What is the African blob found below the continent?

Peculiar formation may help explain why the continent has experienced so many large supervolcano eruptions

An artist's impression of the African blob.
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Scientists say a formation about 1,000km under the surface of Africa may be the reason for its violent geology.

The African blob is one of two discovered in the 1970s, with little known about the anomalous objects.

But now researchers from Arizona State University have published a study saying the blob may help explain why the continent has experienced so many large supervolcano eruptions.

The two blobs in the lower mantle, one beneath Africa and the other beneath the Pacific Ocean, were found using seismic observations.

Now scientists have found the African blob extends far closer to the surface and is more unstable.

But what is it and how did it get there?

What is the African blob?

It is not known exactly what the blob is made of, though recycled oceanic crust or iron-rich material are among the theories.

It is one of two giant blobs sitting deep within Earth's mantle — the second sitting almost directly opposite the first under the Pacific Ocean.

They are known as large low-shear-wave-velocity provinces.

The mantle blobs get their name because when seismic waves generated by earthquakes travel through these deep-mantle zones, the waves slow down, indicating a difference in the mantle such as density or temperature.

What caused the blobs?

Scientists are not sure why the blobs exist, though there are two popular hypotheses, Qian Yuan, a graduate associate in geology at Arizona State University, who led the research, told science news site Live Science.

The first is that they consist of accumulations of crust that have been pushed from the surface to deep in the mantle by tectonic plate convergence.

The second theory is that they are what is left of an ocean of magma that may have existed in the lower mantle during Earth's early history, which has cooled and crystallised, leaving denser areas than the rest of the mantle.

The giant blobs nestled deep in the Earth may influence surface geology.

How are the two blobs different?

The African blob extends about 1,000 kilometres higher than the Pacific blob. It extends about 1,600 to 1,800km upwards from the boundary between the core and the mantle, while the Pacific blob extends about 700 to 800km.

Research shows the two blobs differ in density, with the African blob found to be less dense, or less stable.

And the African blob is rising. But with a speed of about 1cm to 2cm a year, it would take 50 to 100 million years for it to reach the surface, researcher Mingming Li told Newsweek.

How could the African blob affect Earth’s surface?

The difference could help to explain why the crust under Africa has been lifted upwards 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 Mr Yuan.

Despite the African blob’s distance from the Earth's crust ― and the fact the planet’s mantle is 2,900km thick — scientists think the blob’s instability could affect the planet's surface because large low-shear-wave-velocity provinces may be a source of hot plumes, which could cause supervolcano eruptions, tectonic upheaval or even continental break-up, Mr Yuan said.

Updated: March 16, 2022, 10:12 AM