The scientists stuck close to their guides to avoid the landmines that littered their path, slept in huts, and had to secure permission from tribal leaders to gain access to study sites. All of their efforts were not in vain, however, as they led to the discovery that volcanic activity in the Catanda complex, a small cluster of volcanic cones in Angola, is surprisingly recent.
Andrea Giuliani, a member of the Reservoirs and Fluxes Community, and his colleagues at the University of Melbourne and University of Tasmania (Australia) joined the collaboration to analyze the volcanic rock samples collected by his Spanish partners. Using geochemical techniques to determine the age and source of the samples, Giuliani estimates that they originated in the mantle and came to the surface very recently, only about 500 to 800 thousand years ago. Furthermore, by employing geophysical and geochemical modeling, the researchers think that the volcanic activity is due to an upwelling rising up from deep in the mantle underneath Angola. They report their findings in a new paper in Geology .
On their Angola visits, the Spanish researchers had collected carbonatite and related rock samples. These lavas are rich in carbonate minerals and typically found as eruptions from the mantle near continental rift areas, where tectonic plates are splitting apart. Giuliani analyzed the samples for isotopes, which are atoms of an element that carry a different number of neutrons. Certain isotopes are unstable and break down at known intervals, a property that can be used to calculate a mineral’s age. At first, Giuliani selected isotopes (rubidium-strontium and uranium-lead) suitable for dating rocks that are millions of years old. “Initially, we couldn’t get an age, simply because we were not using techniques that were sensitive enough to very young ages,” said Giuliani. Additional testing with argon-argon and uranium-helium systems placed the time of eruption at 500 to 800 thousand years ago, practically “yesterday” in geological history.
“It was exciting,” said Giuliani. “I did a little bit of research and I realized that there was no young volcanic activity in that area of Africa – there is nothing, even within a few million years of present day.”
Giuliani also compared the isotopic fingerprints of the rocks to lava from other regions in Africa and saw that they matched other young, volcanic samples that had originated in the mantle.
After establishing the young age of the carbonatite eruption in Angola, Giuliani teamed up with Juan Carlos Afonso (Macquarie University, Australia) to create a geodynamic model to understand what was happening in the underlying mantle. They used seismic tomography, a technique that uses the waves from earthquakes that travel through Earth to create 3D images of structures and anomalies in the subsurface. Giuliani and Afonso detected an unusually hot region that differs from the surrounding mantle, reaching up from where the mantle meets Earth’s core, to the area beneath Angola and northern Namibia. The researchers suspect that this is a finger of the “South African Superplume” that feeds volcanic activity in the East African Rift and other parts of the continent.
Reports of earthquakes from the U.S. Geological Survey and local tribal leaders support the findings that the area is volcanically active. Giuliani speculates that there will be more volcanic activity in the future and there may be other young volcanic products in unexplored regions of Angola.
There are two schools of thought on what controls where and when these eruptions have occurred in Africa and elsewhere. One school thinks that the main driver is heat in the mantle that causes plumes of hot material to ascend, while the second school thinks that volcanism occurs in response to stress and strain in the rocky lithosphere without any ultradeep feeder. The researchers’ model suggests that there is interplay between these two forces, with the shallow trajectory of mantle upwelling controlled by the lithosphere structure. Giuliani and Afonso will continue modeling the feedback between deep processes, lithosphere, and surface phenomena in the future.
The study’s combined geochemical and geophysical evidence suggest a link between carbonatitic volcanoes at the surface and deep mantle processes in southwestern Africa. “It will be interesting to learn more about the link between the deep carbon cycle and the shallow carbon cycle in this region,” said Giuliani.
At the Catanda complex in Angola, volcanic cones surround a crater formed by a volcanic eruption that occurred about 500 to 800 thousands years ago. Credit: Marc Campeny