Peering into the Earth’s Solid Inner Core Using Seismic Waves…
Earth’s inner core is a hot dense ball of solid iron, which is about the size of the moon. This enigmatic ball of iron is surrounded by a highly dynamic outer-core consisting mainly of a liquid iron-nickel alloy, a highly viscous mantle, and a paperthin solid crust that forms the surface that we live on. Over billions of years, Earth has cooled from inside-out, causing the molten iron core to solidify outward from the centre. As the entire Earth slowly cools, the inner core grows by about a millimeter every year. The inner core grows as the liquid core solidifies or crystallizes. Another word for this is “freezing.”
However, Scientists have recently begun to realize that the inner core may be melting as well as freezing, but there has been much debate about how this is possible when the deep Earth is cooling as a whole.
Using computer models of convection in the outer core, together with seismic data, researchers have now shown that the heat flow in the liquid outer core varies laterally depending on the structure of the overlying mantle. In some regions, this variation is large enough to force heat from the mantle back in to the core, causing localized melting. This suggests that the Earth’s inner core undergoes melting and freezing simultaneously due to variable circulation of heat in the overlying rocky mantle. These findings could help scientists understand how the inner core of Earth was formed and how it is evolving.
Furthermore, at hemispherical-scale, the inner core is divided into eastern and western hemispheres. The two hemispheres do not melt evenly, and each has a distinct seismic structure. The western hemisphere seems to crystallize slowly than the eastern hemisphere. In fact, parts of the western hemisphere of the inner core may actually be melting.
To accurately pinpoint these hemispherical differences and to solve the mystery of how Earth’s inner core was formed, scientists have been working towards finding new evidence that could support the mechanisms they suggested.
Dr. Januka Attanayake is a Sri Lankan solid Earth Scientist who has presented new seismological insights into mechanisms of growth of the Earth’s inner core. His work formerly at the University of Connecticut, USA, and now at the University of Münster, Germany, provides evidence of freezing and melting in the inner core. For more information about his work, we arranged a discussion with Dr. Attanayake. In general, pressure and temperature increase from the surface to the center of the Earth. In the core of the Earth, one can reasonably assume that the composition is nearly uniform although the inner core might have a smaller quantity of impurities such as Silicon and Oxygen relative to the outer core. While an increase in temperature tends to melt iron, an increase in pressure suppresses its tendency to melt. The outer core is liquid because effects of temperature dominate at that depth, whereas effects of pressure dominate at even greater depths corresponding to the inner core, letting it stay in solid state. As you can imagine, there is a delicate balance between temperature and pressure at extreme conditions found in the very deep Earth.
-Sujitha Miranda
No geophysicist of repute I know of subscribes to this hypothesis. That idea is as good as conspiracy theory. First, the Earth is not massive enough to sustain extreme conditions that can produce a plasma core. Only very large planetary bodies like the Sun that can accommodate 1.3 million Earths inside of it can maintain physical conditions required to produce a plasma core. Secondly, radial density distribution of the Earth estimated from astronomical and geological observations is consistent with the existence of an iron core. Thirdly and more importantly, seismological observations such as free oscillations (natural continuous ringing of the Earth) anisotropy (directional dependence of wave speeds) and seismic scattering (re-distribution of wave energy in time and space) along with laboratory experiments conducted at extreme conditions corresponding to those of the core suggest that Earth’s core cannot be a plasma.
In our work, we specifically look at seismic wave speeds and attenuation - how much energy is lost during the passage of waves in the inner core – and the correlation between these two quantities in a given region of the inner core. Sophisticated mineral physics experiments show that seismic waves have unique characteristics when they travel through solid versus liquid media. In solid media, waves tend to speed up and are less attenuated, whereas in liquid media, waves slow down and are more attenuated. By carefully measuring these quantities, it might be possible to tell these two different states (melt and solid) apart.
