Studying the boundary between the Earth’s core and mantle, geophysicists have found that it is not as smooth as previously thought. Surfaces separating the inner layers also have a complex relief. It turns out that our planet is not at all like a set of spheres nested into each other, as is customary to portray it.
Reading the waves. Earth’s crust
The depths of geophysics are judged by the seismic waves generated by earthquakes. There are longitudinal P waves, when elastic mechanical vibrations occur along the propagation direction and transverse S waves, the vibrations in them are perpendicular. At the boundary of layers with different densities, the wave velocity changes dramatically. In the transition from a solid crust to a more plastic upper mantle, it increases. This edge is called the Mokhorovichich surface. The lower mantle is harder than the upper one. The outer core, in which the transverse seismic waves do not propagate, is liquid, and the inner core is again solid, but slightly plastic.
Although the seismograph network was rare, the sections between the inner layers with some degree of conventionality were represented as spheres. As data accumulated, it became clear that each of these boundaries is a complex surface with its relief and internal “mountains” even higher than on Earth’s surface, and the “channels” are deeper. From the top of Everest to the bottom of the Mariana Trench, about 20 kilometers, and, for example, the differences of the Mokhorovichich border, which divides the crust and the upper mantle, reach 40 kilometers. And all this at a depth of five to 70 kilometers.
This wasscientists from China and the United States. They analyzed the results of observations of hundreds of seismic stations obtained from the same events: the 1994 earthquakes in Bolivia and the Okhotsk Sea in 2008 and 2012, as well as seismograph archival records from the National Earthquake Information Center. from the United States Geological Survey.
The study authors found that for the boundary between the upper and lower mantle, located at a depth of approximately 660-670 kilometers, the data from the various stations almost completely coincide. I mean, she has stable relief, which she even managed to map. The Bolivian earthquake signal processing literally created a “topographic map” of the lower mantle surface for an entire region of Southeast Asia.
The most dynamic area. Mantle and core
When talking about Earth dynamics, they generally refer to large-scale surface processes associated with the movement of lithospheric plates. In the ridges and crevices in the mid-ocean, the lithosphere separates, and in the subduction zones on the outskirts of the continents, the oceanic plates sink below the continent.
But no less dynamic processes and surface movements occur within the Earth, only its reflection. First of all, we are talking about mantle convection, which arises due to the difference in temperature in the intestines and on the surface of the planet.
The upward flows of convection cells stretch the lithosphere, the downward flows drag it into the mantle. Furthermore, in the upper parts of the cells, the substance flows in a horizontal plane and these flows cause the lithospheric plates to move. The most dynamic region on Earth is at the edge of the core and mantle, at a depth of approximately 2900 kilometers
Its heterogeneity is believed to affect many geological processes, in particular the oscillation of the Earth’s axis of rotation and the characteristics of the geomagnetic field. Furthermore, convection itself is a consequence of what happens in layer D ”at the border with the nucleus. On its surface, scientists discovered unusually dense hot rock assemblies, abnormally low seismic wave velocity zones (ULVZ – Ultra-low velocity zones). They stretch for hundreds of kilometers and their “height” – tens of kilometers. Above them are hot spots with volcanoes: Hawaii, Marquesas, the Galapagos Islands and the Samoa archipelago in the Pacific Ocean, the Canary Islands and the Azores, Iceland in the Atlantic, the Kerguelen Archipelago in the Indian volcanic zone, far in the Great African Rift .
Using the new machine learning algorithm, American scientists from Johns Hopkins University and the University of Maryland at College Park, along with their Israeli colleagues from Tel Aviv University,Parallel analysis of seven thousand seismograms spanning hundreds of earthquakes from 1990 to 2018, and for the first time compiled a detailed map of the core and mantle section of the Pacific region, in which all ULVZ zones were applied. It turned out that ULVZ are just separate bumps within the larger, low shear rate (LLSVP) provinces, which are also called super feeds. Its branches penetrate the mantle for thousands of kilometers. Scientists now distinguish two of those provinces: Africa and the Pacific.
The circulation of matter in the mantle
Australian scientists from Curtin Universitythat the periods when all the Earth’s land was united in unique supercontinents: Pangea, Rodinia, Colombia and others, coincided with the activity in the deep provinces of LLSVP. They built a dynamic model linking the evolution of super feathers to the assembly and decomposition of the supercontinent. According to this model, LLSVP matrices are formed from lithospheric plates, which, as Sinking, it does not dissolve in the mantle, as previously thought, but descends to the limit of the nucleus. Here they melt, and giant drops of preheated matter – mantle plumes – coming out of LLSVP float to the surface, giving rise to a new geodynamic cycle. The lithosphere rises above the feathers, forming a dome, and then cracks and diverges.
Inside the core
Researchers from the US and China havehow the seismic waves passing through the boundary between the inner and outer nucleus change. For this, we use doublet signals: repeated earthquakes with the same epicenter.
It turned out that these changes have a certain periodicity, which can be explained by two mechanisms: the inner core rotates around 0.05-0.1 degrees per year, or high “mountains” and deep “canyons” appear on its surface. So a dynamically changing relief may also be at the deepest boundary between the shells of the earth.