John Hernlund: Research Summary & Publications
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Scientific Approach & Philosophy

Introduction: I am a natural scientist who is broadly interested in the evolution of interactions between processes and matter. My disciplinary work targets the birth and evolution of planetary processes across deep time and space. Such processes include planet accretion/differentiation, melting/freezing and compositional segregation, mantle convection and plate tectonics, atmosphere and ocean dynamics, planetary dynamos, among others.

Unifying planetary formation and evolution: At ELSI we are working to bridge these research fields, and establish an intellectual framework for understanding planetary diversity from the ground up. The idea is to understand entire planetary systems, as opposed to considering only single planets in isolation from the influence of their sibling planets. This broadens the observational footprint of our theoretical framework, and allows us to think about (for example) habitable stellar systems instead of just habitable planets. Our strategy is to identify important bottlenecks and bifurcations in planet formation and evolution, the inter-dependency between sibling planets (especially during formation), and the degree to which such variations influence planetary state and evolution. We then launch focused efforts to place new constraints on the critical junctures in planet birth and evolution. Ultimately we are also interested in the emergence of more complex geological processes, such as life, as a natural outcome of the formation and evolution of particular planetary families.

Integrative Geodynamical Modeling: While my technical specialty is numerical and mathematical modeling of geological processes, I long ago decided to use models primarily as a tool to unify disparate branches of Earth sciences. I feel that models are not an end in themselves, but rather a means to an end (i.e., building a broader understanding). This is my primary disciplinary work, which I call integrative geodynamical modeling. Integrative models need not be overly complex or require large computational resources, instead the priority is crossing boundaries and building new bridges toward understanding interactions between Earth systems and processes, and how they relate to a variety of observations.

Earth as a Model Planet: A systematic approach to understanding the ecosystem of geological processes on Earth helps to facilitate our ability to place the Earth and its life within the broader context of the huge number of new planets currently being discovered in the Milky Way. Many of our thoughts regarding other planets begins with the Earth as a point of reference (analogous to the zeroth order term in a Taylor series expansion), and it is therefore critical to understand the planet beneath our feet as a foundation for speculating about the potential diversity of other planets in the universe.

Social Chemistry of Scientists: I am also an institutional architect and manager, with interests in the social and logistical aspects of the scientific enterprise. I am a very strong advocate of recognizing and harnessing the diversity of the human brain, and its myriad abilities as they manifest in my scientific colleagues. I believe that nobody has the natural skill to do everything well, we are all better in some areas than others. Some people are excellent technicians, of course we need them! Some people are good at coming up with new ideas, they are of course critically important! Some people are good at selling projects and grant proposals, we need them too! Some people are great writers, some are great teachers, some are great orators, etc.. We each have unique abilities, and I feel that we can achieve much greater science by harnessing synergy through embracing diversity, as opposed to trying to fit every person into the same mold.

Google Scholar Profile: Please click here to visit and view my Google scholar profile, where I maintain an up-to-date list of publications...


Photo by Nerissa Escanlar

Selected Contributions to Integrative Geodynamical Modeling

SiO2 Crystallization in the Earth's Core

Using laser-heated diamond anvil cell experiments that allow us to reproduce the extreme pressures and temperatures at Earth's center, we found that a metal Fe-Si-O alloy crystallizes SiO2 upon cooling, until either all Si and/or O is exhausted from the alloy. Since Si and O were expected to go into the Earth's metallic core during formation, the same process should have gradually played out in Earth's core over billions of years. This discovery completely changes the evolutionary history of the core. The energy released by this process is more than sufficient to account for production of the geomagnetic field since the early Earth, thus offering a new solution to an old paradox.

Reference:
Hirose, K., G. Morard, R. Sinmyo, K. Umemoto, J. Hernlund, G. Helffrich, and S. Labrosse, SiO2 crystallization and compositional evolution of the Earth's core, Nature, 543:99-102, 2017. Link to article at Nature.com


Bridgmanite-Enriched Ancient Mantle Structures

This is a groundbreaking new ELSI model that presents a new scenario for the Earth in which large-scale silica-enriched regions exhibiting a high viscosity stabilize and organize the pattern of convection in the lower mantle. These Bridgmanite-Enriched Ancient Mantle Structures (BEAMS) can explain why hot spots such as Hawaii that are fed by deep-seated plumes could stay geographically fixed over billions of years, why some subducted lithosphere appears to "stagnate" upon descent into the deep mantle at around 1,000 km depth, why the seismic signature of slabs seems to weaken in the mid-lower mantle, why there is an apparent viscosity increase in the mid-mantle, how primitive noble gas and other isotopes could be preserved over the age of the Earth despite whole mantle convection, heterogeneous tectonic coupling with the deep mantle, and many other puzzling observations that are otherwise difficult to explain in the context of a homogeneous well-mixed mantle.