While it is true that the Earth is cooling over geologic time, it is not a uniform process. That is, heat trapped inside the Earth does not move outward in a systematic manner. In fact, recent geodynamic simulations show that thermal convection in the liquid outer core is anything but simple. These models show that lateral temperature variations along the Core-Mantle Boundary estimated to be on the order of 1000°K could produce complex convection patterns in the liquid outer core, which in turn create lateral variations in cooling rates near the Inner Core Boundary. These variations, although expected to be very small in absolute terms, can be actually large enough to generate very different growth conditions along the Inner Core Boundary so that certain regions undergo melting, while other regions undergo solidification. As mentioned earlier, there is a delicate balance between extreme pressure (about 325 million atmospheres) and temperature (possibly as high as 7000°K) at the Inner Core Boundary. Nudging temperature slightly in either direction could produce very different growth conditions.
One of our objectives has been to independently test, using seismic measurements, a recent geodynamic hypothesis that states certain regions in the inner core are undergoing melting, while other regions freeze rapidly. There are two points to be considered here. According to this hypothesis, the inner core can maintain an overall growth rate up to 1 mm per year because the volume of newly solidified iron in regions that undergo fast cooling far exceeds that lost to melting. In addition, we hypothesized in our work that this melting process is episodic. This means that a region undergoing melting today might undergo rapid freezing several tens of millions years from now. This latter hypothesis is based on “jumps” recorded in the geomagnetic field anomalies. So yes, if the melting process is actually consistent with this description, it does not violate the idea that the inner core is progressively increasing its size over geologic time.
Seismic evidence is conclusive about hemispherical variations in the inner core. That is, if you average the seismic structure of the inner core, one half of the inner core is different from the other half. This was discovered exactly 2 decades ago by Japanese seismologists and has since puzzled deep Earth Geophysicists immensely. In the region we identify as the quasi-eastern hemisphere (predominantly the region beneath Southeast Asia), seis- mic waves travel faster and are more attenuated than in the quasi-western hemisphere. Of course, this observation is made when you consider waves that are traveling in the equatorial plane (~90° from the rotation axis of the Earth). If you also consider waves that travel in the poloidal plane (less than about 30° from the rotation axis), the picture is even more complicated. Our work was conducted at what we call mesoscale. That is, we peered into hemispheres rather than trying to fine-tune hemispherical structure. Based on our work, we found that hemispherical structure, in which there is a positive correlation between wave speed and attenuation, is a robust feature. However, the extent of it is less than what was previously thought. We found that the positive correlation between velocity and attenuation breaks down in the region beneath the Pacific Ocean. In fact, we found a reverse correlation between wave speed and attenuation in that region. We believe this is the signature of melting in the inner core and it is consistent with predictions of mineral physics.
As mentioned earlier, inner core growth process seems to be coupled to the temperature variations on the solid side of the Core-Mantle Boundary. Remember, on one side of this boundary is the liquid outer core and on the other side is the solid mantle. Models based on seismic waves very clearly indicate the presence of two continental scale anomalous structures at the bottom of the mantle. These are called Large Low Shear-wave Velocity Provinces or LLSVPs. These are very large structures. They extend laterally ~1500 km and vertically ~1000 km, accounting for roughly 3-5% of the volume of the Earth. One of these structures is located beneath the Pacific and the other is located beneath Africa. One type of seismic waves (Shearwaves) that moves particles perpendicular to the wave propagation direction is particularly sensitive to these structures and shows a reduction in speeds up to about 5%. One explanation for this observation is that LLSVPs are thermal plumes. On the other hand, the colder regions near the Core-Mantle Boundary have been interpreted to be graveyards of subducting slabs. From images of the deep Earth produced by tracking seismic waves in the Earth, we infer that oceanic plates could probably sink as far down as the CoreMantle Boundary as a result of plate tectonics. We see early seismic evidence of these large structures near the CoreMantle Boundary affecting the process of heat extraction near the Inner Core Boundary, which is the driving mechanism of inner core growth. We will need more data in the future to confirm if there is such a direct coupling between the structures near the Core-Mantle Boundary and the growth of the inner core. We are conducting new experiments to further understand this process.
