This article was originally published on The conversation. The publication contributed the article to Space.com magazine. Expert Voices: Op-Ed and Insights.
Nicolas FlamentSenior Professor, University of Wollongong
André MerdithResearcher at the University of Leeds
Ömer F. BodurPostdoctoral Researcher, University of Wollongong
Simon WilliamsResearcher, Northwest University, Xi’an
Deep in the Earth, below us, are two continent-sized bubbles. One is under Africa, the other under the Pacific Ocean.
The bubbles have their roots 2,900 kilometers 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 reach the surface, giant volcanic eruptions occur – the kind that contributed to the extinction of the dinosaurs 65.5 million years ago. The bubbles can also control the eruption of a type of rock called kimberlite, which brings diamonds from depths of 120 to 150 km (and in some cases as much as 800 km) to the Earth’s surface.
Scientists have known that bubbles have been around for a long time, but how they behaved throughout Earth’s history is an open question. In new research, we modeled a billion years of geological history and found that the bubbles come together and break apart like continents and supercontinents.
Related: Layers of the Earth: Exploring our planet inside and out
A model for the evolution of the Earth blob
The bubbles are in the mantle, the thick layer of hot rock between the Earth’s crust and its core. The mantle is solid, but it flows slowly over long timescales. We know the bubbles are there because they slow down waves caused by earthquakes, which suggests that the bubbles are hotter than their surroundings.
Scientists generally agree that the bubbles are linked to the movement of tectonic plates on the Earth’s surface. However, how bubbles have changed throughout Earth’s history intrigued them.
One school of thought has been that the current bubbles acted as anchors, locked in place for hundreds of millions of years as other rocks moved around them. However, we do know that tectonic plates and mantle plumes move over time, and research suggests that the shape of the bubbles is changing.
Our new research shows that Earth’s bubbles have changed shape and location much more than previously thought. In fact, throughout history, they have formed and disintegrated in the same way as continents and supercontinents on the Earth’s surface.
We used Australia’s National Computing Infrastructure to run advanced computer simulations of how the Earth’s mantle flowed over a billion years.
These models are based on the reconstruction of tectonic plate movements. When plates push against each other, the ocean floor is pushed down between them in a process known as subduction. Cold rock from the ocean floor sinks deeper and deeper into the mantle, and when it reaches a depth of about 2,000 km, it pushes the hot bubbles aside.
We found that, like continents, blobs can form – forming “superblobs” as in the current setup – and break down over time.
A key aspect of our models is that while the bubbles change position and shape over time, they still fit the pattern of volcanic eruptions and kimberlite recorded on the Earth’s surface. This pattern was previously a key argument for blobs as immobile “anchors”.
Surprisingly, our models reveal the African bubble assembled 60 million years ago – in contrast to previous suggestions that the bubble could have existed in roughly its current form for nearly ten times longer.
Remaining doubts about blobs
How did blobs come about? What exactly are they made of? We don `t know yet.
The bubbles may be denser than the surrounding mantle and as such may consist of material separated from the rest of the mantle early in Earth’s history. This could explain why Earth’s mineral composition is different from that expected from models based on meteorite composition.
Alternatively, the density of the bubbles can be explained by the accumulation of dense oceanic material from slabs of rock pushed down by the movement of tectonic plates.
Regardless of this debate, our work shows that sinking slabs are more likely to transport continent fragments to the African bubble than to the Pacific bubble. Interestingly, this result is consistent with recent work suggesting that the source of mantle plumes arising from the African bubble contains continental material, while plumes arising from the Pacific bubble do not.
Tracing the bubbles to find minerals and diamonds
While our work addresses fundamental questions about our planet’s evolution, it also has practical applications.
Our models provide a framework to more accurately target the location of minerals associated with mantle upwelling. This includes diamonds brought to the surface by kimberlites that appear to be associated with the bubbles.
Magmatic sulphide deposits, which are the world’s main nickel reserve, are also associated with mantle plumes. 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-emissions economy.
This article is republished from The conversation under a Creative Commons license. read the original article.
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