[Rockhounds] 2 giant blobs in Earth's mantle may explain Africa's weird geology

[Kreigh Tomaszewski]

Deep within Earth's mantle, there are two giant blobs. One sits under
Africa, while the other is almost precisely opposite the first, under the
Pacific Ocean. But these two blobs are not evenly matched.

New research finds that the blob under Africa extends far closer to the
surface — and is more unstable — than the blob under the Pacific. This
difference could ultimately help to explain why the crust under Africa has
been lifted upward and why the continent has seen so many large
supervolcano eruptions over hundreds of millions of years.

"This instability can have a lot of implications for the surface tectonics,
and also earthquakes and supervolcanic eruptions," said Qian Yuan, a
graduate associate in geology at Arizona State University (ASU) who led the
research.

The mantle blobs are properly known as "large low-shear-wave-velocity
provinces," or LLSVPs. This means that when seismic waves generated by
earthquakes travel through these deep-mantle zones, the waves slow down.
This deceleration indicates that there's something different about the
mantle at this spot, such as density or temperature — or both.

Scientists aren't sure why the mantle blobs exist. There are two popular
hypotheses, Yuan told Live Science. One is that they're made up of
accumulations of crust that have subducted from Earth's surface to deep
inside the mantle. Another is that they're the remnants of an ocean of
magma that may have existed in the lower mantle during Earth's early
history. As this magma ocean cooled and crystallized, it may have left
behind areas that were denser than the rest of the mantle.

https://www.livescience.com/mantle-blobs-under-earth