The two main properties we measure are seismic wave speeds and seismic energy loss (attenuation) in the Earth’s inner core. In regions where heat extraction is weak, we expect the inner core to be melting or at near melting conditions, whereas in regions where heat is extracted very efficiently, inner core must be freezing rapidly. Mineral physics experiments show that these two different conditions could impart very different characteristics to seismic waves. Under near melting or melting conditions, Primary waves (P-waves) show a reduction in speed as well as an increase in attenuation, whereas that relationship between wave speed and attenuation is reversed under freezing conditions. We observed the former pattern beneath the Pacific in the inner core and we believe that this is first seismic evidence suggesting melting in the inner core in that region. The rest of the inner core is showing a completely different wave speed-attenuation relationship – higher velocity related to higher attenuation and vice versa. We believe other mechanisms are at play and might be dominant in these freezing regions. Variable impurity incorporation driven by thermal perturbations is one such mechanism. As you rightly point out, Earth’s geo- magnetic field is generated and sustained by the convection in the liquid outer core, a process we call the Geodynamo. Experiments show that liquid iron at these very extreme conditions flow like water at the surface of the Earth. On average, a particle in this liquid iron reservoir could travel about 10 km a year.
What our seismic observations suggest is that Core-Mantle Boundary is coupled to the Inner Core Boundary through thermal convection in the liquid outer core. This would mean that the geomagnetic field is intimately related to the inner core growth process, and its evolution over geologic time might be shaped by what is going on near the inner core boundary.
There is a growing body of literature suggesting that seismic P waves sampling the 150 km region immediately above the inner core (we call this the F-layer) are slowed down. One of the simplest explanations for this observation is that the inner core is surrounded by a dense layer, possibly slurry, in which iron crystals and liquid co-exist. Alternatively, liquid heavier than what is in the outer core can be produced from melting of solid iron in the inner core. Remember that when liquid iron in the outer core freezes into solid inner core, the density of material increases as the lighter elements such as Si, H, and O are pushed out back in to the outer core. Seismic wave speeds are inversely correlated with density, and therefore, such a dense layer can produce the observed signature of seismic waves. Another way to look at this anomalous layer is to determine how seismic waves attenuate in this region. Standard reference Earth models do not have anomalous attenuation associated with the F-layer. A previous study, however, suggested that there is strong attenuation in this layer, which would mean that something in this particular layer is either absorbing seismic energy (intrinsic attenuation) or that it is re-distributing energy in space and time (seismic scattering). This study, however, used very little data and the coverage was sparse. We looked at this problem using different types of seismic phases with greater coverage, and found that seismic wave attenuation is intermediate to what is given in standard reference Earth models and the high values estimated in the study I mentioned earlier. While the exact value of attenuation is not conclusively known, I think there is enough evidence to suggest the presence of an anomalous layer surrounding the inner core. Evidence for a similar layer is found at the top of the liquid outer core immediately below the Core-Mantle Boundary.
Funding Agency: National Science Foundation, United States of America.
Institutional Collaborators: The University of Connecticut, USA, the University of Munster, Germany.
Research Partnerships: Prof. Vernon F. Cormier, USA and Prof. Christine Thomas, Germany. Studentships: Susini de Silva. Dr. Januka Attanayake is a doctoral graduate of the University of Connecticut, USA who is currently working in different areas of Global Seismology and Solid Earth Geophysics. He has been able to publish his discoveries and innovative scholarly works in many international indexed journals including Geophysics Journal International, Earth and Planetary Science Letters, Journal of African Earth Sciences, Journal of Earth Science, Physics of the Earth and Planetary Interiors, etc. In addition Dr. Attanayake has written a book chapter on “Indian Ocean Diffuse Zone (IODZ): Evolution, Structure, Kinematics & Seismicity” that gives a more comprehensive insight into earthquakes in and around Sri Lanka. This chapter will be included in the book “Advancement in Geology of Sri Lanka,” which is scheduled to be published later this year by the Geological Society of Sri Lanka. Further, he is a reviewer of Physics of the Earth and Planetary Interiors Journal and Journal of Geophysics and Engineering. Dr. Attanayake was once an Honorary Research Associate at University College London, UK. In 2010, he was awarded the Outstanding Graduate Student Teacher Award by the University of Connecticut. During his undergraduate times, he received the University Award for Academic Excellence from the University of Peradeniya. At present, Dr. Attanayake serves as a postdoctoral research assistant at the University of Munster, Germany. Discussed & Prepared By, Kusala Madhushani Premaratne For interviews, contact: kusala.educationtimes. scholar@gmail.com T.P. 0767200715, 0717188748, 0772532923