One of the primary processes shaping Earth's surface is the stretching and eventual break-up of continents to create new ocean basins. The processes of stretching and heating of -150 km-thick plates, or layers of rock, are made evident at Earth's surface via earthquakes, volcanoes, hot springs, landslides and rockfalls that occur within and along the margins of narrow, long basins flanked by steep mountain ranges (e.g., East African rift, Gulf of Corinth). Ancient areas of stretching are more difficult to decipher, since the physical processes we wish to understand have since stopped, and competing processes of erosion, sedimentation, and subsidence of the rift zones makes them difficult to image. Finally, studies of continental break-up require an understanding of margins on both sides of ocean basins, which may now be 1000's of km apart (e.g., N. Atlantic). We propose to determine the evolution of continental rifting leading to break-up in the Gulf of Aden, where break-up occurred <20 My ago, and both margins are within a day's shiptime from one another. The short time interval since break-up means that sedimentary strata overlying the stretched, fractured, and heated rock layers of the plate are thin, and we can image clearly using geophysical methods. Our experiment involves the analyses of the travel times of seismic waves through the rock layers of the plates and underlying mantle where rocks are hotter, and where pockets of molten rock may have accumulated. Electronic devices capable of measuring vibrations from earthquakes worldwide (seismometers) will be buried along onshore continuations of profiles across the Gulf of Aden. We intend to use both man-made sound sources generated onboard the French seismic research vessel Marie Dufresne, which will travel along 3 profiles of the Gulf of Aden sending airblasts into the water every 50 metres. Our instruments will also record earthquakes occurring in the Himalayas, the Atlantic mid-ocean ridge, the Mediterranean sea, and the East African rift system. Variations in the arrival times of these sound waves recorded across our array allow us to map out velocity structure of the rocks beneath the array and across the width of the Gulf of Aden. Our aims are to map the geometry of the stretched layers within the plates, as well as their variations in velocity and physical properties. The velocity variations help us detect small variations in temperature and/or composition of the rocks, and help us determine the mechanical properties of the plates as they are stretched. Do the plates come apart along one large fracture, or fault zone, or does the stretching move continually inward to a narrow zone of necking, much like one finds stretching blue tack? Has some of the hot mantle rock which passively rises up to fill the place previously occupied by the plate depressurise and melt to form lavas? Where do these lavas form and rise up? Are these properties continuous along the length of the rift zone, or is the process three-dimensional? All of this information is vitally important to 1) oil explorationists trying to improve predictive models for oil and gas generation and migration 2) planners and government officials who need to evaluate seismic and volcanic hazards in areas of active rifting 3) earth scientists who wish to understand the physical properties of rocks so that we can adequately describe the physics of continental break-up and predict the onset of seafloor spreading. Funds are requested to cover travel to and from Oman to deploy instruments, to download data every 6 weeks (3 months) and every 12 weeks (9 months), plus travel to partner institutions to confer and integrate research results.