CSEDI Collaborative Research: The Origins and Implications of Inner Core Seismic Anisotropy

The Earth's core is a ball of mostly iron metal. It consists of a liquid outer shell - the outer core - enveloping the solid inner core. As the Earth cools down over time, liquid iron freezes, growing the inner core and providing energy to the outer core to generate Earth’s magnetic field.

The inner core is spherical in shape but appears not to be uniform. The speed of seismic waves traveling through it depends on their direction, a feature known as anisotropy. Seismic waves traversing the inner core along a north-south path (near-parallel to the Earth’s rotation axis) go faster than those propagating along an east-west path (in the plane of the equator).

Inner-core seismic anisotropy carries information about the conditions at the time of iron freezing. It has been attributed to alignment of iron crystals in specific directions; but the processes causing this alignment is unclear. Interpreting this feature has been challenging because of the complex processes involved and the extreme pressures and temperatures prevailing in the core.

Here, the researchers test experimentally how samples of iron behave at core conditions. They analyse data from experiments carried out in the diamond anvil cell where extreme conditions are generated at the tips of two opposing diamonds. They use computational models to characterpize iron crystal alignment during the experiments and calculate the resulting seismic velocities.

They use seismic analytical methods to map the velocity structure of the inner core. Combined with geodynamic modeling, the multidisciplinary approach allows simulating inner-core growth and unveiling the processes causing its present-day anisotropy. The project supports an early career scientist.

It promotes the training in a multidisciplinary context of graduate and undergraduate students, notably from underrepresented groups in geosciences. It fosters outreach towards local schools and community colleges. The project outcomes will be broadly and freely distributed to the community.

Understanding inner-core crystallization is central to understanding the geodynamo. Inner-core seismic anisotropy is attributed to alignments of intrinsically anisotropic iron crystals. Here, the researchers investigate the causes and controlling factors of this anisotropy, and how inner-core processes influence outer core processes.

Coupling seismic analysis, mineral physics experiments, and geodynamic modeling, they investigate the dynamics and mineralogical control of crystal alignments within the inner core. They use both laboratory data and computational plasticity models to constrain the behavior of iron at inner-core conditions, placing limits on the mineralogical processes able to generate seismic anisotropy.

Concurrently, with geodynamic models they simulate inner-core growth and determine the pattern and strength of flows and forcing that impact crystal orientation. They test possible feedback of anisotropic thermal and electrical transport properties of aligned crystals on inner-core dynamic evolution. Existing and new seismic measurements provide observational constraints to test possible growth models for the inner core.

Key questions addressed by the team are: how is global anisotropy generated during inner-core growth? What caused the present-day orientation of anisotropy? Can growth models be reconciled with spatial seismic structure across a range of length scales?

What effect does crystallization texture have on final anisotropy? What are the implications of thermal and electrical anisotropy for the rest of the Earth, notably for the geodynamo?

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.