There are two blobs the size of continents beneath us. One is in Africa and the other in the Pacific.
The blobs are almost halfway to the centre of the Earth, with their roots 2,900 km below the surface. They are thought to be the birthplace of the deep mantle, which contains rising columns of hot rock.
The kind of volcanic eruptions that contributed to the extinction of the dinosaurs were the first to reach the surface. The eruption of a kind of rock called kimberlite, which brings diamonds from depths up to 800 km, may be controlled by the blobs.
The blobs have been known to exist for a long time, but how they have behaved over the years has been an open question. We discovered that blobs gather together and break apart like continents and supercontinents in new research.
Bodur.
The blobs are imaged from the data. There are two blobs, one at the top and one at the bottom.
The blobs are in the mantle, a thick layer of hot rock between Earth's crust and core. The mantle flows slowly. The blobs slow down waves caused by earthquakes, which suggests they are hotter than their surroundings.
Scientists agree that the blobs are linked to the movement of plates. How the blobs have changed over time has puzzled them.
One school of thought is that the blobs have been locked in place for hundreds of millions of years while other rock moved around them. The shape of the blobs may be changing due to the movement of the plates and mantles.
Our new research shows that Earth's blobs have changed a lot. In the same way that continents and supercontinents have at Earth's surface, they have assembled and broken up in the same way.
We used Australia's National Computational Infrastructure to run advanced computer simulations of how Earth's mantle has flowed over a billion years.
The models are based on the movements of plates. Subduction occurs when plates push into one another on the ocean floor.
The hot blobs are pushed aside when the cold rock from the ocean floor sinks deeper and deeper into the mantle.
The past 200 million years of Earth's interior. The hot structures are in yellow to red and the cold structures are in blue.
The blobs can form and break up as they are in the current configuration.
Although the blobs change position and shape over time, they still fit the pattern of volcanic eruptions recorded at Earth's surface. The pattern was a key argument for the blobs.
Our models show that the African blob was assembled as recently as 60 million years ago, in stark contrast to previous suggestions that the blob could have existed for nearly ten times as long.
What happened to the blobs? What are they made of? We don't know.
The denser the blobs, the more material they could contain, which could be separated from the rest of the mantle early in Earth's history. This could explain why the composition of the Earth is different from what is expected.
The density of the blobs could be explained by the dense material from the slabs of rock pushed down by the plate movement.
Our work shows that the African blob is more likely to be transported by sinking slabs than the Pacific blob.
This result is consistent with recent work that suggests the source of mantle plumes rising from the African blob contains continental material.
Our work addresses fundamental questions about the evolution of our planet, but also has practical applications.
Our models give a framework to target the location of minerals associated with mantle upwelling. This includes diamonds brought up to the surface by kimberlites.
The world's primary reserve of nickel is associated with magmatic sulfide deposits. Our models can contribute to the transition to a low-emission economy by helping target minerals such as nickel, an essential ingredient of lithium-ion batteries and other renewable energy technologies.
Nicolas Flament, Andrew Merdith, andmer F. Bodur are research fellow, University of Wollongong, and Simon Williams is a research fellow, Northwest University.
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