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2: Earth’s Interior - Geosciences

2: Earth’s Interior - Geosciences


Learning Objectives

Module Objectives

  • At the completion of this module you will be able to:
  • Explain the variations in the composition and characteristics of Earth’s different layers
  • Compare the characteristics, behavior, and velocities of the two types of seismic body waves, and how they can be used to interpret Earth’s interior.
  • Describe temperature variations within the interior and how this relates to mantle convection.
  • Explain the origins of Earth’s magnetic field.
  • Describe the isostatic relationship between the crust and the mantle, and the implications of that relationship for geological processes on Earth.

Superionic iron oxide–hydroxide in Earth’s deep mantle

Water ice becomes a superionic phase under the high pressure and temperature conditions of deep planetary interiors of ice planets such as Neptune and Uranus, which affects interior structures and generates magnetic fields. The solid Earth, however, contains only hydrous minerals with a negligible amount of ice. Here we combine high pressure and temperature electrical conductivity experiments, Raman spectroscopy and first-principles simulations to investigate the state of hydrogen in the pyrite-type FeO2Hx (x ≤ 1), which is a potential H-bearing phase near the core–mantle boundary. We find that when the pressure increases beyond 73 GPa at room temperature, symmetric hydroxyl bonds are softened and the H + (or proton) becomes diffusive within the vicinity of its crystallographic site. Increasing temperature under pressure, the diffusivity of hydrogen is extended beyond the individual unit cell to cover the entire solid, and the electrical conductivity soars, indicating a transition to the superionic state, which is characterized by freely moving protons and a solid FeO2 lattice. The highly diffusive hydrogen provides fresh transport mechanisms for charge and mass, which dictate the geophysical behaviours of electrical conductivity and magnetism, as well as geochemical processes of redox, hydrogen circulation and hydrogen isotopic mixing in Earth’s deep mantle.


Earth interior

The Earth’s interior and its surface are in a constant process of interaction. Often slowly, on very large time scales, but often visible in catastrophic events like earthquakes and volcanism. And human society changes the Earth’s surface in an even faster pace than natural processes do. We utilize the subsurface for public infrastructure and rapidly exhaust the known mineral and energy resources that have been stored for millions of years in Earth’s interior.

Connecting the interior to the surface

A central theme is quantifying the interaction between deep (mantle) and shallow (crust/surface) processes. Through this, we aim to understand the present-day structure and dynamic state of our planet. Our strong mathematical and physical background leads to new imaging and modelling techniques usable for geo-exploration and mitigation of natural disasters.

Understanding inner-Earth processes

In trying to understand the inner-Earth processes, we engage questions such as: how did planet Earth evolve from its early formation until its present-day complexity? Why is Earth so different from other planets? How do mountain belts and sedimentary basins form and shape? How can we image what’s under our feet, but too deep to reach (or too costly just to try)? How and where can we best search for our valuable and scarce resources?


Earth


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Circulation of water in deep Earth's interior

Water is transported into Earth’s deep interior by dense hydrous magnesium silicates (DHMSs). Credit: Ehime University

Phase H is a hydrous mineral that is considered to be an important carrier of water into deep Earth. We determined the dissociation condition of phase H by a theoretical calculation based on quantum mechanics. Phase H decomposes at approximately 60 GPa at 1000 K. This indicates that the transportation of water by phase H may be terminated at a depth of approximately 1,500 km in the middle of the lower mantle.

The existence of water in deep Earth is considered to play an important role in geodynamics, because water drastically changes the physical properties of mantle rock, such as melting temperature, electric conductivity, and rheological properties. Water is transported into deep Earth by the hydrous minerals in the subducting cold plates. Hydrous minerals, such as serpentine, mica and clay minerals, contain H2O in the form of hydroxyl (-OH) in the crystal structure. Most of the hydrous minerals decompose into anhydrous minerals and water (H2O) when they are transported into deep Earth, at 40-100 km depth, due to the high temperature and pressure conditions.

However, it has also been reported that some hydrous minerals, called dense hydrous magnesium silicates (DHMSs), may survive in the deeper part of Earth's interior if the subducting plate is significantly colder than the surrounding mantle. DHMS is a series of hydrous minerals which have high stability under the pressure of deep Earth's interior. DHMS is also referred to as "alphabet phases": phase A, phase B, phase D, etc.

Until recently phase D (chemical composition: MgSi2O6H2) was known to be the highest pressure phase of DHMSs. However, Tsuchiya 2013 conducted first principles calculation (a theoretical calculation method based on quantum mechanics) to investigate the stability of phase D under pressure and found that this phase transforms to a new phase with a chemical composition of MgSiO4H2 (plus stishovite, a high pressure form of SiO2, if the system keeps the same chemical composition) above 40 GPa (GPa=109 Pa). This predicted phase has been experimentally confirmed by Nishi et al. 2014 and named as "phase H" (Figure 1). The theoretical calculation by Tsuchiya 2013 also suggests that phase H finally decomposes into the anhydrous mineral MgSiO3 by releasing H2O by further compression.

