From a distance, active and ancient volcanoes in East Africa and across the central Indian Ocean may seem isolated from one another. But they could carry signals from much deeper inside the planet, from regions close to the boundary between Earth’s core and mantle; the vast layer of hot rock between the crust beneath our feet and the planet’s deep core.
In a new study published in Science, researchers sought to trace these signals by combining two different tools. The first was seismic imaging, which uses waves generated by earthquakes to create an indirect picture of Earth’s interior, somewhat like a medical CT scan. The second was the chemical analysis of volcanic rocks, because lava reaching the surface carries traces of the material that melted deep underground before rising.
A map built from earthquakes and chemistry
Shiyuan Bao, the study’s lead author and a researcher in the Department of Earth and Planetary Sciences at Harvard University, said a long-standing challenge in understanding some volcanoes is identifying where their material actually comes from.
Volcanic rocks show that material emerging at different locations is not chemically identical, suggesting that it may originate from different sources within the mantle. But locating those sources deep inside Earth has remained difficult because molten material does not necessarily travel upwards along simple, straight paths. Instead, it can bend, branch and be redirected by the complex structure of the planet’s interior.
To picture this, Bao suggests imagining the roots of a huge tree underground. Some roots branch, others meet, while some bend around obstacles that cannot be seen from the surface. The study suggests something similar may happen with “mantle plumes” — rising columns of hot material from deep inside Earth that can feed some volcanoes.
These plumes do not always appear as neat vertical pipes. Instead, they may form part of a broader network of deep regions where seismic waves travel more slowly. Such slower speeds can often indicate higher temperatures or differences in the composition of the rocks.
To account for this complexity, the researchers treated Earth’s interior almost like a three-dimensional road map. They then searched for the most likely routes connecting eight volcanic regions to possible sources near the bottom of the mantle.
The locations included the Afar region of Ethiopia, other parts of East Africa, the Comoros Islands, Réunion, Marion, Crozet, Kerguelen and Amsterdam–Saint Paul.
The results did not produce a single definitive route for each volcanic region. Instead, they revealed a range of possible pathways. Some were shorter, some travelled through areas where seismic waves move more slowly, and others clearly changed direction before reaching the deepest parts of the mantle.
This uncertainty matters, Bao explained, because Earth’s interior does not provide a single, simple answer. Nevertheless, a broader pattern emerged: these volcanic regions do not all appear to trace back to one uniform deep source. Instead, they seem to be connected to different regions deep inside the planet.
Three deep sources
The researchers also compared the chemistry of volcanic rocks using isotopes of elements including strontium, neodymium and lead. Isotopes are different forms of the same chemical element, and their proportions can act as fingerprints of where geological material came from and how it changed over time.
If volcanic rocks from distant places have similar isotope fingerprints, this can suggest that they originated from similar deep sources or from material that shared a similar geological history.
When the researchers compared their seismic results with the chemical evidence, they found a clear agreement between the two.
The volcanic locations fell into three main groups. The first included Afar, East Africa and the Comoros. A second was associated with Réunion, while a third included Marion, Crozet, Kerguelen and Amsterdam–Saint Paul.
The finding suggests that the eastern part of a huge structure deep beneath Africa and the Indian Ocean, known as the African Large Low Shear Velocity Province, may not be one simple, uniform mass. Instead, it appears to contain at least three distinct deep regions.
The structure sits near the base of the mantle and is identified because some seismic waves travel through it more slowly than through surrounding material.
According to Bao, the findings address a much larger question about Earth’s interior: Is this enormous deep structure simply unusually hot, or does it also contain ancient material with a different chemical composition?
The study points towards the second possibility.
If temperature alone explained the differences, the researchers would expect the deep sources feeding the volcanoes to look more similar. Instead, they found separate regions in both the seismic images and the chemical fingerprints of volcanic rocks. Together, these lines of evidence suggest that Earth’s deep interior may preserve chemically different materials that have not completely mixed, even over immense spans of geological time.
The study, however, does not provide a precise map of narrow pipes running from the bottom of the mantle to volcanoes at the surface. The researchers identify broad source regions that can be around 1,000 kilometres across, rather than proving the exact route taken by each mantle plume.
The findings also depend on the quality of seismic imaging models, which varies from place to place. The picture is less detailed in regions where fewer seismic stations are available, leaving some uncertainty about exactly how material travels from the deepest parts of Earth to the volcanoes we see at its surface.
