Starting to understand two strange hot islands of the size of continents deep in the Earth’s lower mantle

Amazing stuff! Plate tectonics and a well flowing mantle are apparently not the whole story!

"Deep inside Earth there are mysterious and fascinating regions the size of continents at the boundary between the core and mantle, and earth scientists have just cracked the mystery of what they’re made of.

Known as “Large Low Seismic Velocity Provinces”, or LLSVPs, the regions cause seismic waves to slow down. ...

One is found under Africa, the other under the Pacific Ocean.

“These 2 large islands are surrounded by a graveyard of tectonic plates which have been transported there by a process called ‘subduction’, where one tectonic plate dives below another plate ...

study looked at whole-Earth oscillations – seismic waves caused by very large earthquakes, which make the planet ring like a bell for weeks or even months – to learn more about LLSVPs. ..."

"... In this type of research, seismologists make good use of oscillations caused by really large earthquakes, preferably quakes that take place at great depths, such as the great Bolivia earthquake of 1994. “It never made it into the newspapers, because it took place at a large depth of 650 km and luckily did not result in any damage or casualties at the Earth’s surface,” ... The whole Earth oscillations, or tones, are mathematically described in such a way that we can easily ‘read’ the damping (i.e. how loud the oscillation is) due to a specific structure and separate it from the wave speed (i.e. how much out of tune it is). “Which is impressive, because the damping of the signal is only one-tenth of the total amount of information that we can unravel from these oscillations.” For this type of research, it is not necessary to wait until another earthquake occurs. The data from previous earthquakes is just as useful. “We can go back to 1975, because from that year onwards, seismometers became good enough to give us data of such high quality that they are useful for our research.” ..."

From the abstract:

"Seismic tomographic models based only on wave velocities have limited ability to distinguish between a thermal or compositional origin for Earth’s 3D structure. Complementing wave velocities with attenuation observations can make that distinction, which is fundamental for understanding mantle convection evolution. However, global 3D attenuation models are only available for the upper mantle at present.

Here we present a 3D global model of attenuation for the whole mantle made using whole-Earth oscillations, constraining even spherical harmonics up to degree four. In the upper mantle, we find that high attenuation correlates with low velocity, indicating a thermal origin, in agreement with previous studies.

In the lower mantle, we find the opposite and observe the highest attenuation in the ‘ring around the Pacific’, which is seismically fast, and the lowest attenuation in the large low-seismic-velocity provinces (LLSVPs). Comparing our model with wave speeds and attenuation predicted by a laboratory-based viscoelastic model suggests that the circum-Pacific is a colder and small-grain-size region, surrounding the warmer and large-grain-size LLSVPs. Viscosities calculated for the inferred variations in grain size and temperature confirm LLSVPs as long-lived, stable features."

Starting to understand strange islands in the Earth’s mantle

Subterranean ‘islands’: strongholds in a potentially less turbulent world (original news release) "Deeply hidden in Earth’s mantle there are two huge ‘islands’ with the size of a continent. New research from Utrecht University, published in Nature, shows that these regions are not only hotter than the surrounding graveyard of cold sunken tectonic plates, but also that they must be ancient: at least half a billion years old, perhaps even older. These observations contradict the idea of a well-mixed and fast flowing Earth’s mantle, a theory that is becoming more and more questioned. “There is less flow in Earth’s mantle than is commonly thought.”"

Global 3D model of mantle attenuation using seismic normal modes (no public access)

Location of the LLSVPs and a schematic representation of the Earth's cross-section for speed and damping of the seismic waves.

Schematic representation of the process of subduction of tectonic plates and of a mantle plume rising from an LLSVP. In the latter, the mineral grains are larger than those in the subducted plates.