Reproducing Rock Deformation Occurring at Ultra-high Pressure and Temperature Conditions inside the Earth

My research focuses on experimental rock deformation and rock rheology (i.e., flow of rocks). I reproduce high-pressure and high-temperature conditions in the laboratory to study earthquakes, mantle convection, and other phenomena occurring inside the Earth and other planetary bodies.

Because Mars, Venus, and the Moon are made of minerals similar to the Earth, one of my research goals is to accumulate our knowledge of these planetary bodies by studying the Earth’s interior.

Why do I reproduce deep-underground environments in the lab? Because it is extremely difficult to investigate the Earth’s interior directly, that’s why. Japanese deep-sea drilling scientific vessel Chikyu has a special equipment for drilling down into the seafloor, and we can collect samples from beneath the seafloor and install observation equipment into boreholes using this drilling vessel.

These researches are quite important because it allows for direct sampling from underground of the Earth, and I also use these samples in my research. However, human technology can only drill to around 10 km deep, while the Earth has a radius of around 6,400 km. Another issue with this research method is that it involves significant time and geographical constraints, and the research projects themselves require tremendous manpower and financial commitments. Therefore, for studying of Earth’s interior, it is vital to use various research techniques, including geophysical observations using seismography and GPS, in combination with chemical analysis of rocks and minerals delivered from deep in the Earth to near the surface by events like crustal deformation and volcanic eruptions.

My research involves reproducing the high-temperature and high-pressure environments exist deep in the Earth—anywhere from dozens to thousands of kilometers underground—and observing how rocks and minerals deform and what kind of reactions occur at these environments. In this way, I strive to boost understanding of what it’s like down there. Compared with other methods, this kind of research requires only a small number of people, and can repeat trial-and-error many times in the lab. I feel this way suits my personality, too.

Rock and Water Circulation Deep Inside the Earth

New Planet Rheology: From Earth to Mars and Other Icy Planets, my research project selected for the Japan Science and Technology Agency’s (JST) Fusion Oriented Research for Disruptive Science and Technology program, is aimed at studying the time scale at which rocks and water circulate inside the Earth and other planets. We know that rocks in the Earth convect slowly in solid form—a few centimeters per year—but our knowledge of rock rheology in the Earth’s deep interior is limited to the relatively shallow region—the top 10% or so from the surface.

Also, research over the past 30 years has changed our knowledge of water in the Earth. Ocean water is only about 0.2% of volume of the Earth, and we now know that many times more water than that of the oceans circulates deep in the Earth, not just as a fluid, but also as hydroxyl (-OH), and hydrogen (H). It is thought that the presence of such water enhances the flow of rocks and triggers earthquakes in the Earth’s deep interior; one aim of my research is to determine the roles of water in deformation of the Earth through rock deformation experiments.

Almost all current rock deformation test equipment is unable to deform rocks under the high-temperature, high-pressure conditions found thousands of kilometers below the Earth’s surface; nor can it inject water into rock while controlling the extreme high pore fluid pressures thought to exist deep underground. But my colleagues and I are working to develop new apparatuses that can withstand those extreme conditions so as to undertake new research. Recently, we have successfully reproduced the temperature and pressure conditions between 30 km and 700 km below the Earth’s surface. This has enabled us to study how olivine undergoes phase transition* and turns into other minerals when exposed to those temperatures and pressures, and how its rheological properties change with the progress of reaction with and without water under those extreme conditions.

Olivine is a mineral that composes the Earth’s upper mantle. Deformation of the mantle can be estimated by observing how olivine deforms and changes under high temperatures and pressure. Similarly, the Earth’s inner core is mainly composed of solid iron, so eventually we would like to reproduce the pressures and temperature condition found there so as to study how iron deforms at the center of the Earth.

*Phase transition: A phenomenon in which the material structure changes with changing pressure and/or temperature, such as water turning into ice or graphite turning into diamond.

The laboratory. Five different rock deformation apparatuses are maintained with Prof. Ikuo Katayama, also of Hiroshima University.

Researchers involved in ultra-high pressure rock deformation experiments at SPring-8, a synchrotron radiation facility in Hyogo Prefecture. The project involves Asst. Prof. Okazaki’s research lab, Prof. Shintaro Azuma’s research lab at the Institute of Science Tokyo, and Dr. Kentaro Uesugi and Dr. Masahiro Yasutake of SPring-8.

These studies may help improve our understanding of the mechanism of earthquakes. Most earthquakes occur in subduction zones, where oceanic tectonic plates subduct beneath continental tectonic plates. But “slow earthquakes,” in which faults move more slowly than in normal earthquakes, have been observed in some areas of subduction zones. Some believe that there is a relationship between water in rocks and the mechanism triggering slow earthquakes, so I hope to use the outcomes of our experiments to shed light on deformation behavior in rocks with large amounts of water, and on the connection with normal and slow earthquakes.

Put simply, I hope that this research project will result in discovery of the “metabolism” of planets. Understanding how water in the Earth’s rivers, oceans, atmosphere, and the most voluminous of all bodies of water—i.e., that inside the Earth—circulate together with underground rocks will provide clues to help us better grasp our planet’s “metabolism,” which includes plate tectonics (planetary activity caused by interaction between plates), seismic and volcanic activities, and environmental changes throughout the Earth’s history.

A Fascinating Field of Research that Looks into the Distant Past and the Future

The main thing that inspired me to pursue this area of research was the fact that it was not likely to be immediately useful. The more immediately useful the knowledge, the more widely known it’s likely to be, and the more it’s liable to become obsolete quickly. Originally, I was going to teach science at high school; my thinking was that if I’m going to teach, I at least want to teach something that can continue to stimulate lifelong curiosity. So, naturally, if I was going to study specialist knowledge at university, it had to be something that has a long focus. Studying the Earth and the universe fits the bill aptly, with its extremely long-term perspectives, stretching from billions of years in the past and into the future.