Their roots reach 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 extinction of the dinosaurs 65.5 million years ago. The plumes can also control the eruption of a rock called kimberlite, which brings diamonds from 120-150 km (and in some cases up to 800 km) to the Earth’s surface.
Scientists have long known about the existence of these spots, but the question of how they behaved throughout the history of the Earth remained open.
In a new study, we simulated a billion years of geological history and found that plumes come together and break apart, much like continents and supercontinents.
Images of the Earth’s formations obtained using seismic data. The African ball is at the top, and the Pacific one is at the bottom
Earth Bubble Evolution Model
The clots are found in the mantle, a thick layer of hot rock between the earth’s crust and core. The mantle is firm but flows slowly over long periods. We know about the existence of the mantle because it slows down the waves caused by earthquakes, which means that the mantle is hotter than its surroundings.
Scientists generally agree that the appearance of spots is associated with the movement of tectonic plates on the Earth’s surface. However, the way these spots have changed over the course of Earth’s history has puzzled them.
One school believes that the current spots acted as anchors, fixed in place for hundreds of millions of years as other rocks moved around them. However, we know that tectonic plates and mantle plumes move over time, and studies show that the shape of the clumps changes.
Our new research shows that clumps of land are changing shape and location much more frequently than previously thought. In fact, over the course of history, they have gathered and disintegrated in the same way that continents and supercontinents do on the surface of the Earth.
We used Australia’s National Computing Infrastructure to run advanced computer simulations of the movement of the Earth’s mantle over a billion years.
These models are based on the reconstruction of the movement of tectonic plates. When the plates collide with each other, the ocean floor is pushed between them in a process known as subduction.
Cold rock from the bottom of the ocean sinks deeper and deeper into the mantle and, reaching a depth of about 2,000 km, pushes hot clots to the side.
We have found that, like continents, clumps can assemble – forming “super clumps” as in the current configuration – and break apart over time.
A key aspect of our models is that, despite the change in position and shape of the bubbles over time, they still fit the pattern of volcanic and kimberlite eruptions recorded on the Earth’s surface. Previously, this pattern was a key argument in favor of the fact that the balls are motionless “anchors”.
Strikingly, our models show that the African Orb was assembled only 60 million years ago – in stark contrast to previous suggestions that the orb could have existed in its current form roughly ten times longer.
Remaining Bubble Questions
How did the clots come about? What exactly are they made of? We still don’t know.
The patches may be denser than the surrounding mantle, and so they may be composed of material separated from the rest of the mantle early in Earth’s history. This may explain why the mineral composition of the Earth differs from that expected from models based on the composition of meteorites.
Alternatively, patch density can be explained by the accumulation of dense oceanic material from slabs of rock pushed downward by tectonic plate movement.
Regardless of these debates, our work shows that sinking plates are more likely to carry continental fragments into the African patch than into the Pacific patch.
Interestingly, this result is consistent with recent work suggesting that the source of mantle plumes rising from the African Clot contains continental material, while plumes rising from the Pacific Clot do not.
Bubble tracking for minerals and diamonds
While our work raises fundamental questions about the evolution of our planet, it also has practical applications.
Our models provide the basis for more precise localization of minerals associated with mantle uplift. These include diamonds brought to the surface by kimberlites, which are apparently associated with these clots.
Igneous sulphide deposits, which are the world’s main nickel reserves, are also associated with mantle plumes. By helping to find minerals such as nickel, an important component of lithium-ion batteries and other renewable energy technologies, our models can facilitate the transition to a low-emission economy.
Contact us: [email protected]