Reference:
Ballmer, M.D., C. Houser, J.W. Hernlund, R. Wentzcovitch & K. Hirose, Persistence of Strong Silica-Enriched Domains in the Earth's Lower Mantle, Nature Geoscience, 10:236–240, 2017. Link to article at Nature.com


The Basal Magma Ocean

The basal magma ocean was the first model to fully couple Earth’s core and mantle thermal evolution together with geomagnetic constraints, seismological observations of slow velocity features in the core-mantle boundary region, thermodynamical limits on dynamos, physical properties of rocks and metals at high pressure and temperature from laboratory experiments, and melting behavior of rocks at extremely high pressure. The model predicted that the bottom of the mantle would have been extensively molten in the past, gradually crystallizing to produce structures that are seismically observed today in the deep mantle.

This work is an outgrowth of modeling efforts I began as a graduate student, and then came to fruition during my first post-doc at the Institut de Physique du Globe de Paris in France. We had been trying to produce partially molten layers at the core-mantle boundary (CMB) called ultralow-velocity zones (ULVZs, see below for more) in our models by melting ordinary mantle. This ran into myriad problems, such as keeping melt and solids from separating. This issue became more acute if we accounted for the fact that the CMB was itself hotter (and hence more molten) in the past. The idea hit us one day, that we should make ULVZs by freezing molten mantle, instead of melting solid mantle! This was precisely the conceptual Copernican inversion that allowed us to move forward with a new model where ULVZs and many other observed features could naturally be produced as by-products of a basal magma ocean. We have also shown that fractional crystallization can produce more dramatic composition variations than partial melting, such as iron oxide enrichments great enough to produce the ~10% density anomalies inferred for ULVZs.

References:
Labrosse, S., J.W. Hernlund, and N. Coltice, A crystallizing dense magma ocean at the base of the Earth’s mantle, Nature, 450:866-869, 2007. (Click for PDF)
Nomura, R., H. Ozawa, S. Tateno, K. Hirose, J. Hernlund, S. Muto, H. Ishii, and N. Hiraoka, Spin crossover and iron-rich silicate melt in the Earth's deep mantle, Nature, 473:199-202, 2011. (Click for PDF)


The Post-Perovskite Double-Crossing

The post-perovskite double-crossing is a hypothesis I proposed in 2005 that integrated geodynamical models of Earth's heat flow and internal boundary layer dynamics with seismic observations attributed to a newly discovered post-perovskite phase transition. With seismologist Christine Thomas, we used mineral physics data to calculate a model in the context of a thermal boundary layer, propagated seismic waves through the model structure, and compared it with observed data. We showed how the validity of this model makes specific predictions about the temperature of the core and the heat flux passing between the core and mantle, which are 2 of the most important (and uncertain) parameters in the Earth affecting its thermal and magnetic evolution. The simple figure I made (shown here) has been shown hundreds (if not thousands) of times in presentations at conferences and workshops.

Reference:
Hernlund, J.W., C. Thomas, and P.J. Tackley, A doubling of the post-perovskite phase boundary and structure of the Earth’s lowermost mantle, Nature, 434:882-886, 2005. (Click for PDF)


Asthenospheric Decompression Melting Instability

I developed models and theory for a kind of convective instability that occurs in Earth's asthenosphere when it undergoes partial melting. The instability can produce a large quantity of melt, and interestingly it is suppressed by rapid upwelling induced by lithospheric extension. This is a process that takes place at mid-ocean spreading ridges, or in regions undergoing distributed extensional deformation. It explains the peculiar observation that fast spreading mid-ocean ridges have smooth variations in magmatism relative to slow-spreading ridges, and why volcanism peaked in the Western USA after extension ceased.

References:
Hernlund, J.W., P.J. Tackley, and D.J. Stevenson, Buoyant melting instabilities beneath extending lithosphere, 1. Numerical models, J. Geophys. Res., 113:B04405, 2008. (Click for PDF)
Hernlund, J.W., D.J. Stevenson, and P.J. Tackley, Buoyant melting instabilities beneath extending lithosphere, 2. Linear analysis, J. Geophys. Res., 113:B04406, 2008. (Click for PDF)


Foundered Melt at the Lithosphere-Asthenosphere Boundary

Using data from a combination of mineral physics experiments at high pressures, we proposed that melts forming in the shallow mantle could percolate upward and become trapped at depths of 80 km in the Earth, perhaps freezing to form a deep sill-like structure. This model can explain seismic observations of a reflecting layer buried at this depth range which is called the "Lithosphere-Asthenosphere Boundary," which has been a very hot topic in geophysics in recent years.

