When continents break apart to form new oceans, the rupture process known as rifting is often accompanied by the production of large volumes of magma. Decades of petrological and geophysical studies converged in a theoretical model whereby magma is generated in the Earth’s mantle and then intruded at shallower depths in the crust, where it ponds in the form of interconnected transient pockets called sills. Sills are key elements in the evolution of magma-rich rifts, as they shape the physical and chemical structure of the crust. In addition, they feed phases of long-lasting volcanic activity, like the eruptions that are still ongoing in Iceland since 2021, or that occurred in Ethiopia during 2005-2010. However, our understanding of magmatic processes beneath rifts is partial and we still know little about the basic mechanism by which magma is extracted from the mantle to form the networks of sills in the crust.

Our Solid Earth Geophysics group at the Earth Sciences Department of University of Pisa (Italy), together with researchers from other institutions in Italy, China, Iceland, France, and UK tackled this open question in a scientific article published in Nature Communications (https://doi.org/10.1038/s41467-024-47136-4). Our international team used satellite-based techniques to explore the surface deformation induced by deep magma motions beneath the central portion of the Afar rift, developing across the countries of Ethiopia, Djibouti, Eritrea, and Somalia. The Afar rift is a key area for tectonics and volcanology studies since it is one of the few sub-aerial rift zones on Earth where we can directly observe the incipient formation of an ocean. The region is ideal to use satellites to explore the subtle signals related to magma motions beneath the surface during the breakup of a continent.

Our team adopted a main technique called Interferometric Synthetic Aperture Radar (InSAR) that allows for measuring millimetric motions of the Earth’s surface from space during a certain time-period. This technique was then combined with Global Navigation Satellite System (GNSS) measurements to understand how the Earth’s surface moved in three-dimensions. InSAR observations during 2014-2021 showed that a region about 100 km wide in central Afar rose up with rates of about 5 mm/yr. The most interesting observation was that the uplift started simultaneously at the end of 2016 at four areas separated by tens of km. We modeled the observed surface deformation and found that the uplift was induced by the simultaneous pressurization of four sills. The sills that we modeled are located at various crustal depths, ponding near the Mohorovičić discontinuity that separates the upper mantle and the lower crust (30 km to 22 km in the study area) or at the depth where the crust rheology change from ductile to brittle (10-15 km). These layers of the crust are known to be preferred location for the magma to accumulate due to changes in the physical property of the rocks.

Our results called for a mechanism that acts at the regional scale: how can distant sills start inflating at the same time? Our explanation is that these four sills shared a pressure connection with a common source and they responded to a sudden arrival of magma from the upper mantle (Figure 1), a mechanism that has been recently proposed for big volcanic islands such as Hawaii and Galapagos. Our study thus unveiled one of the possible mechanisms controlling the deep dynamic of magma during rifting and demonstrated that magma can be supplied in the crust trough episodic pulses from the mantle, forming the network of sills characterizing extensional plate margins.

Figure 1. Model of simultaneous pressurization of sills (red ellipses) in the Central Afar rift following the inflow of magma from the upper mantle