All schoolchildren know that the planet Earth is divided into three (or four) large layers: the crust, mantle and core. In general, this is true, although this generalization does not take into account several additional layers that scientists have identified, one of which, for example, is the transition layer inside the mantle.
In a study published February 15, 2019, geophysicist Jessica Irving and graduate student Wenbo Wu of Princeton University, in collaboration with Sidao Ni of the Geodetic and Geophysical Institute in China, used data from the massive 1994 Bolivian earthquake to locate mountains. and other topographic features on the surface of the transition zone deep within the mantle. This layer, located 660 kilometers underground, separates the upper and lower parts of the mantle (without having a formal name for this layer, the researchers simply called it "660 km boundary").
In order to "look" so deep underground, scientists used the most powerful waves on the planet, caused by the strongest earthquakes. “You need a big, deep earthquake to shake the planet,” said Jessica Irving, assistant professor of geophysical sciences.
Large earthquakes are much more powerful than normal ones - the energy of which increases 30 times with each additional step up the Richter scale. Irving gets his best data from earthquakes of magnitude 7.0 and above, because the seismic waves sent out by such powerful earthquakes travel in different directions and can travel through the core to the other side of the planet and back. For this study, key data was obtained from seismic waves that were recorded from an earthquake of magnitude 8.3 - the second deepest earthquake ever recorded by geologists - that rocked Bolivia in 1994.
“Earthquakes of this magnitude do not happen often. We are very fortunate that there are now many more seismometers installed around the world than 20 years ago. Seismology has also changed a lot in the last 20 years thanks to new tools and computer power.
Seismologists and data scientists use supercomputers such as the Princeton Tiger Cluster Supercomputer to simulate the complex behavior of scattering seismic waves deep underground.
The technologies are based on the fundamental properties of waves: their ability to reflect and refract. Just as light waves can bounce (reflect) off a mirror or bend (refract) when they pass through a prism, seismic waves travel through homogeneous rocks but are reflected or refracted when they encounter rough surfaces along the way.
"We know that almost all objects have uneven surfaces and therefore can scatter light," said Wenbo Wu, lead author of the study, who recently received his Ph.D. in geonomy and is now a postdoctoral fellow at the California Institute of Technology. “Thanks to this fact, we can “see” these objects - scattering waves carry information about the roughness of the surfaces that they encounter on the way. In this study, we studied scattering seismic waves propagating deep inside the Earth to determine the “roughness” of the found 660-kilometer boundary.”
The researchers were surprised how "rough" this boundary is - even more than the surface layer on which we live. “In other words, this underground layer has a more complex topography than the Rocky Mountains or the Appalachian mountain system,” Wu said. Their statistical model was unable to determine the exact heights of these underground mountains, but there is a strong possibility that they are much higher than anything on the surface of the Earth. Scientists also noticed that the 660-kilometer border is also unevenly distributed. In the same way that the land layer has a smooth ocean surface in some parts and massive mountains in others, the 660 km boundary also has rough zones and smooth layers on its surface. The researchers also studied underground layers at a depth of 410 kilometers and at the top of the middle layer of the mantle, but could not find a similar roughness of these surfaces.
"They found that the 660-kilometer boundary is as complex as the surface layer of the earth," said seismologist Kristina Hauser, an assistant professor at the Tokyo Institute of Technology, who was not involved in the study. “Using seismic waves created by powerful earthquakes to find a 3 kilometer difference in elevation 660 kilometers deep underground is an unimaginable feat.… Their discoveries mean that in the future, using more sophisticated seismic instruments, we will be able to detect previously unknown, subtle signals which will reveal to us new properties of the inner layers of our planet.
Seismologist Jessica Irving, assistant professor of geophysics, holds two meteorites from the Princeton University collection that contain iron and are believed to be part of planet earths.
Photo taken by Denis Appelwhite.
What does this mean?
The existence of rough surfaces at the 660 km boundary is essential to understanding how our planet is formed and functions. This layer divides the mantle, which makes up about 84 percent of our planet's volume, into upper and lower sections. For years, geologists have debated how important this boundary is. In particular, they studied how heat is transported through the mantle - and whether heated rocks move from the Gutenberg boundary (the layer separating the mantle from the core at a depth of 2900 kilometers) up to the top of the mantle, or if this movement is interrupted at the 660-kilometer boundary. Some geochemical and mineralogical evidence suggests that the upper and lower layers of the mantle have different chemical compositions, supporting the idea that both layers do not mix thermally or physically. Other observations suggest that the upper and lower mantles have no chemical difference, giving rise to the so-called "well-mixed mantle" controversy, where both mantle layers participate in an adjacent heat exchange cycle.
“Our study provides a new perspective on this controversy,” Wenbo Wu said. The data from this study suggests that both sides may be partially right. The smoother strata of the 660 km boundary may be due to thorough, vertical mixing, where rougher, mountainous zones may have been formed where the mixing of the upper and lower mantle layers did not proceed as smoothly.
In addition, the "roughness" of the layer at the found boundary was found on large, medium and small scales by research scientists, which in theory could be caused by thermal anomalies or chemical heterogeneity. But due to the way heat is transported in the mantle, Wu explains, any small-scale thermal anomaly would be smoothed out within a few million years. Thus, only chemical heterogeneity can explain the roughness of this layer.
What could be the reason for such a significant chemical heterogeneity? For example, the appearance of rocks in the layers of the mantle that belonged to the earth's crust and moved there over many millions of years. Scientists have long argued about the fate of the plates on the seafloor that are pushed into the mantle in subduction zones that collide around the Pacific Ocean and elsewhere on the globe. Weibo Wu and Jessica Irving suggest that the remnants of these plates may now be above or below the 660 km boundary.
“Many believe that it is quite difficult to study the internal structure of the planet and its changes over the past 4.5 billion years, only using seismic wave data. But this is far from the case! - Irving said - this study has given us new information about the fate of ancient tectonic plates that descended into the mantle over many billions of years.
At the end, Irving added, "I think seismology is most interesting when it helps us understand the internal structure of our planet in space and time."
From the author of the translation: I always wanted to try my hand at translating a popular science article from English into Russian, but did not expect as far as it's complicated. Great respect for those who regularly and efficiently translate articles on Habré. In order to professionally translate a text, one must not only know English, but also understand the topic itself by studying third-party sources. Add a little “gag” to sound more natural, but don’t overdo it so as not to spoil the article. Thank you very much for reading 🙂
Source: habr.com