Reference:
Sakamaki, T., A. Suzuki, E. Ohtani, H. Terasaki, S. Urakawa, Y. Katayama, K. Funakoshi, Y. Wang, J.W. Hernlund, M.D. Ballmer, Ponded melt at the boundary between the lithosphere and asthenosphere, Nature Geoscience, 10.1038/NGEO1982, 2013. (Click for PDF)


Partially Molten Ultra-Low Velocity Zones (ULVZ)

20 years ago seismologists detected thin veneers of material at the very bottom of the mantle where seismic waves slow down dramatically, and proposed the presence of partial melt. Through a series of numerical modeling investigations I showed that the observed ULVZ could not be produced due to partial melting of ordinary mantle as it circulates through the CMB region. The results instead favored compositionally distinct ULVZ which are dense and have a lower melting point than ordinary mantle owing to enrichment in iron oxide. Although early models suggested that melt percolation poses a challenge to melt retention, my later more sophisticated mush dynamics model calculations showed that partial melt could be prevented from draining out of ULVZ by the effects of viscous stirring driven by convection in the overling mantle.

References:
Hernlund, J.W. and P.J. Tackley, Some dynamical consequences of partial melting at the base of Earth’s mantle, Phys. Earth Planet. Inter., 162:149-163, 2007. (Click for PDF)
Hernlund, J.W. and A.M. Jellinek, Dynamics and structure of a stirred partially molten ultralow velocity zone, Earth Planet. Sci Lett., 296:1-8, 2010. (Click for PDF)


Selected Contributions to Numerical Model Development

3D Mantle Convection in the Cubed Sphere

Summary: As a graduate student I developed a method for numerically modeling mantle convection in a 3D spherical shell using a non-orthogonal coordinate system called the "cubed sphere." This method uses overlap between adjacent regular meshes to achieve a more uniform resolution on the surface of the sphere than spherical polar coordinates. It was the groundwork for later models developed by other research groups which are now considered the cutting edge of numerical techniques in 3D mantle convection.

Reference:
Hernlund, J.W. and P.J. Tackley, Three-dimensional spherical shell convection at infinite prandtl number using the 'Cubed Sphere' method. Proceedings of the Second M.I.T. Conference on Computational Fluid and Solid Mechanics, 2003. (Click for PDF)


Solid Cell Assembly Thermal Model

I developed a rapid, highly accurate, flexible, fine resolution model to compute temperatures inside solid medium high pressure resistance-heated experiments. The code was applicable to piston-cylinder-type and multi-anvil-type experiments, which are widely used worldwide to produce high pressure/temperature conditions for sample volumes ranging from 1mm to several cm. Supplemented by a beautiful user interface composed by my colleague Kurt Leinenweber, this code is still in use today by the high pressure community, and provides a state-of-the-art tool to design experiments with better control and characterization of temperature gradients.

Reference:
Hernlund, J.W., K. Leinenweber, D. Locke, and J.A. Tyburczy, A numerical model for steady-state temperature distributions in solid-medium high-pressure cell assemblies, American Mineralogist, 91:295-305, 2006. (Click for PDF)


The Spherical Annulus

In many geophysical scenarios it is often useful to employ two-dimensional models that allow one to capture higher resolution details and to push parameter limits beyond what is possible in 3D models. But the inherent spherical nature of planets is difficult to capture in two dimensions, and there are many problems with models that employ cylindrical coordinates because expressions do not scale spherically in radius. In this paper, I proposed a way to make a 2D cut through a 3D spherical coordinate system that allows one to completely capture spherical scaling while utilizing a convenient 2D annular domain. Paul Tackley added this option to his convection code "STAGYY" and ran test cases to show comparisons to full 3D spherical calculations and 2D cylindrical models. This method has become very popular and is now widely used by researchers in computational geophysics.

Reference:
Hernlund, J.W. and P.J. Tackley, Modeling mantle convection in the 'Spherical Annulus', Phys. Earth Planet. Inter., 171: 48-54, 2008. (Click for PDF)


TempDAC

I developed a fast and accurate 3D multigrid code for modeling ultra-high pressure experiments in the laser-heated diamond anvil cell (DAC). DACs are a very important tool that allow scientists to study material behavior to pressures exceeding the center of the Earth, and are responsible for many of the major scientific breakthroughs in studies of Earth and planet interiors in recent decades. However, these experiments are plagued by problems with extreme temperature gradients. The model I developed was adapted by graduate student Emma Rainey (Ph.D. UCLA, 2014) which she compared to experimental data. This model is currently being expanded to better quantify the effects of temperature gradients in the DAC on chemical processes between materials at high pressures and temperatures.

Reference:
Rainey, E.S.G., J. Hernlund, and A. Kavner, Temperature distributions in the laser-heated diamond anvil cell from 3-D numerical modeling, J. Appl. Phys., 114:204905, 2013. (Click for PDF)