Two giant blobs lurk deep within the earth and it appears they are shapeshifters

Deep in the earth below us lie two blobs the size of continents. One is under Africa, the other under the Pacific Ocean.

The blobs have their roots 2,900 km (1,800 miles) below the surface, almost halfway to the center of the earth. They are believed to be the birthplace of rising columns of hot rock called “Deep Mantle Plumes” that reach the earth’s surface.

When these plumes first reach the surface, giant volcanic eruptions occur — the kind that contributed to the volcano’s extinction dinosaur 65.5 million years ago. The blobs could also control the eruption of a rock called kimberlite, which brings diamonds to the surface from depths of 120 to 150 km (and in some cases up to about 800 km).

Scientists have long known the blobs exist, but how they behaved throughout Earth’s history has been an open question. In new research, we’ve modeled and discovered a billion years of Earth’s history the blobs gather together and break apart similar to continents and supercontinents.

(Omar Bodur)

Above: Earth’s blobs mapped from seismic data. The African spot is at the top and the Pacific spot is at the bottom.

A model for the development of clods of earth

The blobs are in the mantle, the thick layer of hot rock between the Earth’s crust and its core. The mantle is solid but flows slowly over long periods of time. We know the blobs are there because they slow earthquake-induced waves, suggesting the blobs are hotter than their surroundings.

Scientists generally agree that the blobs are associated with the movement of tectonic plates at the Earth’s surface. However, how the blobs have changed throughout Earth’s history has confused her.

One school of thought was that the present clumps acted as anchors locked in place for hundreds of millions of years while other rocks moved around them. However, we do know that tectonic plates and mantle plumes move over time, and research suggests the shape of the blobs changes.

Our new research shows that the blobs of Earth have changed shape and location much more than previously thought. In fact, they have assembled and disintegrated throughout history in much the same way that continents and supercontinents have on Earth’s surface.

We used Australia’s National Computing Infrastructure to run advanced computer simulations of how Earth’s mantle has flowed over a billion years.

These models are based on Reconstruction of the movements of tectonic plates. When plates collide, the seafloor is pushed down between them in a process known as subduction.

The cold rock from the sea floor sinks deeper and deeper into the mantle and pushes the hot blobs aside from a depth of around 2,000 km.

Above: The last 200 million years of the Earth’s interior. Hot features are yellow to red (darker is shallower) and cold features are blue (darker is deeper).

We found that just like continents, the blobs can accumulate – forming “superblobs” as in the current configuration – and dissipate over time.

A key aspect of our models is that while the blobs change position and shape over time, they still fit the pattern of volcanic and kimberlite eruptions recorded at Earth’s surface. This pattern has been an important argument for the blobs as unmoving “anchors”.

Strikingly, our models show the African blob forming as little as 60 million years ago, in stark contrast to previous assumptions that the blob might have existed in roughly its present form almost ten times as long.

Remaining questions about the blobs

How did the blobs come about? What exactly are they made of? We still don’t know.

The blobs can be denser than the surrounding mantle and could therefore consist of material separate from the rest of the mantle early in Earth’s history. This could explain why the Earth’s mineral composition differs from what is expected from models based on the composition of meteorites.

Alternatively, the density of the blobs could be explained by the accumulation of dense oceanic material from rock plates that have been pushed down by tectonic plate movements.

Separate from this debate, our work shows that sinking plates transport continents to the African blob rather than the Pacific blob.

Interestingly, this result is consistent with recent work suggesting that the source of the mantle plumes rising from the African blob contains continental material, while the plumes rising from the Pacific blob do not.

Follow the blobs to find minerals and diamonds

Our work addresses fundamental questions about the development of our planet, but also has practical applications.

Our models provide a framework to more accurately determine the location of minerals related to mantle buoyancy. These include diamonds brought to the surface by kimberlites that appear to be associated with the blobs.

Magmatic sulphide deposits, which represent the world’s most important nickel reserves, are also associated with mantle plumes. By helping to target minerals like nickel (an essential ingredient in lithium-ion batteries and other renewable energy technologies), our models can contribute to the transition to a low-carbon economy.The conversation

Nicholas FlamentSenior lecturer, University of Wollongong; Andrew MerdithScientific Assistant, University of Leeds; Omer F. Bodurpostdoc, University of Wollongongand Simon WilliamsScientific Assistant, Northwest University, Xi’an.

This article is republished by The conversation under a Creative Commons license. read this original article.

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