models of core growth, each involving various sizes of iron grains.
They settled on a model that takes into account the movement of iron crystals from the solid central core to the liquid outer core—and they demonstrate how this convection of iron grains would lead to the permanent crystallization of the iron on one side of the core and continuous melting of the iron on the opposite side.
The researchers’ model shows exactly how this slow migration of iron in the inner core could lead to a melting Eastern hemisphere and a solid Western hemisphere in order to create the lopsided core we experience today. Since the process is still ongoing, this research also supports the theory of a relatively young inner core of the Earth.
In a separate study, Arwen Deuss and colleagues determined the structure of the Earth’s core using seismic recordings of 90 large earthquakes that occurred between 1976 and 2009.
Their data pinpoints specific regional variations in the structure of the core that are in addition to the hemispheric east-west lopsidedness, and may likewise be attributed to the preferential alignment of iron. On top of that, the patterns of variations these researchers found in the Earth’s core match up directly with the shape of the Earth’s magnetic field—a finding that suggests the core’s growth has been coupled to the planet’s magnetic field since it first formed.
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-- Lopsided Growth of Earth's Inner Core, Marc Monnereau et al., Published Online April 15, 2010, Science DOI:10.1126/science.1186212
-- Regional Variation of Inner-Core Anisotropy from Seismic Normal-Mode Observations, Published Online April 15, 2010, Science DOI:10.1126/science.1188596