The thick red line indicates the calculated dissociation phase boundary of phase H. Ehime Univeristy

Although the theoretical calculation estimated the decomposition pressure of phase H around the middle of the lower mantle (from 660 km to 2900 km depth), a detailed determination has not yet been achieved, because the estimation of the Gibbs free energy of H2O was needed to determine the decomposition pressure of phase H. The Gibbs free energy is a thermodynamic potential that can determine the stability of a system. At lower mantle conditions, the H2O phase has a crystal structure with disordered hydrogen positions, i.e. hydrogen positions are statistically distributed among several different positions. In order to calculate the disordered state of hydrogen, Tsuchiya and Umemoto 2019 calculated several different hydrogen positions and estimated the Gibbs free energy of H2O using a technique based on statistical mechanics.

As a result, they estimated the decomposition pressure of phase H at around 62 GPa at 1000 K, corresponding to the

1500 km depth (Figure 2). This result indicates that the transportation of water by subducting plate terminates at the middle of the lower mantle in the Mg-Si-O system. Tsuchiya and Umemoto 2019 also suggested that superionic ice may be stabilized by the decomposition of phase H in the subducted plate. In superionic ice, oxygen atoms crystalize at lattice points whereas hydrogen atoms are freely mobile. The chemical reactions between superionic ice and surrounding minerals have not been identified yet, but high diffusivity of hydrogen in superionic ice may produce reactions faster than that in solid ice, but different from water, the liquid phase of H2O.

M. Nishi et al. Stability of hydrous silicate at high pressures and water transport to the deep lower mantle, Nature Geoscience (2014). DOI: 10.1038/ngeo2074

Jun Tsuchiya et al. First‐Principles Determination of the Dissociation Phase Boundary of Phase H MgSiO4H2, Geophysical Research Letters (2019). DOI: 10.1029/2019GL083472


Acknowledgements

We thank E. Ohtani and W. F. McDonough for advice and guidance. The KamLAND experiment is supported by a Grant-in-Aid for Specially Promoted Research under grant 16002002 of the Japanese Ministry of Education, Culture, Sports, Science and Technology the World Premier International Research Center Initiative (WPI Initiative), MEXT, Japan and the US Department of Energy (DOE) grants DEFG03-00ER41138 and DE-AC02-05CH11231, as well as other DOE grants to individual institutions. The reactor data are provided by courtesy of the following electric associations in Japan: Hokkaido, Tohoku, Tokyo, Hokuriku, Chubu, Kansai, Chugoku, Shikoku and Kyushu Electric Power Companies, Japan Atomic Power Company and Japan Atomic Energy Agency. The Kamioka Mining and Smelting Company has provided service for activities in the mine.


Earth and Environmental Sciences

The Department of Earth and Environmental Sciences at UIC offers education and research opportunities in the dynamic processes that modify our environment, including those influenced by human activity, and that shape the earth and other planetary bodies. Our students receive broad training in interdisciplinary earth science and, with faculty, conduct research that involves field work around the globe, laboratory investigations on campus and at national facilities, and computational modeling and simulations of big data.

We offer three degree programs: Bachelor of Science (BS), Master of Science (MS), and Doctor of Philosophy (PhD). A minor in Earth and Environmental Sciences is also available to undergraduate students. As a department, we prioritize supporting diverse identities, creating inclusive environments, and ensuring education and research opportunities are equitable. We invite you to explore our programs and consider the opportunities they provide for future careers.


Author information

Affiliations

School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, GA, USA

Kazumi Ozaki & Christopher T. Reinhard

NASA Astrobiology Institute, Alternative Earths Team, Mountain View, CA, USA

Kazumi Ozaki & Christopher T. Reinhard

NASA Postdoctoral Program, Universities Space Research Association, Columbia, MD, USA

Department of Earth and Planetary Science, Graduate School of Science, The University of Tokyo, Bunkyo-ku, Tokyo, Japan

Department of Systems Innovation, School of Engineering, The University of Tokyo, Bunkyo-ku, Tokyo, Japan

Analysis Engineering Department, Hitachi Power Solutions Co., Ltd., Hitachi-Shi, Ibaraki, Japan

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Contributions

K.O. and E.T. developed the hypothesis. K.O., E.T. and C.T.R. designed the study. K.O. constructed the quantitative framework and performed experiments with the sGRB model. P.K.H. and Y.N. carried out the experiments with the coupled model. K.O., P.K.H., Y.N. and C.T.R. analysed the results. K.O., E.T. and C.T.R. wrote the paper with input from P.K.H. All authors discussed and contributed intellectually to the interpretation of the results.

Corresponding author


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