Imaging the African Superplume

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Since the Earth formed some 4.5 billion years ago, it has had one objective, to cool down. The Earth's mantle, found between the 5-50 km thick crust on which we live, and the iron core at 3000 km depth, is rocky in nature. However, over hundreds of millions of years cold material at the surface sinks into the Earth, while hot material from the interior rises. This convective process is the engine for plate tectonics, and has shaped the Earth as we know it today. One, often debated suggestion is that some of the upward flow of material is focused in cylindrical columns, 'mantle plumes', rising from the edge of the core. Mantle plumes have been proposed to cause present-day volcanism away from plate boundaries, at so-called hotspots like Hawaii and Afar in eastern Africa. Plumes have also been held responsible for much larger past eruptions that may have split continents, and changed the climate. To find plumes, we can use seismology to look inside the Earth. Earthquakes occurring around the world send vibrations (like sound waves) through our planet. We record these vibrations all over the world using sensitive instruments called seismometers. If earthquake energy passes through a hot region, it will slow down, and if it passes through a cold region it will speed up. If we record enough earthquake waves from different parts of the world, we can build up a 3D image of the Earth's interior (much like a medical doctor uses X-rays). With global networks of seismometers, we have successfully imaged cold material sinking to large depths in the Earth, as these bodies are hundreds of kilometers in size. However, to date no one has conclusively imaged hot rising plumes, as these are thought to be very narrow (10s of kms across), too small to see with the sparse global distribution of seismometers. Dense regional seismic networks would help, but most hotspots are located in the oceans where deploying seismometers is challenging and expensive. The aim of the proposed project is to study one of the few major continental hotspot regions, the East African Rift. Over the past two decades, many temporary seismic networks have been deployed across the area from South Africa to Saudi-Arabia, to understand continental break-up. Due to their small size, each individually can only image the top few hundred kilometers of the Earth. To look deeper, the different network groups have agreed to share their data with me to create one large array. This means I will be able to image a possible plume from its source deep in the Earth to the surface. These images will allow me to investigate what the plume is made of, how hot it is, and what role it plays in plate tectonics and the cooling of the Earth. A direct benefit of producing high resolution models of the Earth's interior is that we will be able to better locate earthquakes and nuclear explosions globally. Mantle plumes are also held responsible for Africa's unusually high elevation, kilometers above sea level. By better understanding plumes, we can predict uplift history, which in turn controls where oil may collect in East Africa's river basins. The seismic images will also help to answer the fundamental question of what role plumes play in the Earth's internal engine, and how they affect the breaking up of the African continent along the rift. With this understanding, we can improve assessment of the volcanic and seismic hazards associated with this process, a current concern to collaborators in East Africa.