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 pillars of hot rock called “deep mantle flags” that reach the earth’s surface.
Once these flags reach the surface, giant volcanic eruptions occur – the kind that contributed to the extinction of dinosaurs 65.5 million years ago. The flaps can also control the eruption of a kind of rock called kimberlite, which brings diamonds from depths of 120-150 km (and in some cases up to about 800 km) to the Earth’s surface.
Scientists have known that blobs existed for a long time, but how they have behaved throughout Earth’s history has been an open question. In new research, we modeled a billion-year geological history and discovered that the cliffs are gathering and breaking like continents and supercontinents.
Above: Earth’s blobs as depicted from seismic data. The African blob is at the top and the Pacific blob at the bottom.
A model for the Earth’s blob development
The flaps 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 time scales. We know the blobs are there because they slow down waves caused by earthquakes, suggesting that the blobs are warmer than their surroundings.
Scientists generally agree that the blobs are associated with the movement of tectonic plates on the Earth’s surface. But how the blobs have changed over the history of the Earth has puzzled them.
One thought has been that the current blobs have served as anchors, locked in place for hundreds of millions of years, while other rocks move around them. We do know, however, that tectonic plates and mantle planes move over time, and research suggests that the shape of the blobs is changing.
Our new research shows that Earth’s blobs have changed shape and location far more than previously thought. In fact, throughout history, they have accumulated and broken up in the same way that continents and supercontinents have done on the Earth’s surface.
We used Australia’s National Computational Infrastructure to run advanced computer simulations of how the Earth’s mantle has flowed over a billion years.
These models are based on reconstructing the movements of tectonic plates. When plates push into each other, the ocean floor is pushed down between them in a process known as subduction.
The cold rock from the seabed sinks deeper and deeper into the mantle, and when it reaches a depth of about 2,000 km, it pushes the warm blobs aside.
Above: The last 200 million years of the Earth’s interior. Warm structures are in yellow to red (darker is shallower) and cold structures in blue (darker is deeper).
We found that like continents, blobs can accumulate – and form “superblobs” as in the current configuration – and break up over time.
A key aspect of our models is that even though the blobs change position and shape over time, they still fit the pattern of volcanic and kimberlite eruptions recorded on the Earth’s surface. This pattern was previously a central argument for the blobs as motionless “anchors”.
Strikingly enough, our models reveal the African blob collectively as late as 60 million years ago – in stark contrast to previous proposals, the blob could have existed in roughly its current form for almost ten times as long.
Remaining questions about the blobs
How did the blobs originate? What exactly are they made of? We still do not know.
The valves may be denser than the surrounding mantle, and as such they could consist of material separated from the rest of the mantle early in Earth’s history. This could explain why the Earth’s mineral composition is different from that expected from models based on the composition of meteorites.
Alternatively, the density of the cliffs could be explained by the accumulation of dense oceanic material from rock slabs pushed down by tectonic plate motion.
Regardless of this debate, our work shows that sinking plates are more likely to transport fragments of continents to the African blob than to the Pacific blob.
Interestingly, this result is consistent with recent work suggesting that the source of coat flags rising from the African blob contains continental material, whereas flags rising from the Pacific blob do not.
Tracing blobs to find minerals and diamonds
While our work addresses fundamental questions about the evolution of our planet, it also has practical applications.
Our models provide a framework for more accurately targeting the location of minerals associated with mantle swelling. This includes diamonds brought up to the surface by kimberlites that appear to be associated with the blobs.
Magmatic sulfide deposits, which are the world’s primary reserve of nickel, are also associated with mantle flags. By helping to target minerals such as nickel (an essential ingredient in lithium-ion batteries and other renewable energy technologies), our models can contribute to the transition to a low-emission economy.
Nicolas Flament, Lecturer, University of Wollongong; Andrew Merdith, Research Fellow, University of Leeds; Ömer F. Bodur, Postdoctoral Fellow, University of Wollongong, and Simon Williams, Research Fellow, Northwest University, Xi’